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Chapter 10: Animal Tissue

In the crowded hospitals of Paris in the late 1700s, a young French anatomist and pathologist named Marie François Xavier Bichat spent his nights doing something most physicians of his time found pointless. He dissected the bodies of patients who had died, over and over again, looking not for damaged organs but for damaged layers.

The medicine of that period treated the body as a collection of organs — heart, liver, lung, stomach. Bichat noticed something that did not fit. When disease struck, it often did not ruin an entire organ. It attacked one particular membrane or layer inside that organ, and the very same kind of layer could be found in other, completely unrelated organs. A disease of “the chest” and a disease of “the abdomen” could, in fact, be a disease of the same material.

So he took organs apart by hand — peeling, drying, boiling, soaking them in acids — to see which parts behaved alike. His conclusion overturned the older view. Organs, he argued, are not the basic building units of the body at all. They are assembled from a small number of simpler materials, which he called tissues, and he described about twenty-one of them.

Remarkably, he did almost all of this without a microscope, an instrument he distrusted. He died at about thirty years of age, but his idea reshaped medicine: to understand a disease, look at the tissue. He is remembered today as the Father of Histology.

The four kinds of animal tissue you are about to study are shown together in Fig. 10.0.

Diagram

Fig. 10.0 – Opening illustration: outline of the human body with four zoom-in circles, each labelled — a sheet of epithelial cells lining a surface, connective tissue with cells and fibres in a matrix, striated muscle fibres, and a single neuron with dendrites and axon.

NoteCuriosity Corner

Q1. A cut on your skin heals within a few days, but an injured nerve may never fully recover. What is it about these two tissues that makes such a difference? Q2. Your heart beats all night without you ever ordering it to, yet your arm moves only when you decide to move it. What makes the muscle in the heart so different from the muscle in your arm? Q3. If you fold your ear pinna it springs back into shape, but a bone will not bend at all. Both are supporting tissues — so why do they behave so differently? Q4. Animals living in very cold regions carry a thick layer of fat under their skin. Which tissue is this, and how exactly does it keep them warm? Q5. Blood is a liquid that flows through vessels. How can something that flows be called a tissue at all?

Keep these five questions in mind as you read. By the end of this chapter, you will be able to answer every one of them.

10.1 Histology

What Exactly Is a Tissue?

Your body is not a random crowd of cells. Cells of the same kind are grouped together, and each group is given one particular job to do. Such a group is called a tissue.

A tissue is a group of cells that share the same embryonic origin, the same basic structure, and the same function. “Embryonic origin” simply means that all these cells were formed from the same layer of the developing embryo, which is why they end up looking alike and behaving alike.

NoteDefinition

A tissue is a group of cells having the same embryonic origin, the same structure and the same function.

Q. Why does the definition insist on a common embryonic origin, and not just a similar shape?

A. Because shape alone can mislead. Cells from different embryonic layers may look similar under a microscope but behave very differently. A shared origin is what guarantees that the cells of a tissue truly work as one functional team.

From Cells to the Whole Body

Tissues do not stay isolated. Several tissues combine in an orderly, definite arrangement to form a large functional unit called an organ. The stomach, for example, is not made of one tissue but of epithelial, connective, muscular and nervous tissue working together.

Organs in turn combine to form an organ system, and all the organ systems together make up the body. This step-by-step build-up is the organisational ladder of every animal, and it is summarised in FC 10.1.

Flowchart

FC 10.1 – Levels of organisation in an animal body: Cells → Tissues → Organs → Organ systems → Body

a tissue Zoom in: a tissue is made of cells, and each cell of molecules. Zoom out: tissues build organs, organs build organ systems, and organ systems build the whole organism.

The human body is built from roughly 37 trillion cells. All of them are sorted into just four basic tissue types — an extraordinary amount of order from an extraordinary number of parts.

Two Kinds of Cells

The cells of an animal body are of two types: somatic cells and germ cells. The word “somatic” comes from the Greek word soma, meaning “body”.

All the body cells of an organism except sperm and ova are somatic cells. The sperm and the ova are the germ cells, and they belong to the reproductive system. Every tissue you will study in this chapter is built from somatic cells.

somatic cells vs germ cells Somatic cells are all the ordinary body cells and are not passed on to the next generation. Germ cells are only the sperm and ova, and they alone carry the parent’s genetic material into the offspring.

The Study of Tissues

The study of the structure and arrangement of tissue is called histology.

NoteDefinition

Histology is the study of the structure and arrangement of tissues.

The name comes from the Greek histos, meaning “tissue” or “web”, and logos, meaning “study”. The scientist you met in the chapter opener, Marie François Xavier Bichat, is honoured as the Father of Histology for establishing that tissues, not organs, are the body’s building materials.

Histology is not a museum subject — it is used in hospitals every single day. When a doctor suspects cancer, a small piece of the suspected tissue is removed. This is called a biopsy. The sample is cut into extremely thin slices, stained with dyes, and examined under a microscope by a pathologist. The pathologist is reading exactly what Bichat was after: has the normal arrangement of this tissue broken down? Cancerous tissue loses its orderly arrangement — cells crowd together, change shape, and cross boundaries they should respect. In most cancers, the final and deciding diagnosis is made not by a scan but by a histologist looking at the tissue.

The Four Animal Tissues

There are four types of tissues present in animals: epithelial, connective, muscular and nervous. This classification is shown in FC 10.2, and it is the framework for the rest of this chapter.

Flowchart

FC 10.2 – Animal tissues: Animal tissue → branches into four types → Epithelial tissue (covering and lining); Connective tissue (binding and support); Muscular tissue (contraction and movement); Nervous tissue (communication and control)

Notice that these four names are really four jobs. Epithelial tissue covers and lines, connective tissue binds and supports, muscular tissue contracts to produce movement, and nervous tissue carries messages. Once you hold on to the job, the structure of each tissue starts to make sense, because in biology structure almost always follows function. Let us now study each type in detail.

the four animal tissues E-C-M-N — Every Cell Makes Networks: Epithelial, Connective, Muscular, Nervous.

  1. In your own words, what is a tissue?
  2. Where in the body would you expect to find squamous epithelial tissue?
  3. List the functions of bone.

10.2 Epithelial Tissue (epi : above, thelium : layer of cells)

The name itself tells you where this tissue sits. Epi means “above” and thelium means “layer of cells” — a layer of cells lying above something else. Epithelial tissue forms a covering on the inner and the outer surfaces of the body and of the organs.

Think of it as the body’s wrapping and lining material. Your skin’s outer layer, the lining of your windpipe, the inner lining of your intestine and the wall of a blood capillary are all epithelial tissue. Wherever the body has a boundary — with the outside world or with the space inside a hollow organ — an epithelium marks that boundary.

How Epithelial Cells Are Arranged

The cells of this tissue are compactly arranged, with very little intercellular matrix between them. This tight packing is not accidental. A covering layer must not have gaps in it, otherwise the very substances it is meant to keep out would leak straight through.

The cells rest on a non-cellular basement membrane. “Non-cellular” means the basement membrane is not made of cells at all; it is a thin sheet of secreted material lying underneath the epithelium, and it acts as the foundation on which the cell layer is anchored.

Depending on the type, the cells are polygonal, cuboidal or columnar in shape. A single nucleus is present in each cell, placed either at the centre or at the base.

the epithelial layer (cells packed tightly with almost no matrix → no gaps for materials or microbes to slip through → the layer works as a true barrier; cells anchored on a basement membrane → the sheet stays firmly in place under friction and stretching → the covering is not torn away by everyday wear)

Two Properties Worth Understanding

Epithelial tissue is avascular. This means it has no blood vessels of its own running through it. Its cells are nourished by materials that diffuse across the basement membrane from the blood vessels of the connective tissue lying below. This also explains why an epithelium is always thin enough for diffusion to reach every one of its cells.

It has a good capacity of regeneration. Epithelia line the surfaces that face constant friction, chemicals, heat and microbes, so their cells are damaged and lost all the time. To survive, the tissue keeps a population of actively dividing cells that continuously replace the lost ones. This is exactly why a scraped knee grows a new skin surface within days.

Q. Why is a tissue with no blood supply of its own placed at the most exposed surfaces of the body?

A. Because blood vessels would be fragile at a surface that is constantly rubbed, dried and attacked. Instead, the epithelium stays thin and gets its nourishment by diffusion from below, while the vessels stay safely in the connective tissue beneath.

What Epithelial Tissue Does

Its major function is protection. Along with this it also helps in absorption, transport, filtration and secretion.

Notice that these are all “boundary jobs”. Anything that enters or leaves the body, or moves between two compartments inside the body, has to cross an epithelium — which is why this one tissue carries such a varied list of functions.

Simple and Compound Epithelium

Epithelial tissue is classified into two types: simple epithelium and compound epithelium. This classification, along with the types under each, is set out in FC 10.3.

Flowchart

FC 10.3 – Classification of epithelial tissue: Epithelial tissue → branches into (A) Simple epithelium (single layer of cells) → Squamous → Cuboidal → Columnar → Ciliated → Glandular → Sensory → Germinal; and (B) Compound epithelium (two or more layers of cells) → Stratified → Transitional

Simple epithelium is made up of a single layer of cells. Compound epithelium is made up of two or more layers of cells, and here only the lowermost layer lies on the basement membrane.

Simple epithelium vs Compound epithelium | Feature | Simple epithelium | Compound epithelium | |—|—|—| | Number of layers | One layer of cells | Two or more layers of cells | | Contact with basement membrane | Every cell rests on it | Only the lowermost layer rests on it | | Thickness | Thin | Thick | | Best suited for | Absorption, secretion, filtration, transport | Protection against wear and tear | | Examples | Lining of intestine, alveoli, nephron tubules | Epidermis of skin, urinary bladder |

Once you see this, the logic behind the two designs becomes clear. A single layer is thin enough to let substances pass through, so simple epithelium is placed where materials must be exchanged. Several layers are too thick to pass anything through, so compound epithelium is placed where the main need is protection.

A. Simple Epithelium
1. Squamous Epithelial Tissue

The cells of this tissue are flat, thin and polygonal with a serrated margin — that is, the edges are wavy rather than straight. The cells fit together like the tiles of a footpath, each wavy edge interlocking with its neighbour. Because of this appearance the tissue is also called pavement epithelium. A prominent spherical or oval nucleus is present at the centre of the cell, as seen in Fig. 10.1.

Diagram

Fig. 10.1 – Squamous epithelial tissue: flat interlocking polygonal cells seen from above and in section; label apical surface, nucleus, basolateral surface, basement membrane.

Function: protection, absorption, transport, filtration and secretion. It is found in blood vessels, alveoli, coelom, etc.

squamous epithelium in the alveoli (cells extremely flat and thin → the distance a gas has to cross is tiny → oxygen and carbon dioxide diffuse rapidly between air and blood → efficient gas exchange in the lungs)

The same thin design explains a serious clinical problem. In pulmonary oedema, fluid collects in the tissue around the alveoli. The gas now has to cross not just the paper-thin squamous layer but an added layer of fluid as well. The diffusion distance increases, oxygen uptake falls, and the patient becomes breathless even at rest. The tissue has not been destroyed — only its thinness has been lost, and that alone is enough to cripple its function.

2. Cuboidal Epithelium

In this tissue the cells are cuboidal in shape — roughly as tall as they are wide — with a spherical nucleus at the centre, as shown in Fig. 10.2.

Diagram

Fig. 10.2 – Cuboidal epithelium: a single row of cube-shaped cells; label nucleus, cytoplasm, basement membrane, connective tissue.

Function: absorption and secretion. It is found in the lining of the pancreatic duct, the salivary duct, and the proximal and distal convoluted tubules of the nephron.

A cuboidal cell has more cytoplasm than a flat squamous cell, and therefore room for more mitochondria and more secretory machinery. That is why it is found where work has to be done — pushing substances in or out — rather than where thinness is required.

3. Columnar Epithelium

Columnar epithelial cells are tall and pillar-like. The inner ends of the cells are narrow while the free ends are broad and flat. The free surface shows a large number of microvilli — fine finger-like projections of the cell membrane. The nucleus is oval and is present in the lower half of the cell. Fig. 10.3 shows this arrangement.

Diagram

Fig. 10.3 – Columnar epithelium: tall pillar-like cells with microvilli at the free surface; label microvilli, mucus in goblet cell, absorptive cell, basement membrane, connective tissue.

Function: secretion and absorption. It is found in the inner lining of the intestine, gall bladder, gastric glands, intestinal glands, etc.

microvilli of the intestinal lining (thousands of microvilli on every free surface → total absorbing surface increases enormously without the intestine becoming longer → far more digested food is absorbed in the same length of gut)

4. Ciliated Epithelium

The cells of this tissue are cuboidal or columnar. Their free ends are broad while the narrow ends rest on a basement membrane. The free ends show hair-like cilia. The nucleus is oval and is placed at the basal end of the cell, as in Fig. 10.4.

Diagram

Fig. 10.4 – Ciliated epithelium: columnar cells bearing cilia at the free surface; label cilia, cytoplasm, nucleus, basement membrane.

Function: to create a movement of materials in contact with it in a specific direction, and thus to prevent the entry of foreign particles in the trachea and to push the ovum through the oviduct. It is found in the inner lining of the buccal cavity of frog, the nasal cavity, the trachea and the oviduct of vertebrates, etc.

The key word here is “specific direction”. Cilia do not wave randomly; all the cilia of the layer beat together in one direction, like a field of wheat bending under a single gust. This is why the tissue can transport something along a definite path.

cilia vs microvilli Cilia are longer, fewer, and they move — they beat to push materials along a surface. Microvilli are much shorter, far more numerous, and they do not move — they only increase the surface area for absorption. Both appear at the free surface, but one transports and the other absorbs.

Tobacco smoke paralyses and eventually destroys the cilia lining the trachea and bronchi. Dust, tar and trapped microbes can no longer be swept upward and instead settle in the airways. The body’s only remaining way to clear them is forceful coughing — which is precisely what a long-term smoker’s persistent morning cough is, and it is also why smokers pick up chest infections far more easily.

5. Glandular Epithelium

Here the cells may be columnar, cuboidal or pyramidal in shape. The nucleus is large and situated towards the base. Secretory granules — small packets of the substance the cell will release — are present in the cytoplasm, as seen in Fig. 10.5.

Diagram

Fig. 10.5 – Glandular epithelium: columnar absorptive cells with a mucus-filled goblet cell among them; label microvilli, mucus, cytoplasm, goblet cell, nucleus, cell membrane, absorptive cell, basement membrane.

The glands may be unicellular (goblet cells of the intestine) or multicellular (salivary gland), depending on the number of cells. Depending on the mode of secretion, multicellular glands can be classified as duct-bearing glands (exocrine glands) and ductless glands (endocrine glands). This classification is summarised in FC 10.4.

Flowchart

FC 10.4 – Classification of glands: Glands → by number of cells → Unicellular (goblet cells of intestine) or Multicellular (salivary gland); Multicellular glands → by mode of secretion → Exocrine, duct-bearing (salivary gland, sweat gland) or Endocrine, ductless (thyroid gland, pituitary gland)

Exocrine glands pour their secretions at specific sites, e.g. salivary gland, sweat glands, etc. Endocrine glands release their secretions directly into the blood stream, e.g. thyroid gland, pituitary gland, etc.

NoteDefinition

An exocrine gland is a gland that carries its secretion to a specific site through a duct; an endocrine gland is a ductless gland that releases its secretion directly into the blood stream.

Function: to secrete the mucus that traps dust particles, to lubricate the inner surface of the respiratory and digestive tracts, and to secrete enzymes and hormones.

Exocrine glands vs Endocrine glands | Feature | Exocrine glands | Endocrine glands | |—|—|—| | Duct | Present (duct-bearing) | Absent (ductless) | | Where secretion goes | To a specific site | Directly into the blood stream | | Speed and reach | Local and quick | Carried all over the body | | Typical secretion | Enzymes, mucus, sweat | Hormones | | Examples | Salivary gland, sweat gland | Thyroid gland, pituitary gland |

The pancreas is unusual because it is both. Its exocrine part sends digestive enzymes through the pancreatic duct into the small intestine. Its endocrine part, scattered in small clumps of cells, releases insulin straight into the blood. This is why one organ can be involved in two very different problems: a blocked pancreatic duct causes trouble with digestion, while failure of the insulin-releasing cells causes diabetes mellitus.

a speck of dust breathed into the nose 1. The dust particle enters the nasal cavity with inhaled air. 2. It sticks to the sticky mucus secreted by glandular cells (goblet cells) of the lining. 3. The cilia of the ciliated epithelium beat in one direction and move the mucus sheet, with the dust in it, towards the throat. 4. The mucus reaches the pharynx and is swallowed. 5. The trapped dust and microbes are destroyed by the acid of the stomach — the particle never reaches the alveoli.

Flowchart

FC 10.5 – Mucociliary clearance: dust enters nasal cavity → trapped in mucus from goblet cells → cilia beat in one direction → mucus sheet moves towards pharynx → swallowed → destroyed in the stomach

6. Sensory Epithelial Tissue

It is composed of a modified form of columnar cells and elongated neurosensory cells. Sensory hairs are present at the free end of the cell, as shown in Fig. 10.6.

Diagram

Fig. 10.6 – Sensory epithelium of the olfactory region; label olfactory bulb, olfactory tract, mitral cell, cribriform plate, axon, supporting cell, receptor cell, dendrite, epithelium, cilia.

Function: it perceives external as well as internal stimuli. These are found in the nose (olfactory cells), the ear (auditory hair cells) and the eye (photoreceptors).

Sensory epithelium is the point where a covering tissue begins to behave like nervous tissue. The neurosensory cell picks up a stimulus at its free end and passes the message on to a nerve fibre — so it is the first link in the chain of communication you will study as nervous tissue later in this chapter.

7. Germinal Epithelium

The cells of this epithelium divide meiotically to produce haploid gametes. Meiosis is the type of cell division that halves the chromosome number, so a diploid (2n) parent cell gives rise to haploid (n) gametes. This is essential, because at fertilisation two gametes fuse and the original chromosome number must be restored, not doubled.

Example: the lining of the seminiferous tubules and the inner lining of the ovary.

germinal epithelium Zoom in: it is a simple epithelial layer whose cells are capable of meiosis. Zoom out: this single layer of tissue is the starting point of the entire reproductive system, and therefore of the next generation.

B. Compound Epithelium

Compound epithelium is shown in Fig. 10.7, and it exists in two forms.

Diagram

Fig. 10.7 – Compound epithelial tissues: side-by-side sections of stratified epithelium (with stratum germinativum below and flattened stratum corneum above) and transitional epithelium (rounded cells in the relaxed state).

a. Stratified Epithelium

The nucleus is present in the stratum germinativum, the deepest layer of the tissue. This is the layer that keeps dividing and pushing new cells upwards. The cells at the free surface become flat and lack a nucleus, and this outermost layer is called the stratum corneum.

Function: protection.

Examples: epidermis of skin, oesophagus, cornea, vagina, rectum.

Follow the life of one cell here and the design makes sense. A cell born in the stratum germinativum is slowly pushed away from the basement membrane. Since the epithelium is avascular and nourishment reaches it only by diffusion from below, the cell gets less and less supply as it rises, flattens, loses its nucleus, and finally dies and is shed from the surface. The tissue therefore protects the body with a layer of dead, tough cells while staying alive underneath.

This explains why deep burns are so dangerous. If only the surface layers are lost, the stratum germinativum survives and the skin regrows on its own. If the burn destroys the germinativum as well, there are no dividing cells left to rebuild the epithelium, the raw surface loses fluid and admits infection, and a skin graft becomes necessary.

b. Transitional Epithelium

The structure of transitional epithelium is the same as that of stratified epithelium. The special feature is that its cells can undergo a change in their shape and structure depending on the degree of stretch.

Function: distension of the organ — that is, allowing the organ to swell without tearing.

Example: urinary bladder.

An empty urinary bladder has a thick wall of rounded, dome-shaped cells piled several layers deep. As urine collects, the cells slide over one another and flatten out, so the wall becomes thinner and the bladder expands greatly without any tear in its lining. When the bladder empties, the cells return to their rounded shape. No other epithelium in the body can change its own shape and layering to this extent.

Stratified epithelium vs Transitional epithelium | Feature | Stratified epithelium | Transitional epithelium | |—|—|—| | Cell shape | Fixed; flattens towards the surface | Changes with the degree of stretch | | Surface layer | Dead, flat, without nucleus (stratum corneum) | Living, rounded when relaxed, flat when stretched | | Main function | Protection | Distension of the organ | | Example | Epidermis of skin, oesophagus | Urinary bladder |

When do the transitional cells change their shape?

Cell Junctions

A sheet of cells is only useful if the cells are firmly held together, otherwise the layer would come apart under the slightest pull. The epithelial cells are connected to each other laterally as well as to the basement membrane by junctional complexes called cell junctions.

NoteDefinition

Cell junctions are the junctional complexes that connect epithelial cells to one another laterally and to the underlying basement membrane.

There are five types, shown in Fig. 10.8 and classified in FC 10.6.

Diagram

Fig. 10.8 – Types of cell junction between adjacent epithelial cells; label tight junction, adherens junction, gap junction, desmosome and hemidesmosome, showing hemidesmosomes anchoring the cell to the basement membrane.

Flowchart

FC 10.6 – Types of cell junction: Cell junctions → Tight junctions (TJs) → Adherens junctions (AJs) → Gap junctions (GJs) → Desmosomes (Ds) → Hemidesmosomes (HDs)

Tight junctions (TJs): These junctions maintain cell polarity and prevent lateral diffusion of proteins and ions. “Cell polarity” means that the free surface of the cell and its basal surface stay different from each other — the microvilli-bearing top does not become like the bottom, because tight junctions stop membrane proteins from sliding sideways from one region to the other.

Adherens junctions (AJs): These are involved in various signalling pathways and transcriptional regulations. In other words, they do more than stick cells together — they also pass information into the cell that influences which genes it switches on.

Gap junctions (GJs): This intercellular connection allows the passage of ions and small molecules between cells, as well as the exchange of chemical messages between cells. It is essentially a tunnel joining the interiors of two neighbouring cells.

Desmosomes (Ds): These provide mechanical strength to epithelial tissue, cardiac muscles and meninges. They act like rivets holding neighbouring cells together against pulling forces.

Hemidesmosomes (HDs): These allow the cells to strongly adhere to the underlying basement membrane, and they maintain tissue homeostasis by signalling. The prefix hemi, meaning “half”, is the clue — a desmosome joins two cells, while a hemidesmosome joins one cell to the non-cellular basement membrane below it.

desmosome vs hemidesmosome A desmosome anchors one cell to the neighbouring cell. A hemidesmosome anchors one cell downwards to the basement membrane. Same anchoring job, different partner.

what each junction does Tight = seal. Adherens = signal. Gap = talk. Desmosome = rivet between cells. Hemidesmosome = rivet to the floor.

The importance of these junctions shows up clearly when they fail. In pemphigus, an autoimmune disease, the body’s own antibodies attack the proteins of the desmosomes in the skin. With the rivets gone, the epithelial cells separate from one another and fluid collects between them, producing painful blisters at the slightest friction. A single class of junction, present in a layer only a few cells thick, is all that stands between healthy skin and a blistering disease.

Q. Cell junctions are described here for epithelial tissue — do they occur anywhere else?

A. Yes. Desmosomes also provide mechanical strength to cardiac muscle and to the meninges, and gap junctions between cardiac muscle cells let a stimulus pass quickly from cell to cell. You will see this again when you study cardiac muscle later in this chapter.

  1. Explain the basic structure of epithelial tissue and mention its types.
  2. Epithelial tissue has a good capacity of regeneration. Give reason.
  3. Write a note on glandular epithelial tissue.
  4. How do cell junctions help in the functioning of epithelial tissue?

10.3 Connective Tissue

Connective tissue is the most widely spread tissue in the body. It binds, supports and provides strength to other body tissues and organs.

Its design is the exact opposite of the design you have just studied. In epithelial tissue the cells are packed tightly with almost no matrix; in connective tissue the cells are few and scattered, and the material between them is abundant. Connective tissue consists of a variety of cells and fibres, and these are embedded in the abundant intercellular substance called matrix.

NoteDefinition

Connective tissue is a tissue in which a variety of cells and fibres lie embedded in an abundant intercellular matrix, and which binds, supports and gives strength to other tissues and organs.

This is why the properties of a connective tissue depend far more on its matrix than on its cells. A jelly-like matrix gives a soft packing tissue, a matrix packed with parallel collagen gives an unbreakable cord, a stone-hard matrix gives bone, and a liquid matrix gives blood.

Q. If two connective tissues can be as different as blood and bone, what makes them one family at all?

A. The common plan — cells scattered in a large amount of matrix that the cells themselves secrete. Only the matrix changes: fluid in blood, jelly-like in areolar tissue, tough and fibrous in tendon, and hard with mineral salts in bone.

Connective tissue protects the vital organs of the body. It is highly vascular except cartilage. It acts as packing material and also helps in the healing process. The rich blood supply is exactly what makes healing possible, since repair needs a steady delivery of oxygen, nutrients and defensive cells. Cartilage, being the one exception without blood vessels, is also the one connective tissue that heals very slowly.

How Connective Tissue Is Classified

Connective tissue is classified on the basis of the matrix present, as connective tissue proper, supporting connective tissue and fluid connective tissue. The full scheme is shown in FC 10.7.

Flowchart

FC 10.7 – Classification of connective tissue: Connective tissue → (A) Connective tissue proper → Loose connective tissue (areolar tissue, adipose tissue) and Dense connective tissue (dense regular — tendon, ligament; dense irregular — dermis of skin); (B) Supporting connective tissue, also called skeletal tissue → Cartilage (hyaline, elastic, fibrocartilage, calcified) and Bone (spongy, compact); (C) Fluid connective tissue → Blood and Lymph

Connective tissue proper is further classified as loose connective tissue (for example, areolar connective tissue and adipose tissue) and dense connective tissue (for example, ligament and tendon). Supporting connective tissue, also called skeletal tissue, includes cartilage and bone. Fluid connective tissue includes blood and lymph.

A. Connective Tissue Proper

The matrix of loose connective tissue is a semisolid, jelly-like, viscous matter made up of gelatin. Because this matrix is soft and yielding, loose connective tissue can be squeezed, stretched and pushed aside without damage — which is precisely what a packing material must be able to do.

1. Areolar Tissue (Areola : air pockets)

The name comes from the small spaces, like air pockets, seen within the tissue. The matrix of this tissue contains two types of fibres, namely white fibres and yellow fibres, as shown in Fig. 10.9.

Diagram

Fig. 10.9 – Areolar tissue: loose network of fibres in a jelly-like matrix with scattered cells; label macrophage, matrix, fibroblast, collagen fibres (white fibres), yellow fibres, mast cell.

White fibres are made up of collagen. They give tensile strength to the tissue — that is, they resist being pulled apart. Yellow fibres are made up of elastin and are elastic in nature, so they allow the tissue to stretch and then return to its original shape.

Together the two fibres give this soft tissue a useful combination: it can be stretched, but not torn.

The tissue also contains four different types of cells.

Fibroblasts are the large flat cells having branching processes. They produce fibres as well as polysaccharides that form the ground substance or matrix of the tissue. In other words, the fibroblast manufactures almost everything around it.

Mast cells are oval cells that secrete heparin and histamine.

Macrophages are amoeboid, phagocytic cells. Being amoeboid, they can crawl through the tissue, and being phagocytic, they engulf and destroy bacteria and cell debris.

Fat cells, also called adipocytes, have an eccentric nucleus — a nucleus pushed to one side rather than lying at the centre. These cells store fat.

This tissue acts as packing material, helps in the healing process and connects different organs or layers of tissues. It is found under the skin, between muscles, bones, around organs, blood vessels and peritoneum.

The two secretions of the mast cell explain two very familiar things. Heparin prevents blood from clotting inside the vessels, and a purified form of it is used in hospitals to stop dangerous clots forming in patients. Histamine is released during an allergic reaction; it makes small blood vessels leaky, which produces the swelling, redness, itching and running nose of an allergy. This is why the tablets people take for allergies are called antihistamines — they block the action of a chemical released by a connective tissue cell.

areolar tissue Zoom in: it is made of fibroblasts, mast cells, macrophages and adipocytes lying in a jelly-like matrix. Zoom out: this loose tissue is the packing that fills the spaces between skin, muscles and organs and holds them in position.

2. Adipose Tissue (Adipo : fat)

In this tissue a large number of adipocytes are present. The cells are rounded or polygonal. The nucleus is shifted to the periphery because fats are stored in the cell in the form of droplets, and these droplets take up almost the whole of the cell interior. The matrix is less, and fibres and blood vessels are few in number. Fig. 10.10 shows this arrangement.

Diagram

Fig. 10.10 – Adipose tissue: closely packed rounded adipose cells each with a large fat globule and a peripheral nucleus; label empty adipose cell, nucleus, cytoplasm, matrix, fine white fibres, fat globule, blood vessel, yellow fibre, adipose cell.

There are two types of adipose tissue. White adipose tissue appears opaque due to the presence of a large number of adipocytes. It is commonly seen in adults. Brown adipose tissue is reddish brown in colour due to the presence of a large number of blood vessels.

Function: adipose tissue is a good insulator, acts as a shock absorber, and is a good source of energy because it stores fat. The tissue is found in the sole and palm region as well as around organs like the kidney.

Each of these three functions follows directly from fat itself. Fat conducts heat poorly, so a fat layer slows the escape of body heat. Fat droplets are soft and deform under pressure, so a fat pad cushions a blow — which is why fat is placed in the sole and the palm, the two regions that take the most impact. And fat releases more energy per gram than any other stored food, so it is the body’s long-term fuel reserve.

adipose tissue (cell almost entirely filled with a fat droplet → fat is a poor conductor of heat and a soft cushion → the same tissue insulates against cold, absorbs shocks, and stores energy)

Brown adipose tissue is present in large amounts in newborn babies, especially around the neck and between the shoulder blades. A newborn cannot shiver effectively to keep warm, so instead its brown fat burns stored fat directly to release heat. The many blood vessels that give this tissue its brown colour then carry that warmth to the rest of the body.

Q4. Animals living in very cold regions carry a thick layer of fat under their skin. Which tissue is this, and how exactly does it keep them warm?

A. It is adipose tissue, a loose connective tissue packed with fat-storing adipocytes. Fat is a poor conductor of heat, so this layer acts as an insulator and slows the loss of body heat to the freezing surroundings. The same tissue is also a shock absorber and a store of energy, which matters when food is scarce in winter.

White adipose tissue vs Brown adipose tissue | Feature | White adipose tissue | Brown adipose tissue | |—|—|—| | Appearance | Opaque | Reddish brown | | Reason for the colour | Large number of adipocytes | Large number of blood vessels | | Commonly seen in | Adults | Newborns |

B. Dense Connective Tissue

In this tissue, fibres and fibroblasts are compactly arranged. There are two types, namely dense regular and dense irregular tissue. The word “dense” tells you that the fibres are crowded together; “regular” and “irregular” tell you whether or not those fibres are lined up in one direction.

Dense Regular Connective Tissue

In dense regular connective tissue, collagen fibres are arranged in a parallel manner. Two major examples of this tissue are tendons and ligaments.

Tendons connect skeletal muscles to bones. To give tensile strength to the tissue, tendons contain bundles of white fibres, e.g. Achilles tendon, hamstring tendon.

Ligaments are made up of elastic or yellow fibres arranged in a regular pattern. These fibres make ligaments elastic. They are present at joints. They prevent dislocation of bones. Fig. 10.11 shows a ligament at a joint along with the tendon of the muscle.

Diagram

Fig. 10.11 – Ligament at a joint: section through a joint showing muscle, tendon and ligament; label extensor muscle, flexor muscle, tendon, epiphyseal bone, ligament.

the tendon (collagen fibres all laid parallel → every fibre pulls in the same direction as the muscle → almost none of the muscle’s force is wasted → the whole pull is transmitted to the bone without the tendon stretching)

A ligament, by contrast, must give a little. A joint has to move, so the bones must be held together firmly but not rigidly. The elastic yellow fibres of a ligament allow exactly this — enough give for movement, enough hold to prevent dislocation.

Tendon vs Ligament | Feature | Tendon | Ligament | |—|—|—| | Connects | Skeletal muscle to bone | Bone to bone at a joint | | Main fibre | White fibres (collagen), in bundles | Elastic or yellow fibres | | Property | Very high tensile strength, little stretch | Elastic, allows limited movement | | Main role | Transmits the pull of the muscle to the bone | Prevents dislocation of bones | | Example | Achilles tendon, hamstring tendon | Ligaments at the knee and ankle joints |

Achilles tendon. The Achilles tendon connects the calf muscles to the heel bone. Pain at the back of the ankle or the lower calf may signal a problem with the Achilles tendon. Athletes who participate in track and field may face Achilles tendon injury. Most tendon injuries occur near joints such as the shoulder, elbow, knee and ankle.

Dense Irregular Connective Tissue

In dense irregular connective tissue, fibres and fibroblasts are not arranged in an orderly manner. This tissue is found in the dermis of the skin.

Here the lack of order is itself an advantage. Skin can be pulled from any direction, so fibres running every which way give strength against a pull from every side, unlike a tendon which needs strength in one direction only.

C. Supporting Connective Tissue

It is characterised by the presence of a hard matrix. It is classified into two types, cartilage and bone. Because these two form the skeleton, supporting connective tissue is also called skeletal tissue.

Cartilage

This is a pliable yet tough tissue — it can bend without breaking. It forms the endoskeleton of cartilaginous fishes like shark. It is widely distributed in vertebrate animals.

In cartilage, the abundant matrix is delimited by a sheath of collagenous fibres called perichondrium. The matrix is called chondrin. Just below the perichondrium, immature cartilage-forming cells called chondroblasts are present. Chondroblasts mature and get converted into chondrocytes. Chondrocytes are seen scattered in the matrix. They are enclosed in lacunae. Each lacuna contains 2-8 chondrocytes. The general plan is seen in Fig. 10.12.

Diagram

Fig. 10.12 – Hyaline cartilage: chondrocytes lying in lacunae within a gel-like matrix, enclosed by perichondrium; label perichondrium, chondroblast, matrix, lacunae, chondrocyte.

Flowchart

FC 10.8 – Development of a cartilage cell: chondroblast forms just below the perichondrium → secretes matrix (chondrin) around itself → becomes trapped in a small cavity → the cavity is the lacuna → the trapped mature cell is now a chondrocyte

chondroblast vs chondrocyte A chondroblast is the immature, actively matrix-forming cell lying just below the perichondrium. A chondrocyte is the mature cell that has become enclosed within a lacuna in the matrix. Same cell line, two stages.

Cartilage is the one connective tissue without a blood supply, and its cells are further sealed inside lacunae in a solid matrix. Nutrients can only seep slowly through the matrix. This is why a torn cartilage in the knee heals very poorly compared with a broken bone, and why sportspersons with cartilage injuries often need surgery where a fracture would simply have been set and left to mend.

Based upon the type of matrix, there are four types of cartilage, as explained below.

Hyaline Cartilage (Hyline : Glass like)

In this type of cartilage, perichondrium is present. The matrix is bluish white and gel-like, and its glassy appearance is what gives this cartilage its name.

Very fine collagen fibres and chondrocytes are present. Hyaline cartilage is elastic and compressible in nature. It acts as a good shock absorber as well as provides flexibility. It reduces friction. It is found at the ends of long bones, epiglottis, trachea, ribs, larynx and hyoid.

Its two main jobs match its two locations. At the ends of long bones it forms a smooth, slippery, compressible surface, so the bones glide over each other with very little friction and the joint absorbs the jolt of every step. In the trachea and larynx it forms firm but flexible rings that keep the air passage permanently open while still allowing the neck to bend.

Elastic Cartilage

In elastic cartilage, perichondrium is present. The matrix contains elastic fibres. Chondrocytes are few in number. It gives support and maintains the shape of the body part. It is found in the ear lobe, the tip of the nose, etc. Fig. 10.13 shows the dense network of elastic fibres in its matrix.

Diagram

Fig. 10.13 – Elastic cartilage: chondrocytes in lacunae within a matrix crowded with dark, branching elastic fibres, bounded by perichondrium; label perichondrium, matrix, lacuna, chondrocyte, elastic fibre.

The elastic fibres are the whole point of this tissue. They let a structure be bent right out of shape and then snap back to its original form without any damage — which is exactly what happens every time you fold your ear.

Fibrocartilage

Perichondrium is absent in fibrocartilage. The matrix contains bundles of collagen fibres and few chondrocytes, scattered in fibres. Fibrocartilage is the most rigid cartilage. It maintains the position of vertebrae. Intervertebral discs are made up of fibrocartilage. It is also found at the pubic symphysis. Fig. 10.14 shows its bundles of white fibres.

Diagram

Fig. 10.14 – White fibrous cartilage: thick bundles of white collagen fibres with few chondrocytes in lacunae scattered between them; label chondroblasts, matrix, lacuna, white fibres.

Being the most rigid cartilage, it is placed exactly where enormous compressive force has to be borne. Every intervertebral disc is squeezed by the entire weight of the body above it, and the packed collagen bundles allow the disc to take that load without collapsing.

Calcified Cartilage

This type of cartilage becomes rigid due to the deposition of salts in the matrix. This reduces the flexibility of joints in old age, e.g. head of long bones.

The four types of cartilage | Feature | Hyaline | Elastic | Fibrocartilage | Calcified | |—|—|—|—|—| | Perichondrium | Present | Present | Absent | — | | Special content of matrix | Very fine collagen fibres | Elastic fibres | Bundles of collagen fibres | Deposited salts | | Chondrocytes | Present | Few | Few, scattered in fibres | — | | Property | Elastic, compressible, reduces friction | Springy, keeps shape | Most rigid cartilage | Rigid | | Found in | Ends of long bones, epiglottis, trachea, ribs, larynx, hyoid | Ear lobe, tip of nose | Intervertebral discs, pubic symphysis | Head of long bones (in old age) |

As we grow old, cartilage becomes rigid. Give the reason.

Bone

Bone is characterised by a hard matrix called ossein. Ossein is made up of the mineral salt hydroxy-apatite, \(Ca_{10}(PO_4)_6(OH)_2\). It is this mineral, and not the cells, that makes bone hard.

An outer tough membrane called periosteum encloses the matrix. Blood vessels and nerves pierce through the periosteum, which is how a “hard, dead-looking” tissue is in fact kept alive and richly supplied.

The matrix is arranged in the form of concentric layers called lamellae. Each lamella contains fluid-filled cavities called lacunae. Fine canals that radiate from each lacuna are called canaliculi. Canaliculi of adjacent lamellae connect with each other as they traverse through the matrix. The complete structure is shown in Fig. 10.15.

Diagram

Fig. 10.15 – Detailed structure of bone: an osteon with concentric lamellae around a central canal, opened out to show a lacuna and its canaliculi, together with spongy bone; label lacuna, canaliculi, osteocyte, venule, arteriole, central canal, lamellae, external circumferential lamellae, periosteum, perforating fibres, perforating canals, osteon, trabeculae of spongy bone, blood vessels.

lacunae and canaliculi (bone cells sealed inside lacunae in a stone-hard matrix → the fine canaliculi radiate out and link neighbouring lacunae → nutrients and wastes pass from cell to cell along these canals → living cells survive in the middle of solid mineral)

In the lacunae, osteoblasts — the active bone cells — and osteocytes — the inactive bone cells — along with osteoclasts are present. Osteoblasts lay down new bone matrix, while osteoclasts break bone matrix down. Bone is therefore not a finished, unchanging block; it is continuously built and removed, which is what allows it to repair fractures and to reshape itself in response to the forces acting on it.

osteoblast vs osteoclast An osteoblast builds bone matrix. An osteoclast carves it away. The single letter that changes carries the opposite meaning, and this is a very common examination trap.

Mammalian bone shows a peculiar haversian system. The haversian canal encloses an artery, a vein and nerves. Observe Fig. 10.15 to understand the haversian system. Because a blood vessel runs through the centre of each unit, no bone cell is ever left far from a supply line.

The canals running through the centre of these bone units are named after Clopton Havers, an English physician of the late seventeenth century. In the 1690s he published a detailed study of bone in which he described the fine channels running lengthwise through its substance. At the time, bone was widely regarded as a dead, stony support. Havers’ observation pointed to the opposite conclusion — that bone is threaded with passages carrying living supplies — and his name has stayed attached to those canals ever since.

According to the presence of matrix there are two types of bones present in the human body. In spongy bones, the haversian system is absent. The reticular matrix is arranged in the form of trabeculae — a network of thin bony bars with spaces between them. It contains red bone marrow. In compact bones, the matrix shows the haversian system without any space between lamellae.

Spongy bone vs Compact bone | Feature | Spongy bone | Compact bone | |—|—|—| | Haversian system | Absent | Present | | Arrangement of matrix | Reticular matrix in the form of trabeculae | Lamellae with no space between them | | Spaces | Present, filled with red bone marrow | Absent | | Weight for a given volume | Light | Heavy and solid |

The spaces inside spongy bone are not wasted space. They hold red bone marrow, the site where red blood cells are formed. This is why a serious injury to a long bone can be followed by anaemia: damage to the marrow reduces the production of red blood cells, and with fewer red blood cells the blood carries less oxygen, leaving the person pale and easily tired.

Cartilage vs Bone | Feature | Cartilage | Bone | |—|—|—| | Matrix | Chondrin, pliable yet tough | Ossein, hardened by hydroxy-apatite | | Blood supply | Absent (avascular) | Highly vascular | | Covering | Perichondrium | Periosteum | | Cells | Chondroblasts, chondrocytes | Osteoblasts, osteocytes, osteoclasts | | Cells enclosed in | Lacunae, 2-8 chondrocytes per lacuna | Lacunae connected by canaliculi | | Property | Flexible, compressible | Hard and rigid |

Q3. If you fold your ear pinna it springs back into shape, but a bone will not bend at all. Both are supporting tissues — so why do they behave so differently?

A. Because their matrices are completely different. The ear pinna contains elastic cartilage, whose matrix is packed with elastic fibres; it can be bent right out of shape and then returns to its original form. Bone has a matrix of ossein hardened by the mineral salt hydroxy-apatite, \(Ca_{10}(PO_4)_6(OH)_2\); a stone-hard matrix cannot deform, so bone does not bend — it resists the force instead.

  1. Give reasons: bone is stronger than cartilage.
  2. Explain the histological structure of mammalian bone.
D. Fluid Connective Tissue (Vascular)

Blood and lymph are the fluid connective tissue present in the body of an animal. You will study these tissues in detail in the chapter ‘Circulation’ in Class XII.

Blood fits the definition of a connective tissue perfectly, even though it does not look like one. It has cells — the blood cells — scattered in an abundant intercellular matrix, and that matrix simply happens to be liquid instead of jelly-like or hard. Because the matrix flows, this connective tissue can carry out its binding role in a different way: instead of physically holding organs together, it links every organ of the body chemically, delivering oxygen and nutrients and carrying wastes away.

Q5. Blood is a liquid that flows through vessels. How can something that flows be called a tissue at all?

A. Because a connective tissue is defined by its plan, not by its hardness: cells scattered in an abundant intercellular matrix. In blood, the cells are the blood cells and the matrix is fluid. That is why blood is classed as fluid connective tissue, along with lymph — the same family design as areolar tissue and bone, with only the state of the matrix changed.

connective tissue Zoom in: connective tissue is scattered cells plus a matrix that those cells secrete. Zoom out: the same tissue family builds the skeleton that supports you, the tendons that move you, the fat that insulates you and the blood that supplies you — four organ systems built on one basic plan.

10.4 Muscular Tissue

The cells of this tissue are elongated and are called muscle fibres. The word “fibre” is used because these cells are far longer than they are wide — a shape that suits a cell whose whole job is to shorten along one direction.

Muscle cells have their own set of names, all built from sarco, meaning “flesh”. Each muscle fibre is covered by a membrane called the sarcolemma, which is simply the cell membrane of a muscle cell. The cytoplasm of the muscle cell is called sarcoplasm.

A large number of contractile fibrils called myofibrils are present in the sarcoplasm. One or many nuclei are present in the muscle cell depending on the type. Myofibrils are made up of the proteins actin and myosin.

the muscle vocabulary Sarco means flesh. Sarcolemma = the membrane, sarcoplasm = the cytoplasm, sarcomere = the unit of contraction. Learn the prefix once and three terms fall into place.

Why It Is Called Contractile Tissue

Muscle fibres contract and decrease in length on stimulation. Hence, muscular tissue is known as contractile tissue.

Notice what this means. Muscle can only pull, never push. Shortening produces a pull on whatever the muscle is attached to — which is exactly why skeletal muscles work in opposing pairs, one pulling a bone one way and its partner pulling it back.

NoteDefinition

Muscular tissue is a tissue whose elongated cells, the muscle fibres, shorten on stimulation and thereby produce movement; because of this property it is called contractile tissue.

It is a vascular tissue and is innervated by nerves too. Muscle cells contain a large number of mitochondria.

These three facts belong together. Contraction is expensive work, so the fibre needs a great deal of ATP; the many mitochondria supply that ATP, and the rich blood supply keeps delivering the oxygen and glucose the mitochondria need. Being innervated means nerve fibres reach the tissue and deliver the stimulus that starts the contraction — muscle does not act on its own initiative.

the muscle fibre (cell elongated and packed with parallel myofibrils of actin and myosin → shortening of all the myofibrils together shortens the whole fibre → a strong pull along one definite direction → movement of the attached bone or organ wall)

A. Types of Muscular Tissue

There are three types of muscular tissue, shown in FC 10.9.

Flowchart

FC 10.9 – Types of muscular tissue: Muscular tissue → Skeletal muscle (striated, voluntary, attached to bones) → Smooth or non-striated muscle (unstriated, involuntary, in visceral organs) → Cardiac muscle (striated, involuntary, in the heart wall)

1. Skeletal Muscles

These muscles are found attached to bones.

Skeletal muscles consist of a large number of fasciculi, which are wrapped by a connective tissue sheath called epimysium or fascia. Each individual fasciculus is covered by perimysium. Each fasciculus in turn consists of many muscle fibres called myofibres.

So a skeletal muscle is a bundle of bundles. Notice also that the wrappings — epimysium and perimysium — are connective tissue, which is a good reminder that an organ is never made of one tissue alone. The whole hierarchy, right down to the contractile proteins, is set out in FC 10.10.

Flowchart

FC 10.10 – Organisation of a skeletal muscle: Skeletal muscle (wrapped in epimysium or fascia) → Fasciculi (each wrapped in perimysium) → Muscle fibres or myofibres (each bounded by sarcolemma) → Myofibrils → Sarcomeres → Actin and myosin filaments

the skeletal muscle Zoom in: a muscle is built from fasciculi, fasciculi from muscle fibres, fibres from myofibrils, and myofibrils from sarcomeres made of actin and myosin. Zoom out: skeletal muscles together with bones and joints form the locomotory system that moves the whole body.

Inside a single muscle fibre

Each muscle fibre is a syncytial fibre that contains several nuclei. A syncytium is formed when several cells fuse together and their nuclei end up sharing one common cytoplasm — which is why a single skeletal muscle fibre is not a single ordinary cell.

The cell membrane called sarcolemma delimits the cytoplasm called sarcoplasm. The sarcoplasm contains a large number of parallelly arranged myofibrils, hence the nuclei get shifted to the periphery. The myofibrils simply occupy so much of the interior that there is no room left for a nucleus at the centre.

Each myofibril is made up of repeated functional units called sarcomeres. Fig. 10.16 shows how these repeating units create the pattern seen in the tissue.

Diagram

Fig. 10.16 – Skeletal muscular tissue: a group of long striated fibres with peripheral nuclei, and below it a single myofibril opened out to show one sarcomere; label striations, nucleus, myofibril, sarcomere, Z line, A band, I band, H zone, M line.

Reading the bands of a sarcomere

Each sarcomere has a dark band called the anisotropic or ‘A’ band in the centre. In the centre of the ‘A’ band is a light area called the ‘H’ zone or ‘Hensen’s Zone’. In the centre of the ‘H’ zone there is the ‘M’ line.

‘A’ bands are made up of myosin as well as actin. On either side of the ‘A’ band are light bands called isotropic or ‘I’ bands that contain only actin. Myosin filaments are thick and dark coloured while actin filaments are thin and light coloured. Adjacent light bands are separated by the ‘Z’ line (Z — Zwischenscheibe line).

Once you know that myosin is thick and dark and actin is thin and light, the pattern explains itself. Wherever thick myosin is present the region looks dark, so the ‘A’ band is dark; where only thin actin lies the region looks light, so the ‘I’ band is light; and the ‘H’ zone is light because at that spot in the middle of the ‘A’ band, the actin filaments do not reach and only myosin is present.

Dark and light bands on neighbouring myofibrils correspond with each other, hence the muscle gets a striated appearance. In other words, the stripes of the whole fibre appear only because thousands of myofibrils lie side by side with their bands in perfect register.

one sarcomere, from one Z line to the next 1. Start at a ‘Z’ line — this marks one end of the sarcomere. 2. Move inwards through the light ‘I’ band, which contains only thin actin filaments. 3. Enter the dark ‘A’ band, where thick myosin and thin actin overlap. 4. Reach the lighter ‘H’ zone at the centre of the ‘A’ band, where only myosin is present. 5. At the very middle of the ‘H’ zone, cross the ‘M’ line. 6. Continue outwards in mirror image — ‘H’ zone, ‘A’ band, ‘I’ band — until the next ‘Z’ line. That whole stretch is one sarcomere.

‘A’ band vs ‘I’ band The ‘A’ band is anisotropic and dark, and it contains myosin as well as actin. The ‘I’ band is isotropic and light, and it contains only actin. A common examination trap is to state that the ‘A’ band contains only myosin — it does not.

How Skeletal Muscles Behave

Skeletal muscles show quick and strong voluntary contractions. They bring about voluntary movements of the body. “Voluntary” means these contractions are under conscious control — you decide to move, and the muscle obeys. You will study about the working of skeletal muscles in the chapter on movement and locomotion.

Red and white muscles. On the basis of the amount of a red pigment, skeletal muscles are of two main types — red and white. Red muscles contain a very high amount of myoglobin while white muscles contain a very low amount of this pigment.

Myoglobin is an iron-containing red coloured pigment found only in muscles. It consists of one haeme and one polypeptide chain. It can carry one molecule of oxygen. Due to the presence of myoglobin, the muscles can obtain their oxygen from two sources, myoglobin and haemoglobin.

myoglobin in red muscle (one haeme group holding one oxygen molecule, stored inside the muscle itself → the fibre has its own oxygen reserve as well as the oxygen arriving in the blood → it can keep respiring aerobically for long periods → red muscle resists fatigue)

This difference in myoglobin is what separates a marathon runner’s muscles from a sprinter’s. Red fibres, rich in myoglobin and mitochondria, work steadily for hours without tiring and dominate in endurance activity. White fibres, poor in myoglobin, contract very quickly and powerfully but fatigue fast, and they dominate in a short sprint or a heavy lift. The same contrast is on your plate: the dark flesh of a bird’s flight muscles and the pale flesh of its breast muscles differ in exactly this pigment.

Myoglobin has a special place in the history of biology. In the 1950s, the British scientist John Kendrew and his colleagues used X-ray crystallography to work out the three-dimensional shape of the myoglobin molecule — the first protein structure ever solved. For the first time, scientists could see how a folded chain of amino acids cradles a haeme group so that it can hold oxygen. Kendrew shared a Nobel Prize for this work, and the method he helped establish is the reason we now know the shapes of thousands of proteins.

2. Smooth or Non-striated Muscles

These muscles are present in the form of sheets or layers. Each muscle cell is spindle shaped or fusiform — broad in the middle and tapering at both ends. The fibres are unbranched, having a single nucleus at the centre, as shown in Fig. 10.17.

Diagram

Fig. 10.17 – Smooth muscle: spindle-shaped unbranched fibres, each with a single central nucleus and no striations; label plasma membrane, myofibrils, nucleus, sarcoplasm.

The sarcoplasm contains myofibrils. Myofibrils are made up of the contractile proteins actin and myosin. Smooth muscles contain less myosin and more actin as compared to skeletal muscles. Striations are absent.

The absence of striations follows from the way the filaments are laid out. In smooth muscle the actin and myosin are not arranged into neatly repeating sarcomeres in register across the fibre, so no regular pattern of dark and light bands appears under the microscope.

These muscles undergo slow and sustained involuntary contractions. They are innervated by the autonomous nervous system, which is the part of the nervous system that works without conscious control.

“Slow and sustained” is exactly what these organs require. A blood vessel or a gut wall must hold a steady degree of contraction for hours together, and a quick, fatiguing contraction of the skeletal type would be useless there.

These are found in the walls of visceral organs and blood vessels. Hence they are also called visceral muscles. They may be arranged lengthwise (longitudinal muscles) or around the circumference (circular muscles) of any organ.

The two arrangements do opposite things. Circular muscles squeeze the tube narrower, while longitudinal muscles shorten it — and the two acting in turn along the gut is what pushes food forward.

Smooth muscle explains several everyday responses of the body. Circular smooth muscle in the iris contracts in bright light and the pupil becomes smaller, protecting the retina — and none of this is decided by you. Smooth muscle in the walls of the bronchi is the tissue that goes into spasm during an asthma attack, narrowing the air passages and making breathing difficult, which is why an inhaler works by relaxing that muscle.

3. Cardiac Muscles

The muscles of this tissue show the characters of both striated and non-striated fibres. They are striated like skeletal muscle, but involuntary like smooth muscle — a genuine combination of the two.

The sarcolemma is not distinct. Hence uni-nucleate muscle fibres appear to be multi-nucleate: because the boundary between neighbouring cells is hard to see, the nuclei of several cells look as though they belong to a single long fibre.

Adjacent muscle fibres join together to give a branched appearance to the tissue. The points of adhesion of muscle fibres are formed by transverse thickenings of the sarcolemma called intercalated discs, as seen in Fig. 10.18.

Diagram

Fig. 10.18 – Cardiac muscle: branched striated fibres joined end to end by intercalated discs; label striations, nucleus, junction between adjacent cells (intercalated disc).

These junctions at places allow cardiac muscles to contract as a unit, i.e. it helps in quick transfer of stimulus. Recall the gap junctions you studied with epithelial tissue: it is this kind of connection within the intercalated disc that lets a stimulus pass straight from one cardiac fibre to the next.

the intercalated disc (thickened junctions joining neighbouring cardiac fibres end to end → a stimulus passes from fibre to fibre almost instantly and the fibres are also mechanically anchored → the whole mass of muscle contracts as one unit → the heart squeezes as a single pump instead of as scattered twitching fibres)

The cardiac muscles are striated involuntary muscles.

Some mammalian cardiac muscles are modified and are capable of generating impulse on their own. Hence the mammalian heart is a myogenic heart. In some animals, cardiac muscles need a neural stimulus to initiate the contraction. Such a heart is called a neurogenic heart. Cardiac muscles form the myocardium of the heart wall.

NoteDefinition

A myogenic heart is one in which the heartbeat is initiated by the modified cardiac muscle of the heart itself; a neurogenic heart is one in which the beat is initiated by a nerve stimulus reaching the heart.

myogenic vs neurogenic heart Myo points to muscle and neuro to nerve. In a myogenic heart, such as that of a mammal, the impulse starts in the heart’s own muscle. In a neurogenic heart, the impulse must come from a nerve. The mammalian heart is myogenic.

Because the mammalian heart is myogenic, a heart removed for transplantation and kept in a suitable solution can go on beating even though every nerve to it has been cut. For the same reason, a transplanted heart continues to beat rhythmically in its new owner. And when the heart’s own impulse-generating muscle fails, doctors implant an artificial pacemaker — a device that supplies the electrical stimulus the modified muscle can no longer produce.

Q2. Your heart beats all night without you ever ordering it to, yet your arm moves only when you decide to move it. What makes the muscle in the heart so different from the muscle in your arm?

A. The arm is moved by skeletal muscle, which is voluntary — it contracts quickly and strongly, but only when a nerve impulse sent under your conscious control reaches it. The heart wall is made of cardiac muscle, which is striated but involuntary. Its modified fibres generate their own impulse, making the mammalian heart myogenic, and the intercalated discs pass that impulse from fibre to fibre so the whole muscle contracts as one unit — beat after beat, whether you are awake or asleep.

Skeletal vs Smooth vs Cardiac muscle | Feature | Skeletal muscle | Smooth muscle | Cardiac muscle | |—|—|—|—| | Striations | Present | Absent | Present | | Control | Voluntary | Involuntary | Involuntary | | Shape of fibre | Long, cylindrical, unbranched | Spindle shaped (fusiform), unbranched | Branched | | Nuclei | Several (syncytial), at the periphery | Single, at the centre | Single, but fibres appear multinucleate | | Special junctions | Absent | Absent | Intercalated discs present | | Nature of contraction | Quick and strong | Slow and sustained | Rhythmic and continuous | | Location | Attached to bones | Walls of visceral organs and blood vessels | Myocardium of the heart wall |

Q. Cardiac muscle is striated like skeletal muscle, yet it never tires. Why?

A. Because its fibres are extremely rich in mitochondria and are supplied by a dense network of blood vessels, so they respire aerobically without a break. A skeletal muscle can be driven hard for a short burst and then rest, but the heart has no opportunity to rest, so its fibres are built for uninterrupted aerobic work.

  1. How many skeletal muscles are present in the human body?
  2. How can exercise improve your muscular system?
  3. Compare and contrast the various types of muscles.

10.5 Nervous Tissue

Nervous tissue is composed of nerve cells or neurons and neuroglia.

The neuron is the structural and functional unit of the nervous system — it is the cell that actually carries messages. Neuroglial cells are non-nervous supporting cells that fill in the interneuronal space, that is, the gaps between neurons. They do not carry impulses themselves; they support, nourish and protect the cells that do.

Notice one important difference between these two partners. The neuroglial cells are capable of regeneration and division, but neurons are not capable of regeneration because of the lack of centriole. The centriole is the structure that organises cell division, so a cell without one cannot divide to replace itself.

Another feature sets this tissue apart from the one you studied just before it. Intercellular matrix is absent in the neural tissue. Connective tissue was defined by its abundant matrix; nervous tissue has none, and the space between neurons is filled by neuroglial cells instead.

Q1. A cut on your skin heals within a few days, but an injured nerve may never fully recover. What is it about these two tissues that makes such a difference?

A. Skin is covered by epithelial tissue, which has a good capacity of regeneration — it keeps a population of dividing cells that constantly replace those lost, so a fresh surface is rebuilt within days. Neurons cannot do this: they lack centrioles and therefore cannot divide to replace a lost neuron. Only the supporting neuroglial cells can divide. This is why damage to nervous tissue, such as a spinal cord injury, can leave a permanent loss of function while a skin wound closes and disappears.

This single fact shapes modern medicine. A person who breaks a bone will usually walk again, because bone is richly supplied with blood and contains osteoblasts that lay down new matrix. A person whose spinal cord is crushed in the same accident may not, because the neurons destroyed there cannot be replaced. It is also why doctors insist that a person with a suspected spine injury must not be moved carelessly — further damage to those neurons cannot be undone.

What a Neuron Can Do

The neuron is an impulse-generating and impulse-conducting unit. Neurons bring about quick communication within the body.

Neurons change the action potential of their membrane on receiving any external stimulus. This property of a neuron is called excitability. They also carry a wave of electric impulse from dendron to axon, the processes of a neuron. This is called conductivity.

NoteDefinition

Excitability is the property by which a neuron changes the action potential of its membrane on receiving a stimulus.

NoteDefinition

Conductivity is the property by which a neuron carries a wave of electric impulse from the dendron to the axon.

These two properties are the whole basis of rapid control in the body. Hormones also carry messages, but they travel in the blood and take time; a nerve impulse is electrical and reaches its target almost at once, which is why you pull your hand back from a hot object long before you have finished thinking about it.

excitability vs conductivity Excitability is the responding — the membrane reacts to a stimulus. Conductivity is the carrying — the impulse then travels along the neuron. First the neuron is excited, then it conducts.

Structure of a Neuron

A neuron is made up of a cyton or cell body. It contains granular cytoplasm called neuroplasm and a centrally placed nucleus.

The neuroplasm contains mitochondria, Golgi apparatus, RER and granules called Nissl’s granules. Nissl’s granules are made up of RNA. Since RNA is used in making proteins, their presence tells us that the cell body is a busy protein factory — and it has to be, because it must supply proteins for the entire length of the neuron, including a process that may be very long.

The cell body gives out two types of processes, namely dendron and axon. Dendrons are short, branched processes. The fine branches of a dendron are called dendrites. They carry impulse towards the cyton. The complete structure is shown in Fig. 10.19.

Diagram

Fig. 10.19 – Structure of a multipolar neuron: star-shaped cell body with several dendrons and one long axon; label dendrites, dendron, cell body, neuroplasm, nucleus, nucleolus, axon, nucleus of Schwann cell, Schwann cell, neurilemma, node of Ranvier, telodendron, and an arrow showing the direction of the impulse.

dendron vs dendrite A dendron is the whole short, branched process arising from the cyton. The dendrites are the fine branches of that dendron. Dendron is the branch; dendrites are its twigs.

The Axon

An axon is a single, elongated, cylindrical process. The axon is bounded by the axolemma, its own limiting membrane. The protoplasm of the axon is the axoplasm. It contains a large number of mitochondria and neurofibrils. The many mitochondria are needed because keeping a long fibre ready to conduct an impulse is continuous, energy-consuming work.

The axon is enclosed in a fatty sheath called the myelin sheath. The outer covering of the myelin sheath is the neurilemma. The myelin sheath and neurilemma are parts of another cell called Schwann’s cell. The Schwann cell shows its nucleus at the periphery.

The myelin sheath is absent at intervals along the axon, and this place is called the node of Ranvier. The terminal arborization of an axon is called the telodendron — the fine branching at the far end through which the impulse is passed on.

Dendron vs Axon | Feature | Dendron | Axon | |—|—|—| | Number per neuron | One or many | Always single | | Length | Short | Elongated | | Branching | Branched, ending in dendrites | Branches only at the tip, as the telodendron | | Direction of impulse | Towards the cyton | Away from the cyton | | Myelin sheath | Absent | May be present |

the neuron Zoom in: a neuron is a cell body with dendrons, an axon, and organelles including Nissl’s granules. Zoom out: bundles of nerve fibres form a nerve, nerves and the brain and spinal cord form the nervous system, and that system controls the entire organism.

For a long time it was not even clear whether the nervous system was made of separate cells or one continuous network. The question was settled with the help of a staining method developed by the Italian scientist Camillo Golgi, which stained only a small fraction of the nerve cells in a piece of tissue — but stained those few completely, right down to their finest branches. The Spanish scientist Santiago Ramón y Cajal used this method to draw nervous tissue in extraordinary detail and concluded that neurons are separate cells that come close to one another without merging. Golgi himself disagreed, and the two men shared a Nobel Prize in the early 1900s while still holding opposite views. Cajal’s interpretation proved correct, and it is the foundation of everything we know about the nervous system today.

Types of Neurons Based on Function

Based on their functions, neurons are classified into three types, as shown in FC 10.11.

Flowchart

FC 10.11 – Neurons classified by function: Neurons → Afferent (sensory) neuron, sense organ → CNS; Interneuron or association neuron, between sensory and motor neurons; Efferent (motor) neuron, CNS → effector organ

Afferent neuron: It carries impulses from a sense organ to the central nervous system (CNS). Hence it is also called a sensory neuron. It is found in the dorsal root of the spinal cord.

Efferent neuron: It carries impulses from the CNS to an effector organ. Hence it is also called a motor neuron. It is found in the ventral root of the spinal cord. An effector organ is simply the structure that carries out the response — usually a muscle or a gland.

Interneuron or association neuron: These are located between sensory and motor neurons. They perform processing and integration of sensory impulses and activate the appropriate motor neuron to generate a motor impulse.

afferent and efferent Afferent Arrives at the CNS; Efferent Exits from it. Sensory in, motor out.

what happens when you touch a hot plate 1. Heat receptors in the skin of your finger are stimulated. 2. An afferent (sensory) neuron carries the impulse from the skin to the spinal cord, entering through the dorsal root. 3. An interneuron in the spinal cord receives the impulse, processes it and activates the appropriate motor neuron. 4. An efferent (motor) neuron carries the impulse out through the ventral root to the muscle of the arm. 5. The skeletal muscle — the effector — contracts and the hand is pulled away, all before the brain has consciously registered the pain.

Flowchart

FC 10.12 – Path of an impulse in a reflex: stimulus (hot plate) → receptor in skin → afferent neuron → dorsal root → interneuron in spinal cord → efferent neuron → ventral root → effector muscle → hand withdrawn

Types of Nerve Fibres Based on the Myelin Sheath

Depending on the presence or absence of the myelin sheath, neurons are classified into two types, i.e. myelinated and non-myelinated nerve fibre.

Myelinated or medullated nerve fibres have an insulating fatty layer called the myelin sheath around the axon. This makes the fibre appear white in colour. This sheath is secreted by Schwann cells. The sheath is not continuous. It is interrupted at the nodes of Ranvier. Neurilemma surrounds the axon.

The impulse is conducted at a faster rate in such nerve fibres because it jumps from one node to the next. Such transmission of impulse is called saltatory conduction. The myelin sheath prevents the loss of the impulse during conduction. Cranial nerves of vertebrates are myelinated.

NoteDefinition

Saltatory conduction is the transmission of a nerve impulse in a myelinated fibre in which the impulse jumps from one node of Ranvier to the next, instead of travelling continuously along the axon.

myelin sheath with nodes of Ranvier (fatty insulating sheath broken at regular gaps → the impulse cannot escape through the insulated stretches and is regenerated only at the exposed nodes → it leaps from node to node instead of creeping along every point of the membrane → conduction becomes much faster and no signal strength is lost on the way)

Q. If insulation makes conduction faster, why is the myelin sheath not continuous along the whole axon?

A. Because the impulse has to be regenerated at intervals or it would fade out over a long axon. The nodes of Ranvier are the bare points where the membrane is exposed and the impulse is renewed at full strength. Insulation between the nodes gives speed; the nodes themselves keep the signal strong.

The Schwann cell of a non-medullated nerve fibre does not secrete a myelin sheath. These fibres are grey in colour due to the absence of the fatty layer. Conduction of impulse in a non-myelinated fibre is slower as compared to a myelinated nerve fibre. Nerves of the autonomous nervous system are non-myelinated.

This difference in speed matches the work each fibre does. Cranial nerves must deliver information about sight, sound and movement instantly, so they are myelinated. The autonomous nerves control slow, steady processes such as digestion and the tone of blood vessels, where a fraction of a second makes no difference.

Myelinated vs Non-myelinated nerve fibre | Feature | Myelinated (medullated) fibre | Non-myelinated fibre | |—|—|—| | Myelin sheath | Present, secreted by Schwann cells | Absent | | Nodes of Ranvier | Present | Absent | | Colour | White | Grey | | Speed of conduction | Faster, by saltatory conduction | Slower | | Example | Cranial nerves of vertebrates | Nerves of the autonomous nervous system |

In multiple sclerosis, the body’s own immune system attacks and destroys the myelin sheath around nerve fibres in the brain and spinal cord. The axons themselves may still be alive, but without their insulation the impulses leak away and slow down badly. The result is weakness, numbness, blurred vision and unsteady movement — a striking demonstration that a nerve fibre’s covering matters just as much as the fibre itself.

Synapse

The functional contact between the axonal ends and the dendrites of adjacent neurons is called a synapse. You will learn about the synapse in the chapter on control and coordination in Class XII.

The word “functional” is important here. The two neurons do not fuse; they only come close enough for the message to be handed across the gap. This is exactly the arrangement that Cajal’s drawings first revealed.

Types of Neurons Based on the Number of Processes

Based on the number of processes given out from the cyton, neurons are classified into three types, shown in Fig. 10.20 and summarised in FC 10.13.

Diagram

Fig. 10.20 – Types of neurons based on the number of processes: (a) multipolar neuron with many dendrites and one axon, (b) bipolar neuron with one dendrite and one axon at opposite poles, (c) unipolar neuron with a single short process dividing into peripheral and central processes; label cell body, dendrites, axon in each.

Flowchart

FC 10.13 – Neurons classified by number of processes: Neurons → Unipolar or monopolar (one process) → Bipolar (two processes) → Multipolar (more than two processes)

1. Unipolar / Monopolar Neuron

It has a single process originating from the cyton. Both the axon and the dendron arise from the cyton at one point. They conduct impulses to the central nervous system. Example: neurons of the dorsal root ganglion of a spinal nerve.

2. Bipolar Neuron

It has two processes. A single dendron and an axon are given off from opposite poles of the cyton. They bring about the transmission of special senses like sight, smell, taste, hearing, etc. Example: neurons of the retina of the eye and of the olfactory epithelium.

Recall the sensory epithelium you studied earlier in this chapter — the olfactory and retinal receptors sitting in that epithelium hand their message to exactly these bipolar neurons.

3. Multipolar Neuron

The cyton is star shaped and gives out more than two processes. There is only one axon and the remaining are dendrons. The axon initiates from a funnel-shaped area called the axon hillock.

Having many dendrons means such a neuron can receive input from a large number of other neurons at once, while its single axon sends out one combined answer. This is why multipolar neurons are the type that does most of the processing in the brain and spinal cord.

Unipolar vs Bipolar vs Multipolar neuron | Feature | Unipolar | Bipolar | Multipolar | |—|—|—|—| | Number of processes | One | Two | More than two | | Arrangement | Axon and dendron arise at one point | One dendron and one axon at opposite poles | One axon, many dendrons; star-shaped cyton | | Special feature | Conducts impulses to the CNS | Serves the special senses | Axon arises from the axon hillock | | Example | Neurons of the dorsal root ganglion | Neurons of the retina and olfactory epithelium | Neurons of the brain and spinal cord |

  1. Differentiate between medullated and non-medullated fibre.
  2. Classify neurons on the basis of the number of processes given out from the cyton, with examples.
  3. Explain the structure of a nerve, using Fig. 10.21.
  4. Find out how an impulse is transmitted from one neuron to another.

Diagram

Fig. 10.21 – Structure of a nerve: (A) external view of the covering of a spinal nerve, and (B) transverse section showing bundles of nerve fibres; label axon, myelin sheath, endoneurium, perineurium, epineurium, blood vessel, fascicle.

A nerve is not a single neuron but a cable. Individual nerve fibres are wrapped in endoneurium, groups of fibres are bundled into fascicles by perineurium, and all the fascicles together are held by an outer epineurium, with blood vessels running between them. Those three wrappings are connective tissue — one more reminder that even the tissue of communication depends on the tissue of support.

10.6 Summary

  1. A tissue is a group of cells having the same embryonic origin, structure and function. Tissues combine to form organs, organs form organ systems, and organ systems form the body.

  2. The cells of the body are of two types. Somatic cells are all body cells except sperm and ova; the sperm and ova are germ cells and belong to the reproductive system.

  3. Histology is the study of the structure and arrangement of tissues. Animals have four types of tissue: epithelial, connective, muscular and nervous.

Flowchart

FC 10.14 – The chapter at a glance: Animal tissue → Epithelial (simple: squamous, cuboidal, columnar, ciliated, glandular, sensory, germinal; compound: stratified, transitional) → Connective (proper: areolar, adipose, dense regular, dense irregular; supporting: cartilage — hyaline, elastic, fibrocartilage, calcified — and bone — spongy, compact; fluid: blood and lymph) → Muscular (skeletal, smooth, cardiac) → Nervous (neurons and neuroglia)

Epithelial Tissue

  1. Epithelial tissue covers the inner and outer surfaces of the body and organs. Its cells are compactly arranged with little intercellular matrix and rest on a non-cellular basement membrane.

  2. It is avascular and has a good capacity of regeneration. Its major function is protection, along with absorption, transport, filtration and secretion.

  3. Simple epithelium has a single layer of cells; compound epithelium has two or more layers, of which only the lowermost rests on the basement membrane.

  4. The seven types of simple epithelium are: squamous (flat, pavement-like; blood vessels, alveoli, coelom), cuboidal (pancreatic and salivary ducts, convoluted tubules of the nephron), columnar (tall, with microvilli; intestine, gall bladder, gastric and intestinal glands), ciliated (nasal cavity, trachea, oviduct, buccal cavity of frog), glandular (secretes mucus, enzymes and hormones), sensory (nose, ear, eye) and germinal (seminiferous tubules, inner lining of ovary).

  5. Glands are unicellular (goblet cells) or multicellular (salivary gland). Multicellular glands are exocrine, which pour secretions at a specific site through a duct, or endocrine, which are ductless and release secretions directly into the blood stream.

  6. Compound epithelium is of two kinds: stratified, with the dividing stratum germinativum below and the flat, non-nucleated stratum corneum at the surface, meant for protection; and transitional, whose cells change shape with the degree of stretch, meant for distension of an organ such as the urinary bladder.

  7. Cell junctions connect epithelial cells laterally and to the basement membrane: tight junctions maintain cell polarity and prevent lateral diffusion of proteins and ions; adherens junctions take part in signalling and transcriptional regulation; gap junctions allow passage of ions, small molecules and chemical messages; desmosomes give mechanical strength to epithelial tissue, cardiac muscle and meninges; hemidesmosomes anchor cells to the basement membrane and maintain tissue homeostasis by signalling.

Connective Tissue

  1. Connective tissue is the most widely spread tissue. It binds, supports and strengthens other tissues and organs. Its cells and fibres lie embedded in an abundant matrix. It protects vital organs, acts as packing material, helps in healing, and is highly vascular except cartilage.

  2. It is classified on the basis of matrix into connective tissue proper, supporting connective tissue and fluid connective tissue.

  3. Areolar tissue has a jelly-like matrix containing white fibres of collagen, which give tensile strength, and yellow fibres of elastin, which are elastic. Its four cell types are fibroblasts (produce fibres and ground substance), mast cells (secrete heparin and histamine), macrophages (amoeboid and phagocytic) and fat cells or adipocytes.

  4. Adipose tissue is packed with adipocytes whose nuclei are pushed to the periphery by stored fat droplets. White adipose tissue is opaque and common in adults; brown adipose tissue is reddish brown because of its many blood vessels. It acts as an insulator, a shock absorber and an energy store.

  5. Dense regular connective tissue has parallel collagen fibres. Tendons connect skeletal muscles to bones and contain bundles of white fibres. Ligaments are made of elastic yellow fibres, occur at joints and prevent dislocation of bones. In dense irregular tissue the fibres are not orderly; it is found in the dermis of skin.

  6. Cartilage is pliable yet tough, with a matrix called chondrin bounded by a perichondrium. Chondroblasts mature into chondrocytes, which lie in lacunae, each lacuna containing 2-8 chondrocytes. Its four types are hyaline (fine collagen fibres; ends of long bones, epiglottis, trachea, ribs, larynx, hyoid), elastic (elastic fibres; ear lobe, tip of nose), fibrocartilage (bundles of collagen fibres, no perichondrium, most rigid; intervertebral discs, pubic symphysis) and calcified (rigid due to salt deposition; head of long bones).

  7. Bone has a hard matrix of ossein made of hydroxy-apatite, \(Ca_{10}(PO_4)_6(OH)_2\), covered by periosteum. The matrix is arranged in lamellae containing lacunae connected by canaliculi. Osteoblasts are active bone cells, osteocytes inactive, and osteoclasts are also present. Mammalian bone shows a haversian system whose canal encloses an artery, a vein and nerves. Spongy bone lacks the haversian system, has trabeculae and contains red bone marrow; compact bone has the haversian system with no space between lamellae.

  8. Blood and lymph are fluid connective tissue, in which the matrix is liquid.

Muscular Tissue

  1. Muscle cells are elongated fibres bounded by the sarcolemma, with cytoplasm called sarcoplasm containing myofibrils made of actin and myosin. Muscle fibres shorten on stimulation, so this is called contractile tissue. It is vascular, innervated, and rich in mitochondria.

  2. Skeletal muscles are attached to bones. They are built of fasciculi wrapped in epimysium or fascia, each fasciculus covered by perimysium and made of myofibres. Each fibre is syncytial with peripheral nuclei, and each myofibril is made of repeating sarcomeres.

  3. In a sarcomere, the dark anisotropic ‘A’ band contains myosin as well as actin, the light isotropic ‘I’ band contains only actin, the ‘H’ zone lies at the centre of the ‘A’ band with the ‘M’ line in its middle, and adjacent light bands are separated by the ‘Z’ line. Corresponding bands on neighbouring myofibrils give the striated appearance.

  4. Skeletal muscles show quick, strong, voluntary contractions. On the basis of myoglobin content they are red (high myoglobin) or white (low myoglobin). Myoglobin has one haeme and one polypeptide chain and carries one molecule of oxygen.

  5. Smooth or non-striated muscles are spindle shaped, unbranched, with a single central nucleus and no striations. They contain less myosin and more actin than skeletal muscle, contract slowly and continuously, are involuntary, are innervated by the autonomous nervous system, and occur in the walls of visceral organs and blood vessels as longitudinal or circular muscles.

  6. Cardiac muscles are striated but involuntary, branched, with an indistinct sarcolemma so that uninucleate fibres appear multinucleate. Intercalated discs join adjacent fibres, allowing quick transfer of stimulus so the muscle contracts as a unit. The mammalian heart is myogenic, whereas a heart needing a nerve stimulus is neurogenic. Cardiac muscle forms the myocardium.

Nervous Tissue

  1. Nervous tissue consists of neurons and neuroglia. The neuron is the structural and functional unit of the nervous system. Neuroglial cells are non-nervous supporting cells that can divide and regenerate, while neurons cannot regenerate because they lack a centriole. Intercellular matrix is absent in neural tissue.

  2. Excitability is the change in the action potential of the neuron’s membrane on receiving a stimulus; conductivity is the carrying of the electric impulse from dendron to axon.

  3. A neuron has a cyton containing neuroplasm, a central nucleus, mitochondria, Golgi apparatus, RER and Nissl’s granules made of RNA. Dendrons are short and branched and carry impulses towards the cyton. The single axon is bounded by axolemma, contains axoplasm with mitochondria and neurofibrils, is covered by a myelin sheath and neurilemma derived from Schwann cells, is interrupted at nodes of Ranvier, and ends in a telodendron.

  4. Functionally, neurons are afferent (sensory; sense organ to CNS, in the dorsal root), efferent (motor; CNS to effector, in the ventral root) and interneurons, which process and integrate impulses between the two.

  5. Myelinated or medullated fibres are white and conduct fast by saltatory conduction, in which the impulse jumps from node to node; cranial nerves are myelinated. Non-myelinated fibres are grey and conduct slowly; nerves of the autonomous nervous system are non-myelinated.

  6. The functional contact between the axonal ends of one neuron and the dendrites of the next is a synapse.

  7. By the number of processes from the cyton, neurons are unipolar or monopolar (dorsal root ganglion), bipolar (retina, olfactory epithelium) and multipolar (star-shaped cyton, single axon arising from the axon hillock).

10.7 Points to Ponder

  1. Epithelial tissue is packed tight with almost no matrix, while connective tissue is mostly matrix with few scattered cells. Both are called tissues by the same definition. What does this tell you about how biologists decide what counts as a tissue?

  2. A tissue is defined partly by a common embryonic origin. If two groups of cells in the adult body looked identical under the microscope and did the same job, but came from different embryonic layers, would you call them the same tissue? Why?

  3. Epithelium is avascular, yet it is placed at the most exposed and most demanding surfaces of the body. Is having no blood vessels a weakness of this tissue, or is it part of its design?

  4. Cells at the surface of stratified epithelium are dead. Think about what it means that the body protects itself with a layer of dead cells, and where else this strategy might be useful.

  5. Cartilage has no blood supply and heals poorly; bone is richly supplied and heals well. Yet the vertebrate skeleton begins as cartilage before much of it is replaced by bone. Why might a developing body start with the tissue that repairs less easily?

  6. The same tissue family gives rise to something as hard as bone and something as fluid as blood, simply by changing the matrix. What does this suggest about the relative importance of cells and their surroundings in deciding what a tissue can do?

  7. Muscle can only pull, never push. Consider how a body made only of pulling tissues manages to produce movement in every possible direction.

  8. Skeletal muscle is fast, strong and voluntary, but tires quickly. Cardiac muscle is involuntary and never rests. If a single type of muscle had to do both jobs, what would it have to sacrifice?

  9. The mammalian heart generates its own beat, while some animals have hearts that must be told to beat by a nerve. What advantages, and what risks, come with an organ that controls itself?

  10. Neuroglial cells can divide and regenerate; neurons cannot. Reflect on what the body gains, and what it gives up, by building its communication system out of cells that are never replaced.

  11. Nervous tissue carries messages in a fraction of a second, while glandular epithelium sends chemical messages through the blood far more slowly. Why might a body keep both systems instead of relying on the faster one alone?

  12. Four tissue types, in different combinations, build every organ in an animal body. Choose any one organ and think about which of the four it could not possibly do without.

10.8 Check Your Concepts

  1. Define a tissue. Why does the definition insist on a common embryonic origin and not merely a similar appearance?

  2. Arrange the following in the correct sequence of increasing organisation: organ system, cell, body, tissue, organ.

  3. Distinguish between somatic cells and germ cells, giving one example of each.

  4. What is a basement membrane? Why is it described as non-cellular, and which type of cell junction anchors epithelial cells to it?

  5. Squamous epithelium lines the alveoli, while columnar epithelium lines the intestine. Explain how the shape of the cells in each case suits the work done there.

  6. What would happen to gas exchange in the lungs if the squamous epithelium of the alveoli were replaced by a compound epithelium? Give reasons.

  7. Distinguish between cilia and microvilli in terms of length, number, movement and function.

  8. Long-term smoking damages the ciliated epithelium of the trachea. Predict two consequences of this damage and justify each.

  9. Draw a labelled diagram of columnar epithelium and mark the microvilli, nucleus and basement membrane.

  10. Why is the pancreas described as both an exocrine and an endocrine gland? Name the secretion released by each part and state where it goes.

  11. Arrange the following steps in the correct order to describe how a dust particle is cleared from the nasal cavity: swallowed into the stomach; trapped in mucus; cilia beat in one direction; particle enters with inhaled air; mucus moves towards the pharynx.

  12. What happens if a burn destroys the stratum germinativum of the skin? Why is such a burn more serious than one that removes only the stratum corneum?

  13. Explain how transitional epithelium allows the urinary bladder to fill without tearing. How does it differ from stratified epithelium in this respect?

  14. Why does connective tissue depend more on its matrix than on its cells for its properties? Support your answer with two contrasting examples from the chapter.

  15. Name the four types of cells found in areolar tissue and state the function of each.

  16. A person takes an antihistamine tablet for an allergy. Which cell of the areolar tissue is responsible for the symptoms, and which of its secretions is being blocked?

  17. Why is the nucleus of an adipocyte pushed to the periphery of the cell? How does this arrangement relate to the function of adipose tissue?

  18. Distinguish between white adipose tissue and brown adipose tissue, and state which is more abundant in newborns and why.

  19. Why is the dense connective tissue of the dermis irregular, while that of a tendon is regular? Relate each arrangement to the forces acting on it.

  20. Trace the development of a chondrocyte, beginning from the chondroblast, and state where each stage is found.

  21. A torn cartilage in the knee heals far more slowly than a fractured bone. Give two reasons based on the structure of the two tissues.

  22. Compare hyaline cartilage, elastic cartilage and fibrocartilage with respect to perichondrium, the special content of the matrix, and one location of each.

  23. Name the mineral salt that hardens the bone matrix and write its chemical formula. Which membrane encloses the matrix, and what passes through it?

  24. Distinguish between osteoblasts and osteoclasts. Explain why a bone must contain both.

  25. Draw a labelled diagram of the detailed structure of bone showing lamellae, lacunae, canaliculi and the haversian canal. Explain how bone cells sealed in a hard matrix continue to receive nourishment.

  26. Distinguish between spongy bone and compact bone, and state which of them is linked with the formation of red blood cells.

  27. Blood does not bind organs together the way areolar tissue does. Justify its classification as a connective tissue.

  28. Why is muscular tissue called contractile tissue? Explain why muscles must work in opposing pairs.

  29. Arrange the following in order from the largest to the smallest structural unit of a skeletal muscle: sarcomere, fasciculus, myofibril, skeletal muscle, muscle fibre.

  30. Draw one sarcomere and label the ‘A’ band, ‘I’ band, ‘H’ zone, ‘M’ line and ‘Z’ line. State which protein is present in each band.

  31. Explain why skeletal muscle appears striated while smooth muscle does not, even though both contain actin and myosin.

  32. Why do the nuclei of a skeletal muscle fibre lie at the periphery of the cell? What does the term ‘syncytial fibre’ mean?

  33. Differentiate between red muscles and white muscles. Which type would you expect to dominate in a long-distance runner, and why?

  34. State the structure and oxygen-carrying capacity of myoglobin. How does it allow a muscle to obtain oxygen from two sources?

  35. What are intercalated discs? Explain how they allow the heart to contract as a single unit.

  36. Distinguish between a myogenic heart and a neurogenic heart. Why does a heart removed from a mammal continue to beat for some time?

  37. Cardiac muscle shares one feature with skeletal muscle and another with smooth muscle. Identify both features and name the tissue in which each is otherwise found.

  38. What happens if the smooth muscle in the walls of the bronchi contracts strongly? Name the condition in which this occurs.

  39. Distinguish between neurons and neuroglial cells with respect to function and capacity for division.

  40. Define excitability and conductivity. Why are both needed for a neuron to carry a message?

  41. What are Nissl’s granules made of? What does their presence tell you about the activity of the cell body?

  42. Differentiate between a dendron and an axon with respect to number, length, branching and direction of the impulse.

  43. Explain saltatory conduction. Why is it not possible in a non-myelinated nerve fibre?

  44. What would happen to the conduction of impulses if the myelin sheath of a nerve fibre were destroyed? Name a disease in which this occurs.

  45. Arrange the following in the correct sequence of a reflex response to a painful stimulus: efferent neuron; receptor in the skin; effector muscle; interneuron in the spinal cord; afferent neuron.

  46. Classify neurons on the basis of the number of processes arising from the cyton, giving one example of each type.

  47. What is an axon hillock, and in which type of neuron is it found? Why is a multipolar neuron well suited for processing information?

  48. Draw a labelled diagram of the transverse section of a nerve showing endoneurium, perineurium, epineurium and a fascicle. To which tissue type do these three coverings belong?

  49. Name the type of tissue and its exact sub-type in each of the following: the lining of the pancreatic duct; the intervertebral disc; the wall of the urinary bladder; the myocardium; the dorsal root ganglion of a spinal nerve.

  50. Identify and name the type of tissues in the following:

    1. Inner lining of the intestine
    2. Heart wall
    3. Skin
    4. Nerve cord
    5. Inner lining of the buccal cavity

  51. Why do animals in cold regions have a layer of fat below their skin?

  52. What enables the ear pinna to be folded and twisted while the nose tip can’t be twisted?

  53. Sharad touched a hot plate by mistake and took away his hand quickly. Can you recognize the tissue and its type responsible for it?

  54. Priya got injured in an accident and hurt her long bone, and later on she was also diagnosed with anaemia. What could be the probable reason?

  55. Supriya stepped out into the bright street from a cinema theatre. In response, her eye pupil shrunk. Identify the muscle responsible for the same.

  56. What is a cell junction? Describe the different types of cell junctions.

  57. With the help of a neat labelled diagram, describe the structure of areolar connective tissue.

  58. Describe the structure of a multipolar neuron.

  59. Distinguish between smooth muscles and skeletal muscles.

  60. Complete the following table.

Cell / Tissue / Muscles Functions
a. Cardiac muscles ———-
b. ———- Connect skeletal muscles to bones.
c. Chondroblast cells ———-
d. ———- Secrete heparin and histamine

10.9 Objective Questions

  1. The study of structure and arrangement of tissue is called as ____________.

    1. anatomy
    2. histology
    3. microbiology
    4. morphology

  2. ____________ is a gland which is both exocrine and endocrine.

    1. Sebaceous
    2. Mammary
    3. Pancreas
    4. Pituitary

  3. ____________ cell junction is mediated by integrin.

    1. Gap
    2. Hemidesmosomes
    3. Desmosomes
    4. Adherens

  4. The protein found in cartilage is ____________.

    1. ossein
    2. haemoglobin
    3. chondrin
    4. renin

  5. Find the odd one out.

    1. Thyroid gland
    2. Pituitary gland
    3. Adrenal gland
    4. Salivary gland

  6. Assertion: Epithelial tissue is always thin. Reason: Epithelial tissue is avascular and its cells are nourished by diffusion from the connective tissue below. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.

  7. Assertion: A tendon has very high tensile strength but stretches very little. Reason: The collagen fibres of a tendon are arranged in a parallel manner. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.

  8. Assertion: Muscle cells contain a large number of mitochondria. Reason: Contraction of muscle fibres requires a continuous supply of energy. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.

  9. Assertion: Neurons cannot be replaced after they are lost. Reason: Neurons lack a centriole. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.

  10. Match the following and rewrite.

‘A’ Group ‘B’ Group
a. Muscle i. Perichondrium
b. Bone ii. Sarcolemma
c. Nerve cell iii. Periosteum
d. Cartilage iv. Neurilemma
  1. Match the following and rewrite.
‘A’ Group ‘B’ Group
a. Tight junction i. Anchors the cell to the basement membrane
b. Gap junction ii. Maintains cell polarity
c. Desmosome iii. Passage of ions and small molecules between cells
d. Hemidesmosome iv. Provides mechanical strength