Describe spermatogenesis, semen, follicle development, oogenesis, menstrual cycle, lactation, puberty, ageing, sexual response and normal vaginal discharge.

Reproductive Tract Physiology

Clinical Overview

Start with one organising idea: the female reproductive tract is a system built to bring one mature egg and a receptive lining together at the same moment, roughly once a month, and to do so reliably for thirty to forty years. Everything else in this chapter is a servant of that single goal. The brain sets the rhythm, the ovary produces the egg and the hormones, and the genital tract (endometrium, cervix, vagina and tube) turns those hormones into the physical conditions for conception. When conception does not happen, the lining is shed and the whole sequence restarts. Hold that picture, and the detail below stops being a list to memorise and becomes a chain of cause and effect.

From that first principle, the machinery follows in order. The hypothalamus generates pulsatile GnRH; the pituitary releases FSH and LH; the ovary recruits follicles, selects a dominant follicle, ovulates and forms a corpus luteum; the endometrium proliferates, differentiates, bleeds and repairs; the cervix changes its mucus and collagen; the vagina changes its epithelial thickness, glycogen and microbial ecology; and the tubes capture the oocyte, support fertilisation and move the early embryo to the uterus. The clinical mistake is to treat the menstrual cycle as a calendar. It is a sequence of linked physiological events, and the calendar is only a rough surface marker of what the hormones are doing underneath.

Primary FCOG candidates need this chapter because most gynaecology stems are disguised physiology stems. Adolescent heavy bleeding is often immaturity of the hypothalamic-pituitary-ovarian axis, but pregnancy and bleeding disorders must still be excluded. Primary dysmenorrhoea is prostaglandin-rich ovulatory menstruation, not infection. Subfertility after pelvic inflammatory disease is tubal transport failure. Endometrial hyperplasia risk is unopposed oestrogen without progesterone opposition. Menopause symptoms are the systemic consequences of follicle depletion, not a disease in itself.

Mechanism-to-clinical-consequence chains are high-yield:

Low energy availability or severe stress -> reduced GnRH pulsatility -> low LH/FSH stimulation -> anovulation -> amenorrhoea or irregular bleeding.

Persistent anovulation -> no corpus luteum -> low progesterone exposure -> unstable proliferative endometrium -> irregular heavy bleeding and hyperplasia risk.

Chlamydial salpingitis -> epithelial injury and scarring -> impaired ciliary transport -> tubal infertility and ectopic pregnancy risk.

Follicle depletion -> low inhibin B and oestradiol -> high FSH -> vasomotor symptoms, vaginal atrophy and bone loss.

Core Knowledge

Puberty and HPO Axis Activation

Puberty begins when childhood restraint of the hypothalamic-pituitary-ovarian axis is released. The hypothalamus does not simply "turn on". It increases pulsatile GnRH secretion, initially most clearly at night. GnRH pulse frequency and amplitude determine pituitary gonadotrophin output. Faster pulses favour LH secretion; slower pulses favour FSH secretion. Continuous GnRH exposure downregulates the pituitary, which is why GnRH agonists can suppress puberty or ovarian function after an initial flare.

The central gatekeepers include kisspeptin, neurokinin B, dynorphin, leptin, nutrition, sleep, stress pathways and genetic factors. A girl with adequate energy stores, normal hypothalamic signalling and responsive ovaries develops rising gonadotrophin activity. A girl with severe undernutrition, chronic illness, excessive exercise or major psychological stress may have delayed or interrupted cycles because the reproductive axis is energy-sensitive.

Clinically, this sequence is documented with Tanner staging (sexual maturity rating), which scores breast development and pubic hair separately from stage 1 (prepubertal) to stage 5 (adult). Breast staging tracks the oestrogen-driven thelarche; pubic-hair staging tracks the adrenal/ovarian androgen-driven pubarche. Because the two are driven by different hormones, they can advance out of step, and a mismatch (for example, pubic hair well ahead of breast development) is a useful pointer toward an androgen problem rather than ordinary central puberty. Knowing the normal order also tells you when puberty is genuinely abnormal: secondary sexual characteristics before about 8 years of age, or absent breast development by about 13 years with no menarche by about 15 years, both warrant evaluation rather than reassurance.

The adolescent cycle is therefore not expected to be immediately adult-like. In the first years after menarche, many cycles are anovulatory because positive oestrogen feedback and luteal progesterone production are immature. This explains irregularity and heavy bleeding, but it does not make every adolescent bleed benign. In a South African exam stem, always ask: pregnancy? sexual assault? anaemia? coagulopathy? endocrine disease? infection? structural lesion? medication?

Hypothalamic-Pituitary-Ovarian Signalling

The reproductive axis is a feedback loop.

Oestrogen usually exerts negative feedback on FSH and LH. Sustained high oestradiol from a mature follicle reverses this to positive feedback, producing the LH surge. Progesterone after ovulation slows GnRH pulses and stabilises the endometrium. Inhibin B from granulosa cells suppresses FSH in the follicular phase; inhibin A contributes in the luteal phase. Activin promotes FSH synthesis locally, while AMH reflects small growing follicles and restrains early follicular recruitment.