Human Reproduction: Class 12 Biology · Chapter 2

Everyone reading this page started as a single cell about a tenth of a millimetre wide, made when a sperm from the father fused with an egg from the mother. That cell divided, settled into the wall of the mother's uterus and, over roughly nine months, became a baby. Humans reproduce only sexually, and since our young develop inside the mother's body and are born alive, we are viviparous.

This chapter follows that story in order. We meet the organs that make and carry gametes in males and in females, then the cell divisions that produce sperms and eggs. Next come the monthly cycle that readies a woman's body for pregnancy, the meeting of sperm and egg, life in the uterus, birth and breastfeeding. All of this begins only after puberty. One contrast runs through the chapter: a man goes on producing sperms into old age, while a woman's ovaries stop releasing eggs at about fifty.

2.1 The Male Reproductive System

The male reproductive organs lie in the lower abdomen, the pelvic region, with some parts hanging outside it. They form four groups: a pair of testes, which make sperms and hormones; the accessory ducts, which store and carry sperms; the accessory glands, which add fluid; and the external genitalia. Fig. 2.1 follows one side of the system from start to finish.

Why do the testes (वृषण) hang outside the body, where they look exposed? Sperm production works best a little below body temperature, and the scrotum, a pouch of skin below the abdomen, holds them 2–2.5 °C cooler than the body core. Each adult testis is an oval about 4–5 cm long and 2–3 cm wide, enclosed in a tough fibrous coat.

Inside, a testis is divided up rather like a segmented orange: partitions mark off roughly 250 testicular lobules, and coiled up in each lobule are one to three seminiferous tubules, the actual sperm factories.

Now zoom in on the wall of a seminiferous tubule (Fig. 2.2). Two kinds of cells line it. The male germ cells, or spermatogonia, rest against the outer boundary; every sperm starts from one of them. Tall Sertoli cells stretch from the boundary to the hollow centre, the lumen. Developing sperms sit in pockets along their sides, and the Sertoli cells supply them with nutrients.

Then look between the tubules. This packing tissue, the interstitial space, is threaded with fine blood vessels and holds a few immune cells, but its key residents are clusters of Leydig cells (interstitial cells), the hormone factory of the testis. They release the male sex hormones, the androgens, chiefly testosterone, into the blood.

Follow one sperm on its way out (Fig. 2.1, second step):

1. From the lumen of its seminiferous tubule it drains into the rete testis, a meshwork of channels.
2. A handful of fine tubes, the vasa efferentia, carry it out of the testis.
3. It waits in the epididymis, a long, coiled tube lying along the back of the testis, where sperms are stored.
4. The vas deferens takes it up into the abdomen and over the top of the urinary bladder.
5. Behind the bladder the duct of a seminal vesicle joins in; from here the tube is the ejaculatory duct, which runs through the prostate.
6. It enters the urethra, the tube that leads from the bladder along the length of the penis, and leaves through its opening, the urethral meatus.

The urethra is shared with the urinary system: it carries urine at other times, but never urine and semen at once. The penis, the external genital organ, contains spongy erectile tissue that fills with blood during arousal and stiffens the organ for insemination. A loose fold of skin, the foreskin, sheathes its rounded tip, the glans penis.

Along the route, three kinds of accessory glands add fluid: a pair of seminal vesicles, a single prostate wrapped around the urethra just below the bladder, and a pair of pea-sized bulbourethral glands below the prostate. Their combined secretion is the seminal plasma. It supplies fructose, a sugar the sperms burn for energy, along with calcium and several enzymes, and the bulbourethral glands add a slippery fluid that lubricates the penis.

2.2 The Female Reproductive System

In a woman, the organs that make eggs and carry a pregnancy sit inside the pelvis: two ovaries, two oviducts, the uterus with its neck, the cervix, and the vagina, which opens to the outside among the external genitalia. Their jobs follow one another like a relay: releasing an egg, giving sperms a place to meet it, sheltering the baby and then delivering it. The two mammary glands in the chest run the last leg by feeding the newborn.

Start with the ovaries (अंडाशय), the primary female sex organs. Each is an almond-shaped body 2–4 cm long, slung by ligaments between the side wall of the pelvis and the uterus. An ovary does two jobs at once: it ripens the female gamete, the ovum, and as an endocrine gland it releases the ovarian hormones, estrogens and progesterone. In section, a thin skin of epithelium surrounds the ovarian tissue, the stroma. Its outer zone, the cortex, is crowded with follicles at different stages; its inner zone, the medulla, carries the blood vessels.

Trace an oviduct (fallopian tube), 10–12 cm long, outwards from the uterus. The first stretch, the isthmus, has a narrow bore. It widens into the ampulla, the longest and widest stretch, where sperm and egg normally meet. The tube ends in a funnel, the infundibulum, which is not attached to the ovary: finger-like fimbriae on its rim sweep the ovary's surface and draw in the egg after ovulation. The oviducts, the uterus and the vagina together make up the female accessory ducts.

The uterus (womb) is a single, muscular, hollow organ, roughly the size and shape of an upside-down pear, held in the pelvis by ligaments. Its narrow lower end, the cervix, projects into the vagina; the passage through the cervix (the cervical canal) and the vagina together form the route a baby takes at birth, the birth canal. Cut through the uterine wall and you meet three layers (Fig. 2.3, last step). Innermost is the endometrium, the glandular lining that is rebuilt and shed in every cycle. Around it lies the myometrium, a thick coat of smooth muscle whose powerful contractions push the baby out at birth. A thin membrane, the perimetrium, wraps the outside.

The external genitalia, together called the vulva, are listed here from the front and outside inwards.

• Mons pubis: a rounded pad of fat over the pubic bone, with skin and, after puberty, hair over it.
• Labia majora: the outer pair of skin folds on either side of the vaginal opening.
• Labia minora: the inner, thinner pair of folds.
• Clitoris: a small, highly sensitive organ at the front, where the inner folds meet, above the urethral opening.
• Hymen: a thin, often incomplete membrane at the vaginal opening.

One common belief about the hymen needs correcting. Many people assume its state shows whether a woman has had intercourse. It does not. Its shape varies widely between individuals, it can stretch or tear during cycling, horse riding, a fall or the use of a tampon, and it may remain intact after intercourse. The hymen is therefore not a reliable sign of virginity, and judging anyone by it has no scientific basis.

Mammals take their name from these glands: every female mammal can feed her young on milk from her mammary glands. Each breast combines fat, in amounts that vary a lot between women, with milk-producing glandular tissue organised into 15–20 mammary lobes. Every lobe holds clusters of tiny sacs, the alveoli, whose cells secrete milk into their hollow centres. The milk drains along a branching set of tubes (Fig. 2.4): alveoli, mammary tubules, a mammary duct for each lobe, a wider mammary ampulla, then a lactiferous duct that opens at the nipple.

2.3 Gametogenesis

Gametogenesis is the making of gametes: sperms in the testes and ova in the ovaries, the primary sex organs. Both routes use meiosis, the division that halves the chromosome number, so every human gamete carries 23 chromosomes (the haploid number, $n$) instead of the 46 ($2n$) found in a body cell. Fusion of a sperm with an ovum restores 46.

Spermatogenesis

Spermatogenesis runs non-stop from puberty into old age inside the seminiferous tubules (Fig. 2.2). Track one cell through it (Fig. 2.8):

1. Multiply. Spermatogonia, diploid ($2n=46$), sit on the tubule wall and keep dividing by mitosis, so the stock never runs out.
2. Commit. Every so often a spermatogonium grows into a primary spermatocyte ($2n$), the cell that will go through meiosis.
3. Halve. Meiosis I, the reduction division, splits it into two equal secondary spermatocytes, each haploid ($n=23$).
4. Split again. Meiosis II divides each of these, so one primary spermatocyte ends up as four equal, haploid spermatids.

Spermatids are round cells that cannot yet swim. Their reshaping into spermatozoa (sperms), called spermiogenesis, builds a head, a middle piece and a tail and discards most of the cytoplasm, all while the sperm heads are embedded in Sertoli cells. When they are finished, the sperms are set free into the lumen of the tubule, a release called spermiation.

Hormones that switch it on

Why does sperm production wait for puberty? Because it depends on a hormone relay that only gets going then (Fig. 2.5). The starting signal is gonadotropin-releasing hormone (GnRH) from the hypothalamus, whose output rises sharply at puberty. GnRH reaches the anterior pituitary and prompts it to release two gonadotropins. One, luteinising hormone (LH), targets the Leydig cells, which answer by producing androgens, and androgens are what drive spermatogenesis. The other, follicle-stimulating hormone (FSH), targets the Sertoli cells, which then secrete substances that support spermiogenesis.

Structure of a sperm

A sperm is stripped down for one delivery job: getting a haploid nucleus to an egg (Fig. 2.6). Think of it as a payload with an engine. The payload, in the head, is an elongated haploid nucleus. Over its front fits the acrosome, a cap packed with enzymes that will digest a path through the egg's coverings. A short neck joins the head to the middle piece, which is wound with mitochondria, the power supply. Their energy drives the tail, whose whip-like beating moves the sperm, and a sperm that cannot swim cannot fertilise. A single plasma membrane wraps all four regions.

Semen is the sperms plus the seminal plasma. A sperm fresh from a tubule cannot yet fertilise: it matures and gains its swimming power on the way through the ducts, aided by secretions of the epididymis, vas deferens, seminal vesicles and prostate, all of which keep working only while androgens are present. When doctors assess a man's fertility they look at three figures: a single ejaculation normally carries some 200–300 million sperms, of which at least 60 per cent should look normal (correct size and form) and at least 40 per cent should be vigorous swimmers.

Oogenesis

Oogenesis, the making of an egg, keeps a very different timetable from spermatogenesis. Its story is best told by age (Fig. 2.7).

• Before birth. The ovaries of a female foetus produce a couple of million egg mother cells, the oogonia; none are added after birth. They enter meiosis but halt in prophase I, becoming primary oocytes. Each soon gains a single wrapping of granulosa cells and is then a primary follicle.
• Birth to puberty. Most follicles die back, so by puberty an ovary holds only about 60 000–80 000 primary follicles.
• Each cycle after puberty. A few follicles resume growth. Extra granulosa layers plus an outer coat, the theca, make a secondary follicle. A fluid-filled space, the antrum, then opens up, and the theca differentiates into the theca interna (inside) and the theca externa (outside): a tertiary follicle.
• Inside the tertiary follicle. The primary oocyte, paused for years or decades, enlarges and at last completes meiosis I. The split is lopsided: the haploid secondary oocyte keeps almost all the food-rich cytoplasm, while the first polar body gets almost none.
• Just before ovulation. The follicle is now a mature Graafian follicle, and the secondary oocyte inside it is enclosed in a glycoprotein coat, the zona pellucida.11The zona pellucida actually begins to form much earlier, around the growing primary oocyte, and thickens as the follicle develops.
• Ovulation. The Graafian follicle ruptures and the secondary oocyte leaves the ovary: ovulation. The released cell has begun meiosis II but halts again, and completes it only if a sperm enters (Section 2.5).

Comparing the two

Both processes run through the same kinds of division, but the timing and the results are very different. Step through Fig. 2.8 to compare them cell by cell, then use the table that follows.

2.4 Menstrual Cycle

From puberty to about fifty, a woman's body runs a repeating programme that prepares for a pregnancy roughly once a month and dismantles the preparations when no pregnancy begins. This is the menstrual cycle, a pattern humans share with other primates such as monkeys and apes. A cycle is counted from the first day of bleeding to the first day of the next and lasts about 28–29 days on average; the very first bleeding, at puberty, is the menarche. Around mid-cycle, one ovum is normally released. Fig. 2.9 follows one cycle round like a clock.

Menstrual phase (about days 1–5). The bleeding that opens a cycle is the old endometrium being dismantled: the lining and its blood vessels break down, and the resulting fluid leaves through the vagina over 3–5 days. It happens because the previous cycle ended without a fertilised ovum. So when a period does not come, pregnancy is one possibility, but stress, illness and poor nutrition are others.

Follicular phase (about days 6–13). Two things grow at once. In the ovary, gradually rising FSH and LH from the pituitary push a batch of follicles to develop, and one of them reaches the mature Graafian stage. The growing follicles pour out estrogen, and estrogen rebuilds the endometrium by cell division, which is why this stretch is also called the proliferative phase.

Ovulatory phase (about day 14). When estrogen from the ripe follicle peaks, the pituitary answers with a burst of LH, the LH surge, together with a smaller peak of FSH. Within about a day the surge makes the Graafian follicle burst and release its secondary oocyte.

Luteal phase (about days 15–28). The emptied follicle is not wasted: it fills in and becomes the corpus luteum, a temporary gland that releases large amounts of progesterone. Progesterone holds the endometrium in place and switches its glands on, readying it to receive an embryo; this is the secretory phase of the uterus. What happens next depends on fertilisation. If an embryo implants, the corpus luteum is kept going, the lining stays, and cycling is suspended for the whole pregnancy. If not, the corpus luteum withers, progesterone drops, the unsupported lining breaks down, and the bleeding marks day 1 of the next cycle.

The cycles come to a permanent end at about fifty, at menopause. The years between menarche and menopause are a woman's reproductive phase, and regular cycles during them are a sign of good reproductive health. Use the explorer below to move through a cycle day by day and to see what changes when an embryo implants.

2.5 Fertilisation and Implantation

Fertilisation needs a meeting, and timing is everything. In insemination, semen is deposited in the vagina during sexual intercourse (coitus). The sperms then have a long swim: through the cervix, across the uterus and up into an oviduct as far as the ampulla. The egg, meanwhile, is drawn into the same tube by the fimbriae and moved towards the ampulla. An egg stays fertilisable for only about a day after ovulation, while sperms can survive a few days in the female tract. Unless the two are in the ampulla together there is no fertilisation, so not every act of intercourse leads to pregnancy.

Fertilisation (निषेचन) is the union of one sperm and one ovum, and it unfolds in a fixed order (Fig. 2.11):

1. Contact. Many sperms may reach the egg, but the first to touch its glycoprotein coat, the zona pellucida, sets off changes in the egg's coverings.
2. Entry. Enzymes released from that sperm's acrosome digest a way through the zona pellucida; the sperm then fuses with the egg's plasma membrane and its nucleus passes into the cytoplasm.
3. Lock-out. The changes triggered at contact make the coverings impassable, so normally only one sperm gets in.
4. Meiosis II. Sperm entry is the signal for the secondary oocyte to finish its second meiotic division. Again the split is unequal: a haploid ovum, the ootid, and a tiny second polar body.
5. Fusion. The haploid nucleus of the sperm and that of the ovum come together and fuse into one diploid nucleus. The cell is now a zygote.

The sex of the baby

Whether a zygote becomes a girl or a boy is settled at fertilisation, and the deciding vote is cast by the sperm (Fig. 2.12). A woman's body cells carry two X chromosomes; a man's carry one X and one Y. Each egg a woman makes receives one of her two Xs, so every egg has an X. When a man's testes make sperms, meiosis sends the X into half of them and the Y into the other half. An egg fertilised by an X-bearing sperm becomes XX and develops as a girl; one fertilised by a Y-bearing sperm becomes XY and develops as a boy. Blaming a mother for the sex of her child therefore has no scientific basis. Chapter 4 returns to sex determination.

Cleavage and implantation

The zygote divides as it travels (Fig. 2.14). Nudged along the isthmus towards the uterus by beating cilia and gentle muscular waves of the tube, it splits by mitosis into 2, then 4, 8, 16 … cells, the blastomeres. These divisions, called cleavage, carve the zygote into ever smaller cells without making the embryo any bigger overall. At 8–16 blastomeres the embryo is a solid ball called a morula.

Inside the uterus the morula keeps dividing and hollows out into a blastocyst. Its cells now fall into two groups with very different futures: a thin outer shell, the trophoblast, which makes contact with the mother, and a clump on the inner side of the shell, the inner cell mass, which will become the embryo itself. The trophoblast attaches to the endometrium; the uterine cells next to it multiply rapidly and grow over the blastocyst until it lies buried in the lining. This burying is implantation, and with it pregnancy begins.

2.6 Pregnancy and Embryonic Development

Once implanted, the embryo needs a supply line. The trophoblast sends finger-like chorionic villi into the surrounding uterine tissue, where they are bathed in pools of the mother's blood. Villi and uterine tissue grow into each other like interlocked fingers, and together they form the placenta (अपरा), an organ built jointly by the embryo (later called the foetus) and the mother (Fig. 2.15).

Across the thin walls of the villi, the foetus's blood picks up oxygen and nutrients from the mother's blood and hands back carbon dioxide and other wastes; the two bloodstreams themselves normally never mix. The umbilical cord, which carries the foetus's blood vessels, links it to the placenta.

The placenta is also a hormone factory. Two of its products, human chorionic gonadotropin (hCG) and human placental lactogen (hPL), are never found outside pregnancy, and it makes estrogens and progestogens as well. A third pregnancy-only hormone, relaxin, comes from the ovary in the later months and helps loosen the pelvic ligaments for birth. On top of these, the mother's blood carries several times its usual levels of estrogens, progestogens, cortisol, prolactin and thyroxine. This hormonal surge feeds the growth of the foetus, retunes the mother's metabolism and keeps the pregnancy from ending early.

Meanwhile the inner cell mass gets organised (Fig. 2.16). It contains stem cells, unspecialised cells able to form any tissue or organ, and they begin by sorting into layers: first an outer ectoderm and an inner endoderm, then a middle mesoderm between them. Every tissue of the adult body traces back to one of these three germ layers.

A human pregnancy lasts about nine months, often divided into three trimesters of about three months each. The timeline picks out some milestones of development.

2.7 Parturition and Lactation

After about nine months, the gestation period, the baby is born. Birth, or parturition (प्रसव), is the expulsion of the foetus by powerful contractions of the uterus, and it is controlled by nerves and hormones working together, a neuroendocrine mechanism (Fig. 2.17).

Remarkably, the baby gives the starting signal. When the foetus is fully developed, signals from it and from the placenta set off gentle contractions of the uterus, the foetal ejection reflex. Nerve messages from these contractions prompt the mother's pituitary to release oxytocin. Oxytocin makes the uterine muscle squeeze harder, and the harder squeezes release yet more oxytocin. Because each step amplifies the next, a pattern called positive feedback, the contractions build until the baby is pushed out through the birth canal. A little later, further contractions expel the placenta, the afterbirth.

Milk production, lactation, is prepared during pregnancy, as the mammary glands develop further, and begins towards its end. Prolactin from the pituitary keeps the alveoli making milk, and oxytocin, the hormone of labour, squeezes it out when the baby suckles. The first milk, colostrum, is thick and yellowish and rich in antibodies that guard the newborn against infection before its own immune system is ready. That is why doctors advise putting the baby to the breast soon after birth and giving only breast milk for about the first six months.