On a summer night in 1978, in a hospital in England, a baby girl named Louise Brown was born. To everyone in the room she looked like any other healthy newborn. Yet news of her birth travelled across the world, because Louise had begun her life in a way no human ever had before. She was not conceived inside her mother’s body. Her mother’s fallopian tubes were blocked, so the egg and the sperm could never meet the natural way, and the couple had been unable to have a child for years.
Two scientists, the physiologist Robert Edwards and the gynaecologist Patrick Steptoe, tried something bold. They took an egg from the mother, brought it together with the father’s sperm in a glass dish in the laboratory, and allowed fertilization to happen outside the body. The tiny early embryo that formed was then gently placed back into the mother’s uterus, where it settled, grew, and nine months later became Louise. The newspapers called her the world’s first “test-tube baby.” Decades later, Robert Edwards received the Nobel Prize for this work.
Louise’s story sits at the very heart of this chapter. To understand how she could be born, you first need to understand how living things normally make new life — from a simple bud on a Hydra to the remarkable journey of a human sperm, egg, embryo and birth.
As you work through this chapter, you will find the answer to each of these questions.
2.1 Asexual Reproduction in Animals
Every living organism has a fixed lifespan. Individuals grow old and die, yet the species they belong to continues generation after generation. The process that makes this continuity possible is reproduction. Even though a single Hydra or a single human lives for only a limited time, reproduction allows the whole kind to survive across long stretches of time.
Reproduction is the biological process by which new living organisms (offspring) are formed from pre-existing organisms of the same kind.
Reproduction is therefore counted among the major life processes of any organism. In this chapter you will study how animals reproduce, the structure of the human reproductive system, the making of gametes (gametogenesis), fertilization, the early development of the embryo, birth (parturition), and finally reproductive health.
Reproduction in animals takes place mainly by two methods — asexual reproduction and sexual reproduction. We begin with the simpler of the two.
What Makes Reproduction “Asexual”
Asexual reproduction is a common method among lower animals. It has three defining features: it does not involve meiosis, it does not involve the formation and fusion of gametes, and the offspring are produced by a single parent alone.
To understand why these points matter, recall two terms. A gamete is a specialised sex cell (such as a sperm or an egg). Meiosis is the special type of cell division that halves the chromosome number to produce these gametes. In asexual reproduction both of these steps are skipped — new individuals arise from ordinary body-cell divisions of one parent.
Because there is no mixing of genetic material from two parents, the offspring are genetically identical to the single parent. Such genetically identical offspring are called clones.
Asexual reproduction is the formation of offspring from a single parent, without meiosis and without the formation and fusion of gametes, giving progeny that are genetically identical to the parent (clones).
The lower animals reproduce asexually mainly by two methods — gemmule formation and budding. A related process, fragmentation, is also seen in some of them.
Budding
Budding is a simple method of asexual reproduction that normally occurs in favourable conditions, when food and resources are plentiful. It is seen in a variety of animals such as coelenterates (for example Hydra and corals) and in some colonial ascidians (sea squirts).
In Hydra, a small outgrowth called a bud is produced towards the basal (lower) end of the body. This bud grows, develops its own tentacles, and gradually transforms into a small but complete new individual. This whole process is called budding. Finally, the young Hydra gets detached from the parent and becomes an independent new organism. The stages are shown in Fig. 2.2.
The developmental sequence of budding can be followed step by step in FC 2.2.
Gemmule Formation vs Budding | Feature | Gemmule formation | Budding | |—|—|—| | Found in | Sponges only | Coelenterates (Hydra, corals), some colonial ascidians | | Conditions favouring it | Unfavourable (harsh) conditions | Favourable conditions | | Nature of the reproductive body | Internal bud of dormant archaeocytes with a resistant coat | External outgrowth (bud) from the body wall | | Main role | Surviving bad conditions and reviving later | Rapid multiplication when resources are plentiful | | Example | Spongilla | Hydra |
Reef-building corals reproduce partly by budding. When new buds stay attached instead of separating, they build up over time into large coral colonies — the living framework of coral reefs that shelter a huge variety of marine life.
Fragmentation and Regeneration
Some lower animals can also reproduce by fragmentation, in which the body breaks into pieces and each piece grows into a complete new individual. This is well seen in the flatworm Planaria. In Planaria, the anterior end can exert a pull on the posterior end, causing a constriction in the middle and splitting the body into two pieces. Each piece then grows into a new Planaria, so two clones of the original are formed.
Fragmentation is closely tied to the animal’s power of regeneration, but the two ideas must not be confused.
Regeneration, in biology, refers to the process — found in organisms ranging from unicellular bacteria up to complex forms such as humans — by which an organism repairs, regrows or restores a lost or damaged body part. A damaged Hydra, for instance, can regenerate its lost part, and if Planaria is wounded, its cells become active and rebuild the lost part or organ back to its original state. Regeneration involves asexual (mitotic-type) cell activity, yet it differs distinctly from reproduction.
Regeneration vs Reproduction Regeneration only repairs or replaces a lost part of an existing individual — no new organism is created. Reproduction (as in fragmentation) produces a whole new, separate individual. The dividing line is simple: if you end up with one repaired animal, it is regeneration; if you end up with two or more independent animals, it is reproduction.
Recall the common methods of reproduction in unicellular organisms such as Euglena, Amoeba and Paramoecium, and identify which type of asexual reproduction you have just read about occurs in Hydra.
2.2 Sexual Reproduction in Animals
What Makes Reproduction “Sexual”
Sexual reproduction is the process that produces offspring through the formation and fusion of gametes. Because it brings together (mixes) genetic material from two parents, it is also called amphimixis.
In animals, the formation of gametes primarily involves meiosis — the special reduction division that halves the chromosome number. This is a key point: the gametes are haploid (they carry a single set of chromosomes), so that when a male gamete and a female gamete fuse, the normal double set is restored in the offspring.
Sexual reproduction is the production of offspring by the formation and fusion of gametes (amphimixis), the gametes normally being formed by meiosis.
Two Phases of Life
A sexually reproducing animal passes through two main phases during its lifetime. The earlier phase is the juvenile phase, which mainly represents the period of physical growth starting from birth. During this phase the animal cannot reproduce sexually.
The later phase is the reproductive maturity phase, usually reached after physical growth is almost complete. It involves the growth and activity of the sex organs, and only in this phase can the animal reproduce sexually. The duration of both phases varies widely in different animals.
Events After Sexual Maturity
Once an animal attains sexual maturity, reproduction unfolds through a set of events that can be grouped into three stages, summarised in FC 2.1.
Pre-fertilization covers the making of gametes (gametogenesis) and the transfer of gametes so they can meet. Fertilization is the actual fusion of the male and female gametes. Post-fertilization events include the formation of the zygote (the single diploid cell produced when the gametes fuse) and embryogenesis (its development into an embryo).
Breeding Patterns
Sexually reproducing animals differ in when they breed. Some, such as the goat, sheep and donkey, are seasonal breeders, reproducing only in particular seasons of the year. Others, such as humans and apes, are continuous breeders, able to breed throughout the year.
Seasonal Breeders vs Continuous Breeders | Feature | Seasonal breeders | Continuous breeders | |—|—|—| | When they breed | Only in certain seasons | Throughout the year | | Examples | Goat, sheep, donkey | Humans, apes |
Human Reproduction: An Overview
Humans are sexually reproducing animals. The whole process runs through a set of sequential steps: gametogenesis, insemination (the release of semen into the female tract), internal fertilization (fusion of the male and female gametes inside the female body), zygote formation and embryogenesis, gestation (pregnancy) and finally parturition (birth). The details of each step form the rest of this chapter.
Primary and Secondary Sex Organs
The gametes — sperms in the male and eggs (ova) in the female — are produced by the primary sex organs: the testis in the male and the ovary in the female. All other reproductive organs, apart from the testis and ovary, are called secondary sex organs.
Males and females can also be told apart externally by certain features called secondary sexual characters, so they are described as sexual dimorphic characters. In males these include a beard, moustache, hair on the chest, a muscular body and an enlarged larynx (the Adam’s apple). In females they include developed breasts, a broader pelvis and a high-pitched voice.
Primary vs Secondary Sex Organs | Feature | Primary sex organs | Secondary sex organs | |—|—|—| | Main job | Produce gametes (and sex hormones) | Support gamete transfer, fertilization, pregnancy | | Examples in male | Testis | Accessory ducts, glands, penis | | Examples in female | Ovary | Oviducts, uterus, vagina, etc. |
Here we meet the word hormone for the first time — a chemical messenger secreted into the blood by an endocrine gland to control the activity of distant organs. The sex organs are important producers of such hormones.
Male Reproductive System
The male reproductive system consists of the primary male organ (the gonad) called the testes, together with the accessory ducts and glands that form the internal and external genitalia.
The Testes
A pair of testes, mesodermal in origin, are formed in the lower abdominal cavity. They lie in a pouch called the scrotum. During early foetal life the testes develop inside the abdominal cavity and only later descend into the scrotal sac through a passage called the inguinal canal. Each testis is oval, 4 to 5 cm long, 2 to 3 cm wide and 3 cm thick. The internal arrangement of a testis is shown in the longitudinal section, Fig. 2.3.
A few pointers about the testis: the peritoneal covering around it is a clue to its abdominal origin; it is suspended in the scrotum by the spermatic cord; the hormone testosterone stimulates the descent of the testis along a fibro-muscular band called the gubernaculum; and in some males a loop of the intestine may pass through the inguinal canal into the scrotum, causing a condition called inguinal hernia.
Histology of the Testis
Under the microscope, the testis is seen to be covered externally by a collagenous connective tissue layer called the tunica albuginea. Outside this lies an incomplete peritoneal covering, the tunica vaginalis, and inside it a thin, membranous, vascular layer called the tunica vasculosa.
Fibres from the tunica albuginea divide each testis into about 200–300 testicular lobules. Each lobule contains 1–4 highly coiled seminiferous tubules. Each seminiferous tubule is lined internally by cuboidal germinal epithelial cells (the spermatogonia) and a few large pyramidal cells called Sertoli cells (also called sustentacular cells). A transverse section is shown in Fig. 2.4.
The germinal epithelial cells undergo gametogenesis to form the spermatozoa (sperms). The Sertoli cells provide nutrition to the developing sperms. The various stages of sperm formation can be seen within the seminiferous tubules: the innermost spermatogonial cell (2n), then the primary spermatocyte (2n), the secondary spermatocyte (n), the spermatids (n) and finally the sperms (n).
Lying in between the seminiferous tubules are the interstitial cells or Leydig cells. These secrete the male hormone androgen, also called testosterone.
Seminiferous tubule and Sertoli cells (Sperm-forming germinal epithelium lines the tubule, while large Sertoli cells sit among the developing sperms → the Sertoli cells supply nourishment and anchor the immature sperms → so the tubule works as a “nursery” where sperms can be produced continuously and fed until they mature.)
the testis Zoom out: the testis is one organ of the male reproductive system. Zoom in: it is packed with lobules → each lobule holds coiled seminiferous tubules → each tubule is lined by germinal epithelial cells → these cells divide to make sperm cells.
Accessory Ducts
The accessory ducts carry the sperms out of the testis. In order, they are the rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory duct and urethra.
All the seminiferous tubules join at the posterior surface of the testis to form a network of tubules called the rete testis. From the rete testis, 12–20 fine tubules called the vasa efferentia arise. These carry the sperms and open into the epididymis.
The epididymis is a long, highly coiled tube divided into an upper caput, a middle corpus and a lower cauda region. Importantly, the sperms undergo maturation in the epididymis. Posteriorly, the epididymis leads into the vas deferens, which travels up to the abdominal cavity and loops over the ureter to open into the urethra.
Just before opening into the urethra, the vas deferens joins the duct of the seminal vesicle to form the ejaculatory duct. The ejaculatory duct passes through the prostate gland and opens into the urethra. The urethra provides a common passage for both urine and semen, and is therefore also called the urinogenital duct. In males it is long and extends through the penis, opening to the outside at the urethral meatus (urethral orifice). All the accessory ducts except the urethra are present in pairs.
Vasa efferentia vs Vas deferens Vasa efferentia are the 12–20 fine tubules that carry sperms from the rete testis into the epididymis. The vas deferens is the single (paired) larger duct that carries sperms onward from the epididymis towards the urethra. Same job (transporting sperm), different position in the line.
Because the sperm’s route from where it is made to where it leaves the body is a fixed sequence, it is easy to trace, as shown in FC 2.2.
a sperm leaving the male body 1. A sperm is made in the germinal epithelium of a seminiferous tubule. 2. It passes into the network of the rete testis. 3. It travels through the vasa efferentia into the epididymis, where it matures. 4. It moves into the vas deferens, up over the ureter. 5. The vas deferens joins the seminal vesicle duct to form the ejaculatory duct, which passes through the prostate. 6. Finally it enters the urethra and is carried out through the penis.
order of sperm ducts Read the chain as “Seminiferous → Rete → Vasa efferentia → Epididymis → Vas deferens → Ejaculatory duct → Urethra.” A short cue: “Some Rats Very Easily Visit Every Underground-tunnel.”
Accessory Glands
The male accessory glands add fluids to the sperms to make semen. They are as follows.
The seminal vesicles are a pair of glands lying on the posterior side of the urinary bladder. They secrete an alkaline seminal fluid containing fructose, fibrinogen and prostaglandins, and contribute about 60% of the total volume of semen. The fructose provides energy for sperm movement, the fibrinogen coagulates the semen into a bolus for quick propulsion in the vagina, and the prostaglandins stimulate reverse peristalsis in the vagina and uterus, helping the sperms move faster towards the egg.
The prostate gland is a large, single gland made up of 20–30 lobes, located underneath the urinary bladder. It surrounds the urethra and releases a milky-white, slightly acidic prostatic fluid into it, forming about 30% of the volume of semen. It contains citric acid, acid phosphatase and various other enzymes; the acid phosphatase protects the sperms from the acidic environment of the vagina.
The Cowper’s glands (bulbourethral glands) are a small, pea-sized, paired pair of glands situated on either side of the urethra. They secrete an alkaline, viscous, mucous-like fluid that acts as a lubricant during copulation.
Prostate cancer is cancer of the prostate gland. Men who are over 50 years of age and have a daily high consumption of fat have an increased risk of developing it.
Find out the common symptoms of prostate cancer, and note down what role prostaglandins play in reproduction.
Semen
Semen is the viscous, alkaline, milky fluid (pH 7.2 to 7.7) ejaculated by the male reproductive system. Normally, 2.5 to 4.0 ml of semen is given out during a single ejaculation, and it contains about 400 million sperms.
Semen carries the secretions of the epididymis and the accessory glands, which serve three purposes: nourishing the sperms (fructose), neutralizing acidity (calcium ions, \(Ca^{++}\), and bicarbonates), and activating the sperms for movement (prostaglandins).
The Penis (External Genitalia)
The male external genitalia include the penis and the scrotum. The penis is the male copulatory organ. It is cylindrical and muscular, containing three bundles of erectile tissue — a pair of postero-lateral bundles called the corpora cavernosa and a single median bundle called the corpus spongiosum. The swollen tip of the penis is called the glans penis, which is covered by a loose fold of skin called the foreskin or prepuce.
The Scrotum
The scrotum is a loose pouch of pigmented skin lying behind the penis. It is divided into a right and a left scrotal sac by a septum of the tunica dartos, made of smooth muscle fibres.
During development, the foetal testes are guided into and retained in the scrotum by a short fibro-muscular band called the gubernaculum, and are kept suspended by the spermatic cord. If a testis fails to descend into the scrotum, the condition is called cryptorchidism, which also results in sterility.
The scrotum is not just a container — it is a temperature-control device. The cremaster and dartos muscles of the scrotum help draw the testes closer to or further from the body. In this way they keep the temperature of the testes 2–3 °C lower than normal body temperature, which is necessary for spermatogenesis.
the scrotum outside the body (The scrotum holds the testes in a pouch outside the main body cavity, with muscles that adjust their distance from the body → this keeps the testes 2–3 °C cooler than the core → sperm production, which fails at body temperature, can proceed normally.)
In cryptorchidism, a testis remains stuck in the warmer abdomen instead of descending into the cooler scrotum. Because sperm formation needs the lower temperature, an undescended testis often cannot produce healthy sperms, which is why the condition is linked to infertility and is corrected early in childhood when detected.
Female Reproductive System
The female reproductive system consists of the following parts: a pair of ovaries, a pair of oviducts, the uterus, the vagina, the external genitalia (vulva), a pair of vestibular glands and a pair of mammary glands.
The Ovary
The ovary is the primary female sex organ. Its main functions are the production of the egg (ovum) and of the female reproductive hormones. It is a solid, oval or almond-shaped organ, about 3.0 cm in length, 1.5 cm in breadth and 1.0 cm thick, located in the upper lateral part of the pelvis near the kidneys.
Each ovary is held in place by ligaments that attach it to the uterus and the abdominal wall. The largest of these is the broad ligament, a fold of peritoneum that holds the ovary, oviduct and uterus to the dorsal body wall; the ovarian ligament attaches the ovary specifically to the uterus. The ovary produces five hormones: estrogen, progesterone, relaxin, activin and inhibin.
Structure and Development of the Ovary
Each ovary is a compact structure with a central part called the medulla and an outer part called the cortex, and the cortex is covered externally by a layer of germinal epithelium. The stroma (loose connective tissue) of the medulla contains blood vessels, lymph vessels and nerve fibres, while the outer cortex is more compact and granular. The internal structure is shown in Fig. 2.5.
The cortex shows large numbers of tiny masses of cells called ovarian follicles. These are formed collectively from the immature ova, which originate from cells of the dorsal endoderm of the yolk sac. These cells migrate to the gonadal ridge during embryonic development and divide mitotically; they are now called oogonia. As the oogonia grow in size they become surrounded by a layer of granulosa cells and form the rudiments of the ovarian follicles.
Notice how early all this begins: the process of oogenesis (egg formation) starts much before the birth of the female baby, and by the end of about twelve weeks the ovary is fully formed, already containing more than two million primordial follicles.
The cells of the germinal epithelium give rise to groups of oogonia that project into the cortex as cords called the egg tubes of Pfluger. Each cord ends in a round mass of oogonial cells called an egg nest, from which the primordial ovarian follicles develop. Each primordial follicle has, at its centre, a large primary oocyte (2n) surrounded by a single layer of flat follicular cells.
The primary oocyte begins its meiotic division but gets arrested at meiosis I. Of the two million primordial follicles present in the foetal ovary, only about one million remain at birth, and only about 60,000–80,000 remain by the time of puberty. This large-scale destruction of primordial follicles during growth is called atresia.
The development of the primordial follicles into mature (Graafian) follicles restarts only with the onset of puberty. During each menstrual cycle, only one of the primordial follicles starts growing to form the Graafian follicle, and in each cycle the two ovaries take turns — one ovary alternately produces the Graafian follicle.
The first menstrual cycle, called menarche, normally begins at about 13 years, and menopause (the stopping of the cycles) happens at 45 to 55 years. The period between menarche and menopause is the reproductive age of the female, roughly 32 years. During this time a female will produce a maximum of about 416 eggs (\(32 \times 13 = 416\) eggs).
how the egg supply changes with age The follicle numbers tell a clear story: more than 2 million primordial follicles are present in the foetal ovary → only about 1 million remain at birth → only about 60,000–80,000 remain at puberty → and only about 416 are actually released across the whole reproductive life. So a female is born with her entire lifetime stock of eggs already set, and this stock only falls with age — it is never refilled.
Ovarian Histology of a Mature Female
In a mature ovary, the cortex shows the different stages through which an oocyte develops. These changes are cyclic, occurring during each menstrual cycle, and involve the maturation of primordial follicles into primary, secondary and finally Graafian follicles.
Each primary follicle has multilayered cuboidal follicular cells. The stroma cells then add a covering called the theca over the follicle, and it becomes a secondary follicle. In the secondary follicle the oocyte grows, the granulosa cells increase in number, and they begin producing the hormone estrogen.
The secondary follicle grows into the Graafian follicle by the addition of more follicular cells. As the follicles mature, they move towards the surface of the ovary. The Graafian follicle presses against the thin wall of the ovary, giving it a blistered appearance. The egg is then released from the Graafian follicle during ovulation, and the remaining part of the follicle changes into a temporary endocrine gland called the corpus luteum.
If fertilization does not take place, the corpus luteum degenerates into a white scar called the corpus albicans.
Corpus luteum vs Corpus albicans The corpus luteum is the yellow, temporary, hormone-secreting body formed from the leftover Graafian follicle right after ovulation. The corpus albicans is the white, non-functional scar it degenerates into when fertilization does not occur. In short: corpus luteum = active and yellow; corpus albicans = inactive and white.
In a single transverse section of an ovary, can all the stages of follicles be seen at the same time? Think about why the ovary contains many follicles at different stages together.
Structure of the Graafian Follicle
The Graafian follicle is a mature ovarian follicle. Its structure, from inside outwards, is shown in Fig. 2.6.
At its centre lies an eccentrically placed secondary oocyte, surrounded by a non-cellular layer called the zona pellucida, which is secreted by the vitelline membrane of the oocyte. The outermost protective, fibrous covering is the theca externa, and inner to it is the cellular theca interna, which produces the hormone estrogen.
Inner to the theca interna, the follicular cells form the membrana granulosa. From this layer, the cells differentiate into the discus proligerus and the corona radiata cells. The term cumulus oophorus is used for the oocyte together with its surrounding granulosa cells. A fluid-filled cavity called the antrum lies between the oocyte and the membrana granulosa, filled with a fluid called liquor folliculi.
The Oviducts (Fallopian Tubes)
The oviducts, also called fallopian tubes or uterine tubes, are a pair of muscular ducts lying horizontally over the peritoneal cavity. The proximal part lies close to the ovary, and the distal end opens into the uterus. Each tube is 10 to 12 cm long and is lined internally by ciliated epithelium. Each can be divided into three regions.
The infundibulum is the proximal, funnel-like part, with an opening called the ostium. It is surrounded by many finger-like processes called fimbriae, of which at least one is long and connected to the ovary. The cilia and the movement of the fimbriae help drive the ovulated egg into the ostium.
The ampulla is the middle, long and straight part of the oviduct. Fertilization of the ovum takes place in this region. The isthmus (cornua) is the distal, narrow part of the duct that opens into the uterus.
Q1. If the mother’s fallopian tubes were blocked, why did that alone stop a baby from forming — what important event normally happens inside the fallopian tube?
A. Fertilization — the fusion of the sperm and the egg — normally takes place in the ampulla of the fallopian tube. If the tubes are blocked, the sperm and egg simply cannot meet there, so no zygote can form and pregnancy cannot begin naturally. This is exactly why Louise Brown’s parents needed the egg and sperm to be brought together outside the body.
The Uterus
The uterus, commonly called the womb, is a hollow, muscular, pear-shaped organ located above and behind the urinary bladder. It is about 7.5 cm long, 5 cm broad and 2.5 cm thick, and can be divided into three regions.
The fundus is the upper, dome-shaped part, where implantation of the embryo normally occurs. The body is the broad part that gradually tapers downwards. The cervix is the narrow neck, about 2.5 cm long, which extends into the vagina; its passage has two openings — an internal os towards the body and an external os towards the vagina.
The wall of the uterus has three layers. The outermost is the perimetrium. The middle, thick muscular layer is the myometrium, made of thick smooth muscle; the vigorous contractions of these muscles cause labour during parturition (childbirth). The innermost layer is the endometrium (mucosal membrane), made of stratified epithelium; its thickness changes regularly during the menstrual cycle, and it is richly supplied with blood vessels and uterine glands that nourish the developing foetus.
Most uterine cancers begin in the cells that form the lining (endometrium) of the uterus. Symptoms include abnormal bleeding between periods, vaginal bleeding after menopause, an abnormal watery or blood-tinged discharge, and pelvic pain. It is detected by a Pap smear test, biopsy or ultrasound, and treated by chemotherapy, radiation or surgical removal of the uterus (hysterectomy).
The Vagina
The vagina is a tubular, female copulatory organ, 7 to 9 cm long, lying between the cervix and the vestibule. Its wall has an inner mucosal lining, a middle muscular layer and an outer adventitia layer. The mucosal epithelium is stratified and non-keratinised and stores glycogen.
The vagina itself has no glands, but it receives the cervical secretion of mucus. Its opening into the vestibule is called the vaginal orifice, which is partially covered by a fold of mucous membrane called the hymen. The vagina acts as a passage for menstrual flow and also serves as the birth canal during parturition.
External Genitalia (Vulva)
The external genital organs of the female, lying outside the vagina, are collectively called the vulva (meaning covering or wrapping) or pudendum. They include the following parts.
The vestibule is a median vertical depression of the vulva that encloses the urethral and vaginal openings. The labia minora are a pair of thin folds lying inner to the labia majora; posteriorly they merge to form the fourchette, and towards the anterior end they converge into a hood-like covering around the clitoris.
The clitoris is a small, conical, sensitive projection at the anterior end of the labia minora. It contains a pair of erectile tissue, the corpora cavernosa, and is homologous to the penis. The labia majora are a pair of fleshy folds of skin forming the boundary of the vulva; they are homologous to the scrotum, and they surround and protect the other parts of the external genitalia, enclosing the urethral and vaginal openings in the vestibule. The mons pubis is a fleshy elevation above the labia majora; the mons pubis and the outer part of the labia majora bear pubic hair.
Vestibular (Bartholin’s) Glands
The accessory glands, also called vestibular glands or Bartholin’s glands, are a pair of glands homologous to the bulbourethral (Cowper’s) glands of the male. They open into the vestibule and release a lubricating fluid.
Mammary Glands
The mammary glands are accessory organs of the female reproductive system, meant for the production and release of milk after parturition. Development of the mammary gland occurs at puberty under the influence of estrogen and progesterone, while lactotropic hormone (LTH), also called prolactin, helps in the development of the lactiferous (milk-carrying) tubules during pregnancy. Their internal structure is shown in Fig. 2.7.
The mammary glands are a pair of rounded structures present in the subcutaneous tissue of the anterior thorax, in the pectoral region (from the 2nd to the 6th rib). They are actually modified sweat glands. Each mammary gland contains fatty connective tissue and numerous lactiferous ducts.
The glandular tissue of each breast is divided into 15–20 irregularly shaped mammary lobes, each with alveolar glands and a lactiferous duct. The alveolar glands secrete milk, which is stored in the lumen of the alveoli. The alveoli open into mammary tubules, the tubules of each lobe join to form a mammary duct, and many mammary ducts join to form a wider mammary ampulla connected to the lactiferous duct. These converge towards the nipple, near the tip of the breast, which is surrounded by a dark-brown, circular area of skin called the areola.
Breast cancer: its first symptom is usually a painless lump in the breast or underarm. Other signs include swelling of all or part of the breast, skin irritation, breast or nipple pain, nipple retraction, and redness, scaliness or thickening of the nipple or breast skin, sometimes with discharge. It is detected by mammogram (X-ray), ultrasound, MRI, biopsy or blood test, and treated by radiation therapy, chemotherapy, lumpectomy or mammoplasty.
Weaning is the process by which a mother’s milk is gradually replaced with solid food after some time.
Puberty and Sexual Maturity in Males
Puberty is the age at which the reproductive system becomes functional — the sex organs begin to produce gametes and sex hormones. In males, the onset of puberty occurs at 12–15 years. Under the influence of testosterone, the secondary sexual characters appear, and thereafter the system remains functional throughout life.
Puberty and Sexual Maturity in Females
In females too, the reproductive system becomes functional at puberty. This is marked by the onset of the menstrual cycle, also called menarche, which usually occurs at 10–14 years of age.
Unlike males, however, mature females show cyclic changes in their reproductive system — the menstrual cycles. These continue only up to menopause, which normally happens at 45–50 years. The period from menarche to menopause is thus the reproductive age of the female. After menopause, a female is unable to bear children by natural means. Menarche, the menstrual cycles and menopause are all controlled by gonadotropic hormones.
Q4. Why can a woman naturally bear children only between roughly her early teens and her late forties, while a man stays fertile for most of his adult life?
A. A female is born with her entire, fixed stock of eggs already present in the ovary (only about 60,000–80,000 follicles remain by puberty), and this stock is never renewed — it steadily falls until the follicles are used up at menopause, after which no eggs are released. A male, by contrast, keeps making fresh sperms from his germinal epithelium continuously after puberty and so remains fertile for most of his life. The woman’s fertility is therefore bounded by menarche at one end and menopause at the other.
2.3 Menstrual Cycle (Ovarian Cycle)
The menstrual cycle is a characteristic feature of primates, including humans. It involves a series of cyclic changes in the ovary and in the female reproductive tract, mainly in the uterus. The changes in the ovary take place under the effect of gonadotropins, while the changes in the uterus respond to the ovarian hormones.
The word gonadotropin appears here for the first time — these are hormones (from the pituitary gland) that act on the gonads. The two main gonadotropins in this cycle are FSH (follicle stimulating hormone) and LH (luteinizing hormone).
The cycle repeats with a periodicity of approximately 28 days. Its middle is marked by the release of an egg, and this egg comes alternately from one of the two ovaries in successive cycles. The whole cycle is divided into four phases.
The menstrual cycle is the repeating (roughly 28-day) series of cyclic changes in the ovary and uterus of primate females, controlled by gonadotropins and ovarian hormones, in the middle of which an egg is released.
The way the hormone levels, the ovary and the uterine lining change together across the cycle is shown in Fig. 2.8.
Menstrual Phase
The beginning of each cycle is taken as the first day, on which menses (loss of blood, 45–100 ml) takes place; it lasts for approximately five days (average 3–7 days).
During this phase the endometrium of the uterus breaks down. This happens under the effect of prostaglandins, which are released because the levels of progesterone and estrogen have fallen. As a result, blood, tissue fluid, mucus, the endometrial lining and the unfertilized oocyte are all discharged through the vagina. The endometrial lining becomes very thin, about 1 mm.
The menstrual phase occurs when an ovulated egg does not get fertilized and is therefore shed out along with the menstrual flow. For this reason it is sometimes called the “funeral of the unfertilized egg.”
Meanwhile, during these five days, many primordial follicles develop into primary follicles and a few into secondary follicles, under the effect of FSH.
Proliferative Phase (Follicular / Post-menstrual Phase)
This phase, also called the follicular phase or post-menstrual phase, is the duration between the end of menstruation and the release of the ovum (ovulation). Its length is more variable than that of the other phases, but it generally extends from the 5th to the 13th day of the cycle.
During this phase a few secondary follicles (6 to 12) begin to develop, but usually only one of them grows into a Graafian follicle (the mature follicle). The other secondary follicles degenerate, and this degeneration is called atresia. The developing secondary follicles secrete the hormone estrogen. The stimulation for the growth of new follicles is driven by GnRH (gonadotropin releasing hormone, from the hypothalamus), which stimulates the release of FSH.
At the same time, the endometrium begins to regenerate under the effect of the gradually increasing quantity of estrogen. This regeneration involves the formation of endothelial cells, endometrial (uterine) glands and a network of blood vessels. The thickness of the endometrium reaches 3–5 mm.
Ovulatory Phase
This is the shortest phase of the menstrual cycle. It involves the rupture of the mature Graafian follicle and the release of the ovum (the secondary oocyte) into the pelvic cavity, usually on the 14th day of the cycle.
The trigger is a rapid secretion of LH, brought about by a positive feedback mechanism (rising estrogen causes a sharp surge in LH), which makes the mature follicle rupture. This release of the egg is called ovulation. Ovulation may be accompanied by mild or severe pain in the lower abdomen.
Menstruation vs Ovulation Menstruation is the shedding of the uterine lining (blood loss) at the start of the cycle, around days 1–5. Ovulation is the release of the egg from the ovary, around day 14. They occur at opposite parts of the cycle — one marks its beginning, the other its middle.
Secretory Phase (Luteal Phase)
This phase, also called the luteal phase, lies between ovulation and the beginning of the next menses. It is the longest phase, lasting 14 days, from the 15th to the 28th day of the cycle.
After the release of the secondary oocyte, the remaining tissue of the Graafian follicle transforms into the corpus luteum under the effect of LH. The corpus luteum begins to secrete progesterone and estrogen. The ovulated egg may get fertilized within about 24 hours; but if fertilization does not occur, the corpus luteum survives for only about two weeks and then degenerates into a white scar, the corpus albicans.
The corpus luteum releases progesterone, a small amount of estrogen and inhibin. Under the influence of these hormones, the endometrial glands grow, become coiled and start uterine secretions. The endometrium becomes more vascularized and thickens up to 8–10 mm. Inhibin stops the secretion of FSH. All these changes prepare the uterus for fertilization and the subsequent implantation (the embedding of the embryo into the uterine lining).
If the ovulated egg does get fertilized and the embryo is implanted, the developing tissue secretes human chorionic gonadotropin (hCG). This hormone extends the life of the corpus luteum and stimulates its secretory activity, so the endometrium is maintained. In the absence of fertilization, the next menstrual cycle begins.
The presence of hCG in the mother’s blood and urine is an indicator of pregnancy. This is exactly what a home pregnancy test detects: the test strip reacts with hCG in the urine, so a positive result appears only when hCG is being produced — that is, when an embryo has implanted.
Comparing the Four Phases
Phases of the Menstrual Cycle | Feature | Menstrual phase | Proliferative (follicular) phase | Ovulatory phase | Secretory (luteal) phase | |—|—|—|—|—| | Days (of a 28-day cycle) | 1–5 (avg) | 5th–13th | ~14th | 15th–28th | | Ovary event | Follicles start to grow (FSH) | Graafian follicle matures | Graafian follicle ruptures, egg released | Corpus luteum forms | | Uterus (endometrium) | Breaks down, thins to ~1 mm | Regenerates, 3–5 mm | — | Thickens to 8–10 mm | | Dominant hormone(s) | Falling progesterone and estrogen | Rising estrogen (FSH, GnRH) | LH surge | Progesterone (and estrogen, inhibin) |
The whole cycle then repeats itself, which is why it is drawn as a loop in FC 2.1.
reading the hormone graph in Fig. 2.8 Follow the curves together, day by day. FSH rises early and helps follicles grow. Estrogen climbs as the follicle matures, which triggers a sharp LH spike around day 14 — the peak that causes ovulation. After ovulation, progesterone rises steeply (from the corpus luteum) and dominates the second half. If no fertilization occurs, both progesterone and estrogen fall at the end, and this drop is what allows the lining to break down and the next cycle to begin. The key reading skill is to link each hormone peak to the event it causes: FSH → follicle growth; LH surge → ovulation; progesterone → thickened, secretory endometrium.
Hygiene during menstruation matters. Keep the area clean, change the sanitary napkin every 4–5 hours, and dispose of used napkins properly. Maintaining personal hygiene reduces the risk of infection, because using damp, dirty cloth or wearing a napkin for too long creates an ideal environment for harmful bacteria to grow.
Find out which primates, besides humans, show a menstrual cycle, and think about why menstruation is painful for some women.
2.4 Gametogenesis
Gametogenesis is the process of formation of gametes in sexually reproducing animals. The male gamete is the sperm and the female gamete is the ovum (egg). Both types of gamete are formed from the primordial germ cells of the gonads — the special reserve cells set aside to make gametes.
Gametogenesis takes two forms: the formation of sperms, called spermatogenesis, and the formation of ova, called oogenesis. We look at each in turn.
Spermatogenesis
Spermatogenesis is the process of formation of the male gamete (sperm or spermatozoon) from the germinal epithelium of the testis.
The process is switched on at puberty by hormones. At the onset of puberty, the hypothalamus begins secreting gonadotropin releasing hormone (GnRH). This triggers a significant increase in the secretion of follicle stimulating hormone (FSH), which induces spermatogenesis. Each seminiferous tubule is lined by a single layer of cuboidal epithelial cells called the germinal epithelium, and it is these cells that undergo spermatogenesis to produce sperms.
The process involves three phases — multiplication, growth and maturation — as summarised in Fig. 2.9 and FC 2.1.
In the multiplication phase, the primordial germ cells (2n) of the seminiferous tubules undergo repeated mitotic divisions to produce a large number of spermatogonia (2n). Each spermatogonium is diploid, with 46 chromosomes.
In the growth phase, some of the spermatogonia stop dividing and grow in size, becoming primary spermatocytes (2n) due to the accumulation of food materials.
In the maturation phase, meiotic (reduction) division takes place. The primary spermatocyte undergoes the first meiotic division (meiosis I) to form two haploid secondary spermatocytes (n), each with 23 chromosomes. Each secondary spermatocyte then undergoes the second meiotic division (meiosis II) to produce four haploid spermatids.
The spermatid is non-motile and non-functional. It is transformed into a functional spermatozoon by a process called spermiogenesis. During this change, the spermatids remain held to one another and to the Sertoli cells by cytoplasmic bridges — the sperm heads stay attached to the Sertoli cells while their tails hang free in the lumen of the seminiferous tubule. During spermiogenesis, the length of the spermatid increases, the centrioles are rearranged as proximal and distal centrioles, the mitochondria become spirally coiled, and the acrosome is formed from the Golgi complex.
Spermatogenesis vs Spermiogenesis Spermatogenesis is the entire process of making sperms, from spermatogonia right through to spermatozoa. Spermiogenesis is only the last step within it — the transformation of a non-motile spermatid into a motile, functional spermatozoon. Every spermiogenesis is part of spermatogenesis, but not the other way round.
Structure of Sperm
The sperm is the male gamete — a motile, microscopic, elongated cell. It can be divided into three parts: head, middle piece and tail. Its structure is shown in Fig. 2.10.
The head is oval and contains the haploid nucleus. Above the nucleus is a cap-like structure called the acrosome, formed from the Golgi body. The acrosome contains hydrolytic enzymes — hyaluronidase and proteolytic enzymes such as zona lysins and corona-penetrating enzymes.
the acrosome (The acrosome is a cap loaded with hydrolytic and proteolytic enzymes → these enzymes can dissolve the coverings around the egg → so the sperm head can bore through the corona radiata and zona pellucida to reach the egg.)
The neck is a very short region containing two centrioles — the proximal centriole and the distal centriole.
The middle piece has an axial filament surrounded by 10–14 spiral turns of mitochondria, together called the nebenkern. It produces the energy needed for the movement of the sperm.
the middle piece (Many mitochondria are coiled tightly in the middle piece → they generate a large supply of energy (ATP) → this powers the whiplash beating of the tail that drives the sperm forward.)
The tail is a long, slender, tapering part containing cytoplasm and a fine thread, the axial filament. This filament arises from the distal centriole and runs through the whole length of the tail. The tail is partly surrounded by plasma membrane (this covered region is the principal piece); the part without plasma membrane is called the end piece.
Oogenesis
Oogenesis is the process of formation of the haploid female gamete — the egg or ovum — from the diploid germinal epithelium of the ovary. It involves meiosis (along with mitosis in the earlier phase). Like spermatogenesis, oogenesis can be divided into three stages: multiplication phase, growth phase and maturation phase. The lineage is shown in Fig. 2.11 and FC 2.2.
In the multiplication phase, the primary germinal cells (PGCs, 2n) of the ovary undergo repeated mitotic divisions to form millions of gamete mother cells or oogonial cells (2n). In humans, this phase is completed during the embryonic stage — that is, before the female is even born.
In the growth phase, some of the oogonia stop dividing and grow in size to form the primary oocytes (2n). Cellular organelles such as the endoplasmic reticulum, Golgi apparatus and mitochondria increase in number.
In the maturation phase, oogenesis (which takes place in the ovaries) is initiated before the birth of the female baby. The primary oocytes (2n) enter meiosis. The diploid primary oocyte undergoes meiosis I (the reduction division) to form two haploid daughter cells, each with 23 chromosomes. This division is peculiar in females: because the cytoplasm divides unequally, one daughter cell is large (the secondary oocyte, n) and the other is a tiny cell called the first polar body (n). Normally the first polar body does not enter meiosis II.
The secondary oocyte (n) then proceeds into meiosis II but only up to metaphase II, where its division is arrested. At this stage the secondary oocyte is shed from the Graafian follicle and the ovary (this is ovulation). The restart and completion of meiosis II happens only when a sperm enters, usually in the ampulla of the fallopian tube at the time of fertilization. Again the division is unequal — the large cell becomes the ovum (n) and the small cell the second polar body (n). This ovum (n) is the true female gamete, ready for fertilization.
In other words, the completion of meiosis II and the completion of fertilization go hand in hand. If the secondary oocyte does not receive a sperm, it is simply shed off along with the menstrual flow.
Secondary oocyte vs Ovum The “egg” released from the ovary at ovulation is not yet a fully formed ovum — it is a secondary oocyte, arrested at metaphase II. It only completes meiosis II and becomes a true ovum when a sperm enters it. So in humans, the mature ovum actually exists for a very short time, formed at the moment of fertilization.
A primary oocyte begins meiosis I before the female is born and can then stay paused for many years — even decades — until it is ovulated. Because the oocyte is held in this long arrest, the chances of errors during chromosome separation increase as a woman ages, which is one reason the risk of chromosomal disorders such as Down syndrome rises with greater maternal age.
Structure of the Secondary Oocyte
In humans, the unfertilized “egg” released at ovulation is really the secondary oocyte. It is non-cleidoic (without a shell) and microlecithal (yolk is present in very small quantity). It is approximately 0.1 mm (100 microns) in size, and is a rounded, non-motile, haploid female gamete.
The nucleus of the egg appears large and is called the germinal vesicle. A typical nucleus (pronucleus) is formed only at the time of fertilization. The cytoplasm of the egg is called ooplasm and is devoid of centrioles. The egg is surrounded by various coverings, shown in Fig. 2.12.
The egg’s own membrane is the vitelline membrane. It secretes a non-cellular, glycoproteinous membrane, the zona pellucida, on its outside. Adhering to the outer surface of the zona pellucida are several radially elongated cells forming the corona radiata; these are derived from the innermost layer of granulosa cells and are firmly held to the zona pellucida and to one another by hyaluronic acid (a mucopolysaccharide). Between the vitelline membrane and the zona pellucida lies a fluid-filled perivitelline space, in which the first polar body lies.
The egg also shows polarity. The side bearing the germinal vesicle and the first polar body is called the animal pole, while the opposite side is the vegetal pole.
Comparing Spermatogenesis and Oogenesis
Spermatogenesis vs Oogenesis | Feature | Spermatogenesis | Oogenesis | |—|—|—| | Site | Seminiferous tubules of the testis | Ovary | | When it starts | At puberty | Before birth (in the embryo) | | Final products per parent cell | Four functional sperms | One functional ovum + polar bodies (2–3) | | Cytoplasm division in meiosis | Equal | Unequal | | Extra transformation step | Yes — spermiogenesis (spermatid → sperm) | No such step | | Size of gamete | Very small, motile | Large, non-motile |
Using the two diagrams and the table above, prepare your own chart comparing spermatogenesis and oogenesis, showing clearly at which step the cells become haploid in each process.
2.5 Fertilization / Syngamy
Sexual reproduction begins with the formation of gametes; fertilization is the later process in which the gametes actually fuse. Fertilization is the fusion of the haploid male and female gametes to form a diploid zygote (2n).
In humans, as in other mammals, fertilization is internal and usually takes place in the ampulla of the fallopian (uterine) tube. The fertilized egg, or zygote, then develops into an embryo, and this development occurs within the uterus.
Fertilization (syngamy) is the fusion of a haploid male gamete and a haploid female gamete to form a diploid zygote (2n).
Mechanism of Fertilization
The semen released during ejaculation carries sperms along with various secretions. Right after ejaculation the semen is coagulated; it then undergoes liquefaction, and the sperms become active. The mechanism of fertilization can be followed in three steps — movement of the sperm towards the egg, entry of the sperm into the egg, and activation of the ovum — as summarised in FC 2.1.
Movement of Sperm Towards the Egg
The sperms first undergo capacitation after reaching the vagina. At this stage as many as 50% of the sperms are demobilised, broken or destroyed. The remaining sperms undergo capacitation, a process that takes about 5–6 hours. During capacitation the acrosome membrane becomes thin, calcium ions (\(Ca^{++}\)) enter the sperm, and the tails begin to show rapid whiplash movements.
Capacitation is the set of final functional changes a sperm must undergo in the female reproductive tract before it becomes capable of fertilizing the egg.
As a result of capacitation, the sperms become extra active and begin moving upwards from the vagina to the uterus and on to the oviducts. The prostaglandins in the semen help activate the sperms, and the vestibular secretions of the female further enhance sperm motility. The sperms swim at an average speed of 1.5 to 3.0 mm/min.
The sperms reach the ampulla partly by their own swimming and partly through contractions of the uterus and fallopian tubes, which are stimulated by the female’s oxytocin (a hormone that causes muscular contractions). After capacitation, the sperms may reach the ampulla within 5 minutes. Sperms can remain viable for 24–48 hours, while the ovum survives for only about 24 hours.
Entry of Sperm into the Egg
Out of the 200 to 400 million sperms released, only a few hundred manage to reach the ampulla, and although many arrive there, only a single sperm actually fertilizes the ovum.
A sperm that reaches the egg comes to lie against it. Its acrosome releases lysins — hyaluronidase and corona-penetrating enzymes — which separate and dissolve the cells of the corona radiata, allowing the sperm head to pass through towards the zona pellucida. The zona pellucida carries fertilizin receptor proteins (ZP3, ZP2). The fertilizin of the egg binds to a specific acid protein, the antifertilizin, on the sperm. This binding attracts sperms to the egg and enhances fertilization. Because the fertilizin–antifertilizin interaction is species specific, this reaction is also called the compatibility reaction.
As the sperm head touches the zona pellucida in the animal pole region, its acrosome covering ruptures in what is called the acrosome reaction, releasing lytic enzymes — acrosin or zona lysin — which act on the zona pellucida at the point of contact. This causes an egg reaction: a small fertilization cone (cone of reception) forms on the egg membrane. The sperm head comes in contact with this cone, producing a weak wave of depolarisation. The plasma membranes of both cells then dissolve at the point of contact, and the sperm nucleus and centrioles enter the egg, while the other parts remain outside.
As soon as the sperm head touches the vitelline membrane, a cortical reaction is activated. This changes the vitelline membrane into a fertilization membrane and deactivates the sperm receptors of the zona pellucida. A distinct perivitelline space is created around the fertilization membrane. This blocks the entry of any further sperms, so polyspermy (fertilization by more than one sperm) is avoided.
Acrosome reaction vs Cortical reaction The acrosome reaction happens on the sperm side — the acrosome bursts and releases enzymes so the sperm can penetrate the egg’s coverings. The cortical reaction happens on the egg side — the vitelline membrane changes into a hardened fertilization membrane so that no other sperm can enter. One lets the first sperm in; the other keeps the rest out.
Q2. A human male releases about 200 to 400 million sperms at one time, yet only one fertilizes the egg. What happens to all the others, and why does only a single sperm succeed?
A. Most sperms never even reach the egg: about half are demobilised or destroyed soon after being deposited, and of the millions released, only a few hundred manage to swim all the way up to the ampulla. Even then, only one sperm can fertilize the ovum, because the moment the first sperm enters, the egg’s cortical reaction turns its covering into a hardened fertilization membrane. This block to polyspermy shuts out every other sperm, so a single successful sperm is all that gets through.
Activation of the Ovum
Before fertilization, the ovum was arrested at the metaphase II stage. When the sperm head contacts the vitelline membrane of the egg, the egg is activated to resume and complete meiosis II. In doing so it gives out the second polar body. The germinal vesicle now organises into the female pronucleus. Only at this stage is it the true ovum or egg.
Fusion of Egg and Sperm
The coverings of the male and female pronuclei degenerate, allowing their chromosomes to pair. This results in the formation of a synkaryon by the process called syngamy (or karyogamy), and the diploid zygote is thus formed. The whole event is shown in Fig. 2.13.
The proximal centriole received from the sperm then helps to form the synkaryon spindle and the cleavage of the cell into two blastomeres (the first daughter cells of the zygote).
Significance of Fertilization
Fertilization is important for several reasons. The secondary oocyte completes oogenesis and is transformed into a mature ovum (n). The diploid chromosome number is restored in the zygote through syngamy. The ovum lacks the centrioles needed for further divisions, and these are supplied by the sperm during fertilization.
Fertilization also brings together gametes from two different parents, which results in variations that are significant for evolution. Finally, the sex of the offspring is determined at fertilization — this depends on whether the fertilizing sperm carries an X or a Y chromosome.
A few facts to remember: the secondary oocyte (egg) is ovulated after the LH surge, at about day 14 of the menstrual cycle; the egg, arrested at metaphase II, reaches the ampulla 12–24 hours after ovulation, guided by the cilia and fimbriae of the fallopian tube; during coitus, semen is deposited into the vagina — this is called insemination; and a human male during ejaculation gives out about 2–4 ml of semen with an average count of 200–400 million sperms.
Under Indian law (the Hindu Marriage Act), the minimum marriageable age is 21 years for a boy and 18 years for a girl at the time of marriage.
Think about these two questions: What would happen if the sperm fused with the egg before it reached the fallopian tube? And what is an ectopic pregnancy — can such a pregnancy continue up to full term?
2.6 Embryonic Development
The zygote formed as a result of syngamy does not stay a single cell for long — it is activated to divide. From this one cell, a whole sequence of stages leads to the embryo, as summarised in FC 2.1.
Cleavage
Cleavage is the process of early mitotic division of the zygote into a hollow, multicellular blastula. It does not involve any growth of the daughter cells. The cells produced by cleavage are called blastomeres.
Cleavage is the series of rapid mitotic divisions of the zygote, without growth of the daughter cells, that converts the single zygote into a hollow, multicellular blastula.
Because there is no growth phase between successive cleavages, the size of the blastomeres keeps reducing with every division. As the cells become smaller, their metabolic rate increases, so each cleavage is faster than the one before. This rapid dividing requires quick replication of DNA and a high consumption of oxygen.
Process of Cleavage
In humans, cleavage is holoblastic, meaning the whole zygote divides completely. The cleavage planes may be longitudinal (meridional) or equatorial (horizontal). The pattern is radial and indeterminate — that is, the fate of each blastomere is not predetermined.
The first cleavage is meridional and occurs about 30 hours after fertilization, dividing the zygote longitudinally into two blastomeres, one slightly larger than the other. The second cleavage is also longitudinal but at right angles to the first, occurring about 30 hours after the first. The third cleavage is horizontal, and after it the embryo is at the 8-cell stage.
As these cleavages continue, the young embryo is gradually pushed towards the uterus. By the end of the 4th day after fertilization, the embryo is a solid ball of 16–32 cells that externally looks like a mulberry; this stage is called the morula. These stages are shown in Fig. 2.14.
The morula shows two types of cells: smaller, clearer cells on the outer side, and an inner cell mass of larger cells, all compactly arranged. Until the morula stage, the zona pellucida is retained around the embryo, so there is no change in the overall size from the zygote to the morula. The morula reaches the isthmus and gains entry into the uterus by the end of day 4.
Blastulation
Blastulation is the process of formation of the hollow, multicellular blastocyst. The blastocyst that enters the uterus keeps floating in the uterine cavity for 2–4 days after its entry — that is, until about the end of the 7th day after fertilization. The outer layer of cells of the morula now forms a layer called the trophoblast.
Cells of the trophoblast begin to absorb the glycogen-rich “uterine milk.” The blastocyst doubles in size, from 0.15 mm to 0.30 mm, as more fluid enters and the blastocyst cavity forms. The outer cells become flat and are called trophoblast cells, because they help only in absorbing nutrition for the developing embryo. The inner, larger cells form the inner cell mass or embryoblast, from which the embryo proper develops.
Trophoblast vs Inner Cell Mass (Embryoblast) | Feature | Trophoblast | Inner cell mass (embryoblast) | |—|—|—| | Position | Outer layer of the blastocyst | Inner group of larger cells | | Cell size/shape | Small, flattened | Larger cells | | Main role | Absorbs nutrition; helps in implantation and placenta | Forms the embryo proper |
The inner cell mass remains attached to the trophoblast on only one side. The trophoblast cells in contact with the embryonal knob are called the cells of Rauber. At this stage the blastocyst shows polarity: the side with the inner cell mass is the embryonal end, and the opposite side is the abembryonal end.
By the end of the 7th day the blastocyst is fully formed and is ready for implantation and gastrulation. The zona pellucida has an important protective role here — it prevents the embryo from implanting at an abnormal site by not exposing the sticky, phagocytic trophoblast cells until the embryo reaches the correct implantation site inside the uterus. Once there, the zona pellucida ruptures.
Implantation
After it is formed, the blastocyst gets implanted (embedded) into the endometrium of the uterus. This process usually begins on day 7 after fertilization, and by the end of the 10th day the embryo is completely buried inside the endometrium. The embryo usually implants in the region of the fundus of the uterus.
During implantation, the embryo attaches by its embryonic pole, close to the endometrium, because the trophoblast cells of the animal pole have the power to stick to the uterine wall. The trophoblast cells lying against the embryonal knob divide rapidly, forming two distinct layers — the syncytiotrophoblast and the cytotrophoblast.
The outer layer, the syncytiotrophoblast, is a syncytium — a layer of protoplasm with many nuclei. It sends out processes that extensively invade the endometrium, and the lytic enzymes secreted by the trophoblast rupture the endometrial cells, making a burrow into which the embryo settles. By the end of the 10th day the whole embryo is deeply embedded in the endometrium, completing implantation. The inner layer, the cytotrophoblast, is made of cells that retain their defined cell boundaries.
Syncytiotrophoblast vs Cytotrophoblast The syncytiotrophoblast is the outer layer, a multinucleate mass of protoplasm with no clear cell boundaries, that invades the endometrium. The cytotrophoblast is the inner layer, made of ordinary cells that keep their separate membranes. Outer and boundary-less versus inner and cellular.
Gastrulation and the Three Germ Layers
Gastrulation is the process of formation of the gastrula from the blastocyst. In the gastrula stage the rate of cleavage slows down, but two important events take place actively.
Gastrulation is the process in which the cells of the embryoblast rearrange and move to form the three germ layers — ectoderm, mesoderm and endoderm.
The first event is the differentiation of blastomeres, which produces the three germinal (germ) layers — ectoderm, mesoderm and endoderm — from the cells of the embryoblast. The second event is a set of morphogenetic movements, the different cell movements by which cells reach their definite places in the embryo. A section of the late gastrula is shown in Fig. 2.15.
Gastrulation begins in the embryoblast at about the 8th day after fertilization. The cells at the free end of the inner cell mass, called hypoblasts (primitive endoderm), flatten, start dividing and grow downwards towards the blastocoel (the cavity of the blastocyst). This layer, the endoderm, is the first to differentiate; it grows within the blastocoel and forms a sac called the yolk sac. The remaining cells of the inner cell mass, in contact with the cells of Rauber, are called epiblasts (primary ectoderm). Together these two layers form a flat, bilaminar (two-layered) embryonal disc.
After the endoderm, the second layer to differentiate is the ectoderm. The epiblast cells divide and redivide and move so as to enclose the amniotic cavity. The floor of this cavity carries the embryonal disc, and the pyramidal cells of the disc facing the amniotic cavity form the embryonal ectoderm. The roof of the amniotic cavity is lined by amniogenic cells, which later form the amnion — an extra-embryonic membrane that surrounds and protects the embryo. As a result, the bilaminar embryonic disc comes to lie in between the amniotic cavity and the yolk sac.
Actual gastrulation occurs about 15 days after fertilization, when the bilaminar (two-layered) embryonic disc is transformed into a trilaminar (three-layered) one. This happens through division, rearrangement and migration of the epiblast cells. It begins with the formation of a primitive streak, and a shallow groove along it is the primitive groove. The streak progresses from the posterior to the anterior end of the embryo. From the site of the primitive streak, a third layer of cells, the mesoderm, extends between the ectoderm and the endoderm. The anterior end of the primitive groove communicates with the yolk sac through an aperture called the blastopore (the future anus).
The embryonal disc has now differentiated into three layers — ectoderm, mesoderm and endoderm. The process after gastrulation is called histogenesis (formation of tissues), followed by organogenesis (formation of organs).
Blastula vs Gastrula | Feature | Blastula (blastocyst) | Gastrula | |—|—|—| | Number of layers | Not yet differentiated into germ layers (trophoblast + inner cell mass) | Three germ layers (ectoderm, mesoderm, endoderm) | | Main activity | Rapid cleavage; formation of blastocyst cavity | Slowed cleavage; differentiation and morphogenetic movements | | Timing (after fertilization) | Formed by about day 7 | Three germ layers by about day 15 | | Outcome | Ready for implantation | Sets up the body plan for histogenesis and organogenesis |
Each of the three germ layers goes on to form particular tissues and organs, as listed in Table 2.16.
| Epidermis of skin, hair, nails, sweat glands, salivary glands, mammary glands, lacrimal glands, sebaceous glands, cornea, lens, retina, conjunctiva, nasal epithelium, enamel of teeth, internal and external ear, foregut, hindgut, adrenal medulla, anterior and posterior pituitary, pineal gland, and the entire nervous system. |
All types of muscles (except the iris and ciliary muscles of the eye, which come from ectoderm), connective tissues, dermis of skin, adrenal cortex, heart, blood, blood vessels, lymphatic vessels, middle ear, dentine of teeth, urinary and reproductive ducts, gonads, kidneys, and the sclera and choroid of the eye. |
Epithelium of the mid-gut, glands of the stomach and intestine, tongue, tonsils, lungs, trachea, bronchi, larynx, urinary bladder, vagina, liver, pancreas, thyroid gland, parathyroid gland, thymus gland, Eustachian tube, epithelium of the urethra, and lining of the middle ear. |
broad fate of the three germ layers Think of them by “position”: ectoderm = the outer covering and the nervous system (skin, hair, brain, nerves — the “outside and think” layer); endoderm = the innermost linings and their glands (gut, lungs, liver, pancreas — the “innermost tubes” layer); mesoderm = everything in between that gives the body its bulk (muscles, bones-connective tissue, blood, heart, kidneys — the “middle machinery” layer).
Stem cells are undifferentiated body (somatic) cells of a multicellular organism. They can give rise to many more cells of the same type or differentiate into other cell types. Bone marrow cells, blood stem cells and umbilical cord cells are examples. They can be used in the treatment of conditions such as Parkinson’s disease, Alzheimer’s disease, diabetes, leukemia and arthritis.
Find out what meroblastic cleavage is and in which organisms it is seen. Also recall what is meant by monozygotic, dizygotic and conjoined twins, and relate them to when and how the early embryo splits.
2.12 Birth Control
Birth control measures that deliberately prevent fertilization are referred to as contraceptives. Contraceptive methods help to prevent unwanted pregnancies.
Contraceptives are birth control measures that deliberately prevent fertilization, and thereby prevent unwanted pregnancy.
An ideal contraceptive should be easily available, user friendly, effective, and have no or the least possible side effects. Contraceptive methods are of two main types — temporary and permanent — and the whole classification is set out in FC 2.1.
Natural Method (Safe Period / Rhythm Method)
In the natural method, the principle used is that of avoiding the chances of fertilization. A week before and a week after menstrual bleeding is considered the safe period for sexual intercourse. This idea is based on the fact that ovulation occurs on the 14th day of the menstrual cycle, so a couple avoids intercourse around that time. Its drawback is a high rate of failure.
Coitus Interruptus (Withdrawal)
In this method, the male partner withdraws his penis from the vagina just before ejaculation, so as to avoid insemination. This method too has drawbacks, because the pre-ejaculation fluid may already contain sperms, and these can cause fertilization.
Lactational Amenorrhea
This method is based on the fact that ovulation does not occur during the period of intense lactation following parturition. Therefore, as long as the mother breastfeeds the child fully, the chances of conception are almost negligible. However, this method also has high chances of failure. (Amenorrhea simply means the absence of menstruation.)
Chemical Means (Spermicides)
In this method, chemicals such as foams, tablets, jellies and creams are used by the female partner. If these chemicals are introduced into the vagina before sexual intercourse, they adhere to the mucous membrane, immobilize the sperms and kill them. They may cause an allergic reaction, and this method also has chances of failure.
Mechanical Means (Barrier Methods)
In barrier methods, the ovum and the sperm are prevented from physically meeting with the help of a barrier. These mechanical barriers are of three types.
A condom is a thin rubber sheath used to cover the penis of the male during copulation. It prevents the entry of the ejaculated semen into the female reproductive tract, and can thus prevent conception. It is a simple and effective method with no side effects. “Nirodh” is the most widely used contraceptive by males in India; it is easily available and is given free by the government. It should be properly discarded after every use. A condom is also a safeguard against STDs and AIDS.
The condom is the only contraceptive on this list that does double duty — it prevents pregnancy and, because it acts as a physical barrier to body fluids, it also blocks the transmission of sexually transmitted infections such as HIV/AIDS, syphilis and gonorrhoea. This is why public health campaigns promote it so heavily even for couples who use other contraceptives.
Diaphragms, cervical caps and vaults are devices used by the female and made of rubber. They prevent conception by blocking the entry of sperms through the cervix. The device is inserted into the female reproductive tract to cover the cervix during copulation.
Intra-uterine devices (IUDs) are clinical devices made of plastic or metal. A doctor or trained nurse places the IUD into the uterus. These devices include the Lippes loop, copper-releasing IUDs (Cu-T, Cu7, multiload 375) and hormone-releasing IUDs (LNG-20, progestasert). Some of these are shown in Fig. 2.19 and Fig. 2.20.
The three kinds of IUD work in three different ways. The Lippes loop is a plastic double “S” loop; it attracts macrophages and stimulates them to accumulate in the uterine cavity, and these macrophages increase the phagocytosis of sperms within the uterus, acting as a contraceptive. Copper-releasing IUDs suppress sperm motility and the fertilizing capacity of the sperms. Hormone-releasing IUDs make the uterus unsuitable for implantation and the cervix hostile to sperms; they delay pregnancy for a longer period. The drawbacks of IUDs include spontaneous expulsion, occasional haemorrhage and chances of infection.
The Three Types of IUD | Feature | Lippes loop | Copper-releasing IUD | Hormone-releasing IUD | |—|—|—|—| | Made of / releases | Plastic double “S” loop | Copper (Cu-T, Cu7, multiload 375) | Hormone (LNG-20, progestasert) | | How it works | Attracts macrophages → increased phagocytosis of sperms | Suppresses sperm motility and fertilizing capacity | Makes uterus unsuitable for implantation and cervix hostile to sperms |
Physiological (Oral) Devices — the Pills
Physiological devices are used in the form of tablets and are therefore popularly called pills. The pill is an oral contraceptive used by the female. It contains progesterone and estrogen.
The pills work in two ways. They inhibit ovulation, so no eggs are released from the ovary of the female using the pill, and conception cannot occur. They also alter the quality of cervical mucus so as to prevent the entry of sperms.
The pills do have side effects, such as nausea, weight gain, tenderness of the breast, and slight blood loss between menstrual periods. The pill “Saheli” is an oral contraceptive for females which is non-steroidal; it is to be taken once a week, and these pills are sponsored by the Government.
Saheli is now a part of the National Family Programme as an oral contraceptive pill in India.
Other Contraceptives — Implants
The birth control implant is a contraceptive used by the female. It is a tiny, thin rod about the size of a matchstick, implanted under the skin of the upper arm, as shown in Fig. 2.21.
Implants contain progesterone and estrogen, and their mode of action is similar to that of the pills. They prevent pregnancy for 3–4 years.
Q5. How do birth control methods such as the pill or a copper-T actually prevent a pregnancy from happening?
A. They interrupt the chain of events leading to pregnancy at different points. The pill supplies progesterone and estrogen, which inhibit ovulation, so no egg is ever released, and it also thickens the cervical mucus to keep sperms out. A copper-T works inside the uterus instead: the copper it releases suppresses sperm motility and their fertilizing capacity, so the sperms cannot fertilize the egg. Other methods act at still other points — condoms and diaphragms block the sperms physically, spermicides kill them, and hormone-releasing IUDs make the uterus unsuitable for implantation.
Permanent Methods
The permanent birth control method in men is called vasectomy and in women it is called tubectomy. These are surgical methods, also called sterilization, and are shown in Fig. 2.22.
In vasectomy, a small part of the vas deferens is tied and cut. In tubectomy, a small part of the fallopian tube is tied and cut. In both cases the effect is the same in principle — gamete transport is blocked, and pregnancy is prevented.
Vasectomy vs Tubectomy | Feature | Vasectomy | Tubectomy | |—|—|—| | Performed on | Male | Female | | Duct cut and tied | Vas deferens | Fallopian tube | | What it blocks | Passage of sperms out of the testis | Passage of the ovum (and of sperms up the tube) | | Nature | Surgical, permanent (sterilization) | Surgical, permanent (sterilization) |
Sterilization vs Contraception in general All sterilization procedures are contraceptive, but not all contraceptives are sterilization. Temporary methods (pills, condoms, IUDs) can be stopped, and fertility returns. Vasectomy and tubectomy are surgical and are meant to be permanent.
Medical Termination of Pregnancy (MTP)
An intentional or voluntary termination of pregnancy before full term is called Medical Termination of Pregnancy (MTP) or induced abortion.
Medical Termination of Pregnancy (MTP), or induced abortion, is the intentional or voluntary termination of a pregnancy before full term.
MTP is essential in cases of unwanted pregnancies or where there is defective development of the foetus. It is safe during the first trimester of pregnancy. Defective development of the foetus is examined by amniocentesis.
Amniocentesis
Amniocentesis is a process in which amniotic fluid containing foetal cells is collected using a hollow needle inserted into the uterus under ultrasound guidance. The chromosomes of these cells are then studied to look for abnormalities in the developing foetus. The procedure is shown in Fig. 2.23.
But a dangerous trend has been the misuse of amniocentesis to determine the sex of the unborn child. Frequently, if the foetus is found to be female, it is aborted, which is totally illegal. Because of this misuse, the Government of India legalised the MTP Act in 1971, with strict conditions.
The Medical Termination of Pregnancy (Amendment) Act 2017, under section 3 of the MTP Act 1971, was enacted by the Government of India. The intention of the MTP Act is to reduce the incidence of illegal abortion and the consequent maternal mortality. As per the provisions of the Act, only the consent of the woman whose pregnancy is being terminated is required.
According to the MTP Act, a pregnancy may be terminated: (1) within the first 12 weeks; or (2) after more than 12 weeks but less than 20 weeks. In the second case, the registered medical practitioner’s opinion is mandatory, stating that continuing the pregnancy would involve a risk to the life of the pregnant woman, or grave abnormal physical or mental health, or a substantial risk to the child.
The Medical Termination of Pregnancy (MTP) Act 1971 provides the legal framework for the MTP Act 2017.
In sonography clinics one sees a board saying that sex selection and detection is not done there, and is punishable under the PC-PNDT Act. Find out what the PC-PNDT Act is, and explain why such a mandate is essential.
2.14 Infertility
Infertility is defined as the inability to conceive naturally after (one year of) regular unprotected intercourse.
Infertility is the inability of a couple to conceive naturally after one year of regular, unprotected intercourse.
The causes of infertility may be physical, congenital (present from birth), due to diseases, immunological, or even psychological. The common physical causes in females are polycystic ovary syndrome (PCOS), hormonal imbalance and endometriosis, while in males they include a low sperm count and small size of the penis.
Assisted Reproductive Technologies (ART)
Prior to 1978, an infertile couple had only two options — to adopt, or to remain childless. Today, infertile couples have many options for having a child, such as fertility drugs, test-tube babies, artificial insemination, IUI and surrogate motherhood.
Couples can be assisted to have a child through certain special techniques known collectively as Assisted Reproductive Technologies (ART). The main techniques are compared below and set out in FC 2.1.
IVF (In Vitro Fertilization)
IVF is the process of fertilization in which an egg is combined with sperm outside the body — in a test tube or glass plate — to form a zygote under simulated conditions in the laboratory. The zygote, or the early embryo (with up to 8 blastomeres), can then be transferred into the fallopian tube for further development. This is the technique popularly known as the test-tube baby technique.
In vitro vs In vivo fertilization “In vitro” means fertilization brought about outside the body, in the glassware of a laboratory (as in IVF). “In vivo” means fertilization occurring inside the body (as in artificial insemination, where only the sperms are introduced artificially, but fusion still happens inside the female tract).
ZIFT (Zygote Intrafallopian Transfer)
ZIFT is an infertility treatment used when there is a blockage in the fallopian tubes that prevents the fertilization of the egg by the sperm.
In this method, the egg is removed from the woman’s ovary. Fertilization of the egg with the sperms is brought about outside the body under sterile conditions, forming a zygote by the process of in vitro fertilization (IVF). The zygote is then transferred to the fallopian tube for further development.
GIFT (Gamete Intrafallopian Transfer)
GIFT is the transfer of an ovum collected from a donor into the fallopian tube of another female who can provide a suitable environment for its fertilization and development.
This technique has been developed for cases in which only the entrance to the oviducts, or the upper segment of the oviducts, is blocked. In this procedure, ova and sperms are directly injected into regions of the oviduct, where fertilization produces a blastocyst, which then enters the uterus via the normal route. GIFT has a success rate of about 30 percent.
ZIFT vs GIFT | Feature | ZIFT | GIFT | |—|—|—| | What is transferred | The zygote (already fertilized) | The gametes — ova and sperms (not yet fertilized) | | Where fertilization occurs | Outside the body (in vitro) | Inside the oviduct of the female | | Where it is transferred | Into the fallopian tube | Into regions of the oviduct | | Used when | The fallopian tubes are blocked | Only the entrance or upper segment of the oviducts is blocked |
ICSI (Intra Cytoplasmic Sperm Injection)
ICSI is an in vitro fertilization procedure in which a single sperm cell is injected directly into the cytoplasm of an ovum in the laboratory. Here the sperm does not have to naturally penetrate the egg.
Recall from fertilization that a sperm must normally use its acrosomal enzymes to dissolve the corona radiata and zona pellucida before it can enter the egg. If a man’s sperms are too few or too weak to accomplish this, natural fertilization fails even in a laboratory dish. ICSI simply bypasses that entire barrier by placing one sperm directly inside the ooplasm with a fine needle — which is why it is the technique of choice in severe male-factor infertility.
Artificial Insemination (AI)
In some infertility cases, the male partner is unable to inseminate the female due to a very low sperm count. This problem can be solved by artificial insemination.
In this technique, sperms are collected from the male and artificially introduced into the cervix of the female, for the purpose of achieving a pregnancy through in vivo fertilization (that is, fertilization inside the body).
IUI (Intra Uterine Insemination)
In this technique the process is somewhat like that of artificial insemination; the only difference is that the sperms are introduced into the uterine cavity instead of the cervix.
AI vs IUI Both introduce sperms artificially and both rely on fertilization inside the body. The single difference is the site of delivery — AI places the sperms in the cervix, while IUI places them further up, directly in the uterine cavity, giving the sperms a shorter distance to travel.
Sperm Bank (Semen Bank)
A sperm bank or semen bank is a place that collects, stores and provides human sperms or semen. The semen is provided by healthy males called sperm donors. The sperms are stored in the sperm bank by the cryopreservation method — that is, at very low temperature, which halts their metabolic activity and keeps them viable for long periods.
Surrogacy
Some women have a problem in the implantation of the embryo in the uterus. Such a woman can take the help of a modern remedial technique called surrogacy. In this technique, the embryo is implanted in a surrogate mother, who is not the biological mother of the child.
Adoption
Adoption is a legal process by which a couple, or a single parent, gets the legal rights, privileges and responsibilities that are associated with a biological child, for the upbringing of the adopted child.
An adoptive parent should be medically fit and financially able to take care of the adopted child. A person wishing to adopt a child must be at least 21 years old, and there is no legal upper age limit for adoption.
Tobacco, marijuana and other drugs, and smoking, may cause infertility in both men and women. Nicotine blocks the production of sperm and decreases the size of the testicles. Alcoholism in men interferes with the synthesis of testosterone and has an impact on sperm count. The use of cocaine or marijuana may temporarily reduce the number and quality of sperms.
Consider these two cases. Jayesh, a young married man of 26 years, has been suffering from T.B. for the last 2 years; he and his wife want a child but are unable to have one — what could be the possible reason? And Neeta, aged 45, has been advised by her doctor against such a late pregnancy, yet she wants to be the biological mother of a child without getting pregnant herself — is this possible, and how?
Make a list of IVF centres in Maharashtra, and note down which of the ART techniques described above each one offers.