Human Reproduction

The male reproductive system comprises paired testes located in the scrotum (2-3 degrees C below body temperature for optimal spermatogenesis), accessory ducts, glands, and the penis. Each testis contains approximately 250 compartments, each with one to three highly coiled seminiferous tubules where sperm production occurs. Seminiferous tubules are lined with two critical cell types: Sertoli cells (sustentacular cells) that provide nourishment, support, and regulation to developing sperm, and spermatogonia (germ cells) at various stages of maturation. In the interstitial spaces between seminiferous tubules, Leydig cells (interstitial cells) synthesize and secrete testosterone, the primary male sex hormone. Accessory glands include the seminal vesicles (fructose-rich secretion providing energy to sperm), prostate gland (alkaline secretion neutralizing vaginal acidity), and bulbourethral glands (Cowper's glands, lubricating secretion).

Spermatogenesis follows a defined pathway: spermatogonia (2n) undergo mitotic proliferation, then primary spermatocytes (2n) undergo meiosis I to form secondary spermatocytes (n), which complete meiosis II to produce spermatids (n). Spermatids differentiate into spermatozoa through spermiogenesis, taking approximately 64 days. The mature sperm has three regions: the head containing the acrosome (lysosomal enzymes for zona pellucida penetration) and the haploid nucleus; the middle piece packed with mitochondria in a spiral arrangement providing ATP for motility; and the tail (flagellum) for propulsion.

The female reproductive system includes paired ovaries, fallopian tubes (oviducts), uterus, and vagina. The fallopian tube has four regions: fimbriae (finger-like projections collecting the ovulated oocyte), infundibulum, ampulla (where fertilization occurs), and isthmus (connecting to the uterus). The uterus has three layers: perimetrium (outer), myometrium (thick muscular middle layer for contractions), and endometrium (inner glandular lining that undergoes cyclic changes).

Oogenesis begins during fetal development: oogonia (2n) multiply and enter meiosis I to become primary oocytes, which arrest at prophase I. At birth, approximately two million primary oocytes are present, declining to 60,000-80,000 by puberty. Each menstrual cycle, one primary oocyte resumes meiosis I, producing a large secondary oocyte and a small first polar body. The secondary oocyte arrests at metaphase II and is ovulated. Meiosis II is completed only upon sperm penetration.

The menstrual cycle spans approximately 28 days and is regulated by the hypothalamus-pituitary-ovarian axis. The menstrual phase (days 1-5) involves shedding of the endometrial lining. During the follicular/proliferative phase (days 6-13), FSH from the anterior pituitary stimulates follicle development and estrogen secretion, which rebuilds the endometrium. Ovulation occurs around day 14, triggered by a surge in LH (not FSH). During the luteal/secretory phase (days 15-28), the ruptured follicle transforms into the corpus luteum, which secretes progesterone to maintain the thickened, glandular endometrium. If fertilization does not occur, the corpus luteum degenerates, progesterone drops, and menstruation begins.

Fertilization occurs in the ampulla of the fallopian tube. The sperm undergoes the acrosomal reaction, releasing enzymes to penetrate the zona pellucida. Upon sperm entry, the cortical reaction occurs -- cortical granules release their contents, modifying the zona pellucida to block polyspermy. The fertilized egg (zygote) undergoes cleavage divisions as it travels through the oviduct, forming a morula (16-cell stage) and then a blastocyst. The blastocyst, consisting of an outer trophoblast (forms placenta) and an inner cell mass (forms embryo), implants into the endometrium around day 6-7 after fertilization. The trophoblast secretes human chorionic gonadotropin (hCG), the hormone detected in pregnancy tests, which maintains the corpus luteum.

Pregnancy lasts approximately 266 days (38 weeks from fertilization, or 40 weeks from last menstrual period). The placenta serves as the structural and functional connection between the mother and fetus, providing nutrition, oxygen exchange, waste removal, and hormone secretion (hCG, hPL, estrogen, progesterone). Parturition (childbirth) is initiated by signals from the fully developed fetus, triggering release of oxytocin from the posterior pituitary, which stimulates uterine contractions. This operates as a positive feedback loop: contractions stimulate more oxytocin release, causing stronger contractions. After delivery, the hormone prolactin stimulates milk production (lactation), while oxytocin triggers the milk ejection (let-down) reflex. The key testable concept is that oogenesis begins in fetal life with primary oocytes arrested at prophase I, the secondary oocyte is arrested at metaphase II and completes meiosis only upon sperm entry, and fertilization occurs in the ampulla of the fallopian tube (not the uterus).