Human Reproduction

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 2.

Why do the testes hang outside the body?

Every other vital organ is tucked safely inside. The testes, which carry half of the next generation, hang outside in a pouch. This looks like a design flaw, but it is the opposite. Sperm production works only about 2 to 2.5 °C below normal body temperature, so the body keeps the factory in a cooler place. That one fact is a good example of how this chapter works: every structure exists for a reason, and once you see the reason, the facts stop being a list.

Humans reproduce sexually and are viviparous: the baby develops inside the mother and is born live. The events happen in a fixed order, and the whole chapter follows it.

The chapter in one line: gametogenesis → insemination → fertilisation → zygote → cleavage → blastocyst → implantation → gestation → parturition.

The male reproductive system

The male system lies in the pelvic region and has a pair of testes, accessory ducts, accessory glands and the penis as the external genitalia. The testes lie outside the abdominal cavity in a pouch called the scrotum, which keeps their temperature 2 to 2.5 °C lower than the normal body temperature, necessary for spermatogenesis. An adult testis is about 4 to 5 cm long and 2 to 3 cm wide and is divided into about 250 testicular lobules. Each lobule has one to three highly coiled seminiferous tubules, where sperms are made.

Two cell types in the testis are examined often, and they are easy to confuse.

The tubules are lined with male germ cells (spermatogonia) and Sertoli cells.

The sperm's route out in one line: seminiferous tubules → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra → urethral meatus.

The vas deferens loops over the urinary bladder and receives a duct from the seminal vesicle to form the ejaculatory duct, which carries sperms and seminal fluid into the urethra. The urethra runs through the penis and opens at its tip, the urethral meatus. The enlarged tip of the penis is the glans penis, covered by a loose fold of skin called the foreskin.

Their combined secretions form the seminal plasma, rich in fructose, calcium and certain enzymes. The bulbourethral secretion also lubricates the penis. Semen is the seminal plasma together with sperms.

Pause and predict: a boy is born with testes that stay in his abdomen instead of descending into the scrotum. What do you expect for sperm production if this is not corrected?

Check your answer: it will be reduced or absent. At body temperature in the abdomen, spermatogenesis does not proceed normally, since the scrotum exists to keep the testes 2 to 2.5 °C cooler.

Beyond the textbook. For the exam: seminal plasma is rich in fructose, calcium and enzymes. In reality: fructose is the sugar that the sperm cells use as fuel, which links directly to the many mitochondria in the sperm's middle piece. A sperm has almost no cytoplasm and carries almost no food of its own, so it is supplied by the fluid it swims in.

The female reproductive system

The female system has a pair of ovaries, a pair of oviducts (fallopian tubes), a uterus, a cervix, a vagina, the external genitalia and a pair of mammary glands. The ovaries are the primary sex organs and produce the ovum and steroid hormones. Each ovary is 2 to 4 cm long, is attached to the pelvic wall and uterus by ligaments, and is covered by a thin epithelium. Inside is the ovarian stroma, with an outer cortex and an inner medulla.

Each oviduct is about 10 to 12 cm long and extends from the ovary to the uterus. The parts are tested often.

The uterus (womb) is shaped like an inverted pear and opens into the vagina through the cervix. The cervical canal together with the vagina forms the birth canal. The uterine wall has three layers.

The external genitalia include the mons pubis, labia majora, labia minora, hymen and clitoris. The hymen is often torn by many causes, such as a sudden fall, a sport or tampon use, so its presence or absence is not a reliable indicator of virginity or sexual experience.

The mammary glands are paired structures that contain glandular tissue and variable amounts of fat. Each has 15 to 20 mammary lobes with clusters of cells called alveoli that secrete milk.

The milk's route out in one line: alveoli → mammary tubules → mammary duct → mammary ampulla → lactiferous duct → nipple.

Gametogenesis I: spermatogenesis

Gametes are made by gametogenesis. In males it is spermatogenesis, which starts at puberty and takes place in the seminiferous tubules.

In spermiation, the sperm heads are embedded in Sertoli cells, from where the sperms are released into the lumen of the seminiferous tubule. The first haploid cell in the male line is the secondary spermatocyte. One primary spermatocyte gives four sperms.

Hormonal control. At puberty, the hypothalamus increases its release of GnRH. This acts on the anterior pituitary, which releases LH and FSH. LH acts on Leydig cells to produce androgens, which drive spermatogenesis. FSH acts on Sertoli cells to produce factors that help spermiogenesis. See Chemical Coordination and Integration for the hormone side.

The sperm has four parts: a head, a neck, a middle piece and a tail. The head contains a haploid nucleus capped by the acrosome, which is filled with enzymes that help in fertilisation. The middle piece has many mitochondria that supply the energy for the tail's movement, which pushes the sperm forward. A normal ejaculate has 200 to 300 million sperms. For normal fertility, at least 60 per cent should have normal shape and size and at least 40 per cent must show vigorous motility.

Pause and predict: which is the first haploid cell in the male line, and how many sperms does one primary spermatocyte give?

Check your answer: the secondary spermatocyte, formed at the end of meiosis I. The one primary spermatocyte gives four sperms.

Gametogenesis II: oogenesis

The female process, oogenesis, begins much earlier than the male one and is much more stretched out in time. It starts in the embryonic stage, when a couple of million oogonia (gamete mother cells) form in each foetal ovary. No new oogonia are formed or added after birth. The oogonia start meiosis but stop at prophase I, and they are then called primary oocytes, each surrounded by a layer of granulosa cells to form a primary follicle. Many degenerate with time, and by puberty only 60,000 to 80,000 primary follicles are left in each ovary.

Follicle development.

Meiosis I is unequal. It gives one large secondary oocyte, which keeps nearly all the cytoplasm, and a tiny first polar body. At ovulation the Graafian follicle ruptures and releases the secondary oocyte, which is still at the secondary oocyte stage. It completes meiosis II only if a sperm enters.

Pause and predict: why does the egg get nearly all the cytoplasm while the polar bodies get almost none?

Check your answer: the ovum has to feed the zygote and early embryo until implantation, so it keeps the stored food and cytoplasm, and the polar bodies are discarded with the extra chromosomes.

Beyond the textbook. For the exam: the primary oocyte is arrested at prophase I from the foetal stage until puberty. In reality: this means that an oocyte ovulated at 35 has been waiting in the ovary since before its owner was born. This long wait is one reason that the chance of chromosome errors such as Down syndrome rises with maternal age. Sperms are not stored like this, because they are made fresh after puberty.

The menstrual cycle

The menstrual cycle is the reproductive cycle of female primates (monkeys, apes and humans). The first menstruation at puberty is menarche. The cycle lasts about 28 or 29 days on average, one ovum is released per cycle, and the cycles stop around 50 years of age at menopause. Cycles also stop during pregnancy.

The cycle in one line: FSH → follicle grows → estrogen → endometrium rebuilt → LH surge → ovulation → corpus luteum → progesterone → (no fertilisation) corpus luteum degenerates → endometrium breaks → menstruation.

Why it works this way. The corpus luteum is the pregnancy support structure that gets built every month "just in case". If the egg is not fertilised, the corpus luteum degenerates, progesterone falls, and the endometrium that it was maintaining can no longer hold, so it breaks down. If the egg is fertilised, the cycle is not repeated, because hormones from the early embryo and placenta take over.

Pause and predict: a drug blocks the LH surge. Which event is lost, and what does that do to the luteal phase?

Check your answer: ovulation does not happen, because the LH surge is what ruptures the Graafian follicle. Without ovulation no corpus luteum forms, so progesterone does not rise in the usual way.

Beyond the textbook. For the exam: hCG is a hormone made by the placenta only during pregnancy. In reality: hCG is what a urine pregnancy test detects. It rises in early pregnancy and keeps the corpus luteum alive and making progesterone until the placenta can do the job itself.

Fertilisation

Sperms must meet the ovum at the right time, since the ovum survives only briefly after ovulation. Sperms swim through the cervix and uterus and reach the ampulla of the fallopian tube, and fertilisation can happen only when the ovum and sperm reach the ampulla at the same time. The sperm contacts the zona pellucida of the ovum, which triggers changes in the membrane that block the entry of additional sperms. This ensures that only one sperm fertilises an ovum. The acrosome's enzymes help the sperm to penetrate the zona pellucida and plasma membrane. The entry of the sperm induces the secondary oocyte to complete meiosis II, forming a second polar body and a haploid ovum (ootid). The haploid sperm nucleus and the ovum nucleus fuse to form a diploid zygote.

Sex determination. The female has two X chromosomes (XX) and the male has XY. All ova carry an X, while half of the sperms carry X and half carry Y. If an X sperm fertilises the ovum, the zygote is XX, a girl, and if a Y sperm does it, the zygote is XY, a boy. So the sex of the baby is decided by the father's sperm. Principles of Inheritance and Variation covers this in more depth.

Cleavage, blastocyst and implantation

The zygote begins mitotic divisions called cleavage as it moves through the isthmus towards the uterus. It forms 2, 4, 8 and 16 daughter cells, called blastomeres. The embryo with 8 to 16 blastomeres is the morula. As it moves into the uterus, it becomes a blastocyst, in which the blastomeres are arranged in an outer layer, the trophoblast, and an inner group of cells, the inner cell mass, which will form the embryo.

The path in one line: zygote (ampulla) → 2 → 4 → 8 → 16 cells (isthmus) → morula → blastocyst (uterus) → implantation.

Implantation leads to pregnancy. The inner cell mass contains stem cells and differentiates into three germ layers: the outer ectoderm, the inner endoderm and the mesoderm between them. These layers give rise to all tissues and organs.

Pause and predict: at which stage does the embryo implant, the morula or the blastocyst?

Check your answer: the blastocyst. The trophoblast is the layer that attaches to the endometrium, and a morula has no trophoblast.

Placenta and pregnancy

After implantation, finger like projections called chorionic villi appear on the trophoblast and become surrounded by the uterine tissue and maternal blood. The villi and the uterine tissue together form the placenta. It is the structural and functional unit between the developing embryo (foetus) and the maternal body. It supplies oxygen and nutrients, removes carbon dioxide and wastes, and is joined to the foetus by the umbilical cord.

The placenta is also an endocrine tissue.

In addition, during pregnancy the levels of estrogens, progestogens, cortisol, prolactin and thyroxine rise severalfold in the maternal blood. These support the growing foetus and the metabolic changes in the mother.

Beyond the textbook. For the exam: the placenta is the unit linking foetus and mother. In reality: the mother's blood and the baby's blood do not mix. They run side by side, separated by thin walls in the villi, and oxygen, nutrients and wastes cross that barrier. Because other substances may cross too, doctors advise caution with drugs and alcohol during pregnancy.

Foetal development.

Parturition and lactation

The average duration of pregnancy is about nine months, the gestation period. Parturition is the delivery of the baby, caused by vigorous contractions of the uterus. The signals for it come from the fully developed foetus and the placenta, which induce mild uterine contractions called the foetal ejection reflex. This triggers the mother's pituitary to release oxytocin. Oxytocin acts on the uterine muscle and causes stronger contractions, which in turn stimulate more oxytocin release. The coordinated loop continues until the baby is delivered. The placenta is expelled soon after.

The loop in one line: mild contractions → oxytocin release → stronger contractions → more oxytocin → … → delivery.

Why it matters. This is a classic positive feedback loop, where the response intensifies the stimulus instead of damping it, unlike the negative feedback that keeps most hormones in balance. It ends only when the baby is born and the stimulus disappears.

Lactation. The mammary glands differentiate during pregnancy and start making milk towards its end. The milk produced during the first few days is called colostrum, and it contains several antibodies that are essential for the newborn's resistance. Breast feeding during the early period of infancy is therefore recommended for a healthy baby.

Exam traps

60-second revision

Where this chapter connects

• Previous: Sexual Reproduction in Flowering Plants shows the same logic in plants: meiosis makes gametes, fertilisation restores the diploid number, and a nourishing tissue feeds the embryo.
• Next: Reproductive Health builds on this chapter's anatomy and cycle with contraception, infertility and assisted reproduction.
• Related: Chemical Coordination and Integration covers GnRH, LH, FSH, androgens, estrogen and progesterone, and Principles of Inheritance and Variation takes sex determination further.