Human Reproduction

The male reproductive system is built to make and deliver sperm. The paired testes lie outside the body in the scrotum, which keeps them about 2–2.5 °C below body temperature — essential for normal sperm formation. Each testis is packed with coiled seminiferous tubules, the sites of sperm production, lined by two important cell types: the Sertoli (sustentacular) cells, which nourish the developing sperm (stimulated by FSH), and, in the tissue between the tubules, the Leydig (interstitial) cells, which secrete testosterone (stimulated by LH).

Sperm leave the testis and pass in sequence through the rete testis, the epididymis (where they mature and are stored), the vas deferens, the ejaculatory duct and the urethra. Three accessory glands add secretions to form semen: the seminal vesicles, the prostate gland and the bulbourethral (Cowper's) glands.

Spermatogenesis is the formation of sperm, beginning at puberty. The diploid spermatogonia (2n) multiply and some enlarge into primary spermatocytes (2n). Each primary spermatocyte completes meiosis I to give two haploid secondary spermatocytes (n), which complete meiosis II to give four haploid spermatids (n). The spermatids are then transformed into motile sperm (spermatozoa) by a process called spermiogenesis. So one primary spermatocyte yields four functional sperm.

A mature sperm has a head (containing the haploid nucleus, capped by the enzyme-filled acrosome that helps penetrate the egg), a short neck, a middle piece (packed with mitochondria that provide energy for swimming) and a long tail for movement. For NEET, fix the scrotum's cooling role, the Sertoli (FSH) vs Leydig (LH, testosterone) distinction, the spermatogenesis sequence with its meiotic steps, and the four parts of a sperm with the acrosome and mitochondrial middle piece.

The female reproductive system produces eggs and supports pregnancy. The paired ovaries make the eggs and the hormones oestrogen and progesterone. Each oviduct (fallopian tube) begins as a funnel, the infundibulum with finger-like fimbriae, leads into the wide ampulla (the site of fertilisation) and the narrow isthmus, and opens into the uterus (womb). The uterine wall has three layers: the outer perimetrium, the muscular myometrium and the inner glandular endometrium (which thickens and is shed in the cycle). The uterus narrows into the cervix, which opens into the vagina.

Oogenesis, the formation of the egg, has an unusual timing. The oogonia form and become primary oocytes before birth, then stop, arrested in prophase I for years. At puberty, each cycle a primary oocyte completes meiosis I to give a large secondary oocyte and a tiny first polar body. The secondary oocyte then begins meiosis II but is again arrested at metaphase II; it completes meiosis II only if a sperm fertilises it, producing one ovum and a second polar body. So oogenesis makes only one functional egg (plus polar bodies), unlike spermatogenesis.

The menstrual cycle is the roughly 28-day cycle of the human female from menarche (first menstruation) to menopause (its cessation). It has phases. The menstrual phase (days 1–5) is the bleeding caused by breakdown of the endometrium. The follicular (proliferative) phase rebuilds the endometrium under oestrogen while FSH matures a Graafian follicle. Ovulation occurs around day 14, triggered by a surge of LH (the LH surge), which releases the secondary oocyte.

After ovulation comes the luteal (secretory) phase: the ruptured follicle becomes the corpus luteum, which secretes large amounts of progesterone to maintain the endometrium for a possible pregnancy. If fertilisation does not occur, the corpus luteum degenerates, progesterone falls, and menstruation begins again. For NEET, fix the female anatomy (ampulla = fertilisation site; endometrium = shed layer), the two arrest points of oogenesis (prophase I and metaphase II), and the cycle (ovulation ~day 14 by LH surge; corpus luteum → progesterone).

After ovulation, the secondary oocyte is swept into the oviduct, where fertilisation may occur in the ampulla. A sperm reaching the egg undergoes the acrosomal reaction, releasing enzymes that let it penetrate the layers (the zona pellucida) around the oocyte. The entry of the sperm triggers the oocyte to complete meiosis II, forming the ovum and a second polar body; the sperm nucleus and egg nucleus then fuse (syngamy) to form the diploid zygote (2n). Entry of one sperm changes the zona to block other sperm (preventing polyspermy), and fertilisation also determines the sex of the baby (an X-bearing sperm gives a girl, a Y-bearing sperm a boy).

The zygote begins cleavage — rapid mitotic divisions as it travels down the oviduct — forming a solid ball of cells, the morula, which becomes a hollow blastocyst. The blastocyst has an outer layer, the trophoblast (which will attach to the uterus and form part of the placenta), and an inner cell mass (which forms the embryo proper).

The blastocyst reaches the uterus and undergoes implantation: the trophoblast attaches to and embeds in the endometrium, which marks the beginning of pregnancy. After implantation, finger-like chorionic villi from the trophoblast interlock with the uterine tissue to form the placenta.

The placenta is the intimate structural and functional connection between the foetus and the mother. It allows the exchange of nutrients, oxygen and wastes between mother and foetus and acts as an endocrine organ, secreting hormones such as human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens, progesterone and (later) relaxin. (hCG is the hormone detected by pregnancy tests.) For NEET, fix the acrosomal reaction and block to polyspermy, the morula → blastocyst (trophoblast + inner cell mass) sequence, implantation in the endometrium, and the placenta's exchange and endocrine roles with hCG.

After implantation, the inner cell mass develops into the embryo, and cells arrange into the three primary germ layers — the ectoderm (skin, nervous system), mesoderm (muscle, bone, blood, heart, kidney) and endoderm (gut lining, liver, lungs). The period from implantation to birth, called gestation, lasts about nine months in humans and is divided into three trimesters with recognisable milestones: by the end of the first month the heart forms; by the end of the second month limbs and digits develop; by the fifth month the first movements and hair appear; and by the end of the ninth month the foetus is fully developed and ready for birth.

Parturition is the act of giving birth, and it is a neuroendocrine reflex. Signals from the fully grown foetus and the placenta trigger a foetal ejection reflex: this stimulates the release of oxytocin from the mother's posterior pituitary, which causes strong contractions of the uterine myometrium. These contractions in turn stimulate more oxytocin release — a positive feedback loop — building stronger and stronger contractions until the baby is delivered, followed by the placenta. The hormone relaxin helps by softening the pelvic ligaments.

After birth comes lactation, the production of milk by the mammary glands, under the influence of prolactin. The first secretion produced in the days just after delivery is colostrum — a yellowish fluid rich in antibodies (especially IgA) that gives the newborn vital early immunity (passive immunity).

This makes breast-feeding in the first days especially important. For NEET, fix the three germ layers and what they form, the approximate gestation period (~9 months) with its key milestones, the role of oxytocin and positive feedback in parturition, and the importance of colostrum (antibody-rich first milk) for the newborn.