Humans reproduce sexually, with distinct male and female reproductive systems that produce gametes, facilitate fertilisation, and support fetal development. Understanding anatomy and physiology of both systems is central to this chapter.
Male Reproductive System
Key organs: testes (in scrotum, 2-3 degrees below body temperature for spermatogenesis), epididymis (storage and maturation of sperm), vas deferens, seminal vesicles (60% of semen volume), prostate gland (alkaline secretions), bulbourethral glands (Cowper's glands), and penis. Semen = sperm + secretions of accessory glands.
Spermatogenesis
- 1.Formation of spermatozoa in the seminiferous tubules:
- 2.Spermatogonia (2n) proliferate by mitosis
- 3.Primary spermatocytes (2n) undergo meiosis I → two secondary spermatocytes (n)
- 4.Meiosis II → four spermatids (n)
- 5.Spermatids differentiate (spermiogenesis) into mature spermatozoa
A mature sperm has: head (acrosome + nucleus), middle piece (mitochondria for energy), and tail (flagellum for motility).
Female Reproductive System
Key organs: ovaries (produce eggs and hormones), fallopian tubes (oviducts — site of fertilisation), uterus (site of implantation and fetal development), cervix, and vagina. The uterus has three layers: perimetrium (outer), myometrium (muscular middle), and endometrium (inner — shed during menstruation).
Oogenesis
- 1.Formation of ova (eggs):
- 2.Oogonia proliferate by mitosis (before birth)
- 3.Primary oocytes (2n) enter meiosis I but arrest at prophase I
- 4.At puberty, one primary oocyte completes meiosis I per cycle → secondary oocyte (n) + polar body
- 5.Meiosis II is completed only after fertilisation → ovum + second polar body
Menstrual Cycle (28 days)
- Menstrual phase (Days 1-5): shedding of endometrium
- Follicular/Proliferative phase (Days 6-13): FSH stimulates follicle growth; oestrogen causes endometrium proliferation
- Ovulation (Day 14): LH surge triggers release of secondary oocyte
- Luteal/Secretory phase (Days 15-28): corpus luteum secretes progesterone; endometrium thickens
A woman with a regular 28-day cycle ovulates on day 14. If her cycle is 32 days, ovulation likely occurs around day 18 (ovulation is 14 days before the next period).
After ovulation, the ruptured follicle becomes the corpus luteum which secretes progesterone. If fertilisation does not occur, the corpus luteum degenerates (corpus albicans) and progesterone falls, triggering menstruation.
Fertilisation in humans normally occurs in the ampulla of the fallopian tube — the widest part near the ovary end. Sperm capacitation (activation in female tract) is needed before fertilisation.
The zygote undergoes cleavage as it travels down the fallopian tube. By the time it reaches the uterus (~7 days), it is a blastocyst which implants in the endometrium.
The trophoblast cells of the blastocyst secrete human chorionic gonadotropin (hCG) which maintains the corpus luteum, sustaining progesterone production during early pregnancy. hCG is the basis of pregnancy tests.
The placenta acts as the interface between mother and fetus — it allows exchange of nutrients, oxygen, CO2, and waste, but also acts as a barrier to some harmful substances. It secretes oestrogen, progesterone, and hCG.
Parturition (birth) is triggered by oxytocin (from posterior pituitary) acting on myometrium, causing strong contractions. Prostaglandins also play a role. The process is a positive feedback loop.
Hormonal Control
- FSH (Follicle Stimulating Hormone): from anterior pituitary; stimulates follicle development in females and spermatogenesis in males
- LH (Luteinising Hormone): triggers ovulation; stimulates testosterone production in males (Leydig cells)
- Oestrogen: secreted by growing follicle; causes proliferation of endometrium
- Progesterone: secreted by corpus luteum; maintains endometrium for implantation
- Testosterone: produced by Leydig cells; promotes spermatogenesis and secondary sexual characters
Common mistakes
- Fertilisation occurs in the fallopian tube, NOT the uterus.
- The secondary oocyte completes meiosis II only after sperm penetration, not at ovulation.
- Spermiogenesis is the transformation of spermatids into sperm; spermatogenesis is the entire process.
- Corpus luteum is derived from the ruptured follicle after ovulation, not a separate structure.
Summary
Male and female reproductive systems produce gametes through spermatogenesis and oogenesis. Fertilisation in the fallopian tube produces a zygote that implants as a blastocyst. Hormones (FSH, LH, oestrogen, progesterone) regulate the menstrual cycle and pregnancy.