16.1 Male & Female Reproductive Physiology
Key Takeaways
- Hypothalamic GnRH pulses drive pituitary FSH/LH; continuous GnRH down-regulates gonadotrophs. In males LH → Leydig testosterone; FSH → Sertoli support of spermatogenesis. In females FSH drives early follicular growth; midcycle LH surge triggers ovulation.
- Spermatogenesis: spermatogonia → primary spermatocyte (meiosis I) → secondary spermatocyte (meiosis II) → spermatids → spermatozoa (spermiogenesis); full cycle ≈74 days; temperature-sensitive (cryptorchidism impairs it).
- Testosterone is converted by 5α-reductase to DHT (prostate, external genitalia, male hair pattern) and by aromatase to estradiol (bone closure, feedback, libido contributions).
- Menstrual cycle: follicular (FSH-dominant, rising estradiol, proliferative endometrium) → ovulation (LH surge after sustained high estradiol) → luteal (progesterone-dominant, secretory endometrium). Estrogen builds endometrium; progesterone stabilizes and prepares it.
- Menopause: ovarian follicle depletion → low estrogen/inhibin → high FSH (and LH). Puberty follows reactivation of GnRH pulse generator with Tanner staging of secondary sexual characteristics.
16.1 Male & Female Reproductive Physiology
Quick Answer: Pulsatile GnRH → FSH/LH. Male: LH–Leydig–testosterone; FSH–Sertoli–spermatogenesis; DHT via 5α-reductase; estradiol via aromatase. Female cycle: follicular (FSH/E2) → LH surge/ovulation → luteal (progesterone). Estrogen proliferates endometrium; progesterone secretes/stabilizes it. Menopause = high FSH from low inhibin/estrogen. Tanner stages track puberty.
Reproductive physiology on the CBSE is mechanism-first: identify the axis node that is stimulated or blocked, convert steroids with the correct enzyme, and place hormone peaks on the menstrual timeline. Pathology of PCOS, moles, and hormone-driven cancers (next sections) only makes sense if this framework is automatic.
Hypothalamic–Pituitary–Gonadal Axes
GnRH is released in pulses from hypothalamic arcuate/preoptic neurons into the portal system. Pulsatile GnRH stimulates anterior pituitary gonadotrophs to secrete FSH and LH. Continuous GnRH (or long-acting agonists after an initial flare) down-regulates GnRH receptors and suppresses gonadotropins—clinically used in central precocious puberty, endometriosis, and prostate cancer protocols.
| Axis node | Male product / role | Female product / role |
|---|---|---|
| Hypothalamus | Pulsatile GnRH | Pulsatile GnRH (frequency/amplitude shift across cycle) |
| Pituitary LH | Stimulates Leydig cells → testosterone | Midcycle LH surge → ovulation; supports theca androgen production |
| Pituitary FSH | Stimulates Sertoli cells → spermatogenesis support, inhibin B, ABP | Early follicular growth; granulosa aromatase (androgen → estrogen); inhibin |
| Gonadal feedback | Testosterone + inhibin B suppress GnRH/FSH–LH | Estradiol (biphasic), progesterone, inhibin close the loops |
Negative feedback: Sex steroids and inhibin (from Sertoli or granulosa cells) restrain the axis. Inhibin preferentially suppresses FSH. Activin can stimulate FSH (less tested). At midcycle, sustained high estradiol switches to positive feedback, producing the LH (and smaller FSH) surge—the key exception to pure negative feedback.
Prolactin interface: Hyperprolactinemia suppresses GnRH pulse generation → hypogonadotropic hypogonadism (amenorrhea, infertility, low libido). Dopamine agonists restore GnRH tone by lowering prolactin.
Male Reproductive Physiology
Testicular compartments
- Seminiferous tubules: Sertoli cells + germ cells; site of spermatogenesis; blood–testis barrier created by Sertoli tight junctions.
- Interstitium: Leydig cells produce testosterone under LH.
Sertoli functions (high-yield list): nurture germ cells; secrete androgen-binding protein (ABP) concentrating androgens in tubules; produce inhibin B; convert testosterone → estradiol via aromatase; form the blood–testis barrier; phagocytose residual bodies during spermiogenesis.
Spermatogenesis stages
Spermatogenesis is continuous after puberty and highly temperature-sensitive (scrotal temperature a few degrees below core).
- Spermatogonia (diploid stem cells) → mitotic renewal and entry into meiosis.
- Primary spermatocytes (4N DNA after S phase) complete meiosis I → secondary spermatocytes.
- Secondary spermatocytes complete meiosis II → spermatids (haploid).
- Spermiogenesis: spermatids remodel into spermatozoa (acrosome from Golgi, flagellum, residual cytoplasm shed).
- Epididymal maturation confers motility and fertilizing capacity; storage mainly in epididymis.
Full cycle from spermatogonium to mature sperm is on the order of ~74 days; transit and epididymal maturation add further time. Cryptorchidism, varicocele heat effects, and drugs (e.g., alkylators, antiandrogens) impair sperm production.
Testosterone, DHT, and aromatase
Testosterone is the principal Leydig product. Peripheral conversion:
| Enzyme | Reaction | Key tissues / roles |
|---|---|---|
| 5α-reductase | Testosterone → DHT | Prostate growth, male external genitalia differentiation, male-pattern hair, acne sebaceous activity |
| Aromatase (CYP19) | Androgens → estradiol | Bone epiphyseal closure, part of male feedback and libido/bone health; granulosa cells in ovary |
Clinical enzyme pearls: 5α-reductase inhibitors (finasteride, dutasteride) shrink DHT-dependent prostate tissue and reduce male-pattern hair loss but can affect sexual function. 5α-reductase deficiency: XY infant with ambiguous or female-appearing external genitalia at birth, virilization at puberty ("penis at 12"), no breast development, internal male ducts present (because testosterone, not DHT, drives Wolffian structures). Aromatase deficiency / estrogen receptor defects in males: tall stature with delayed epiphyseal closure, osteopenia—showing estradiol is essential for male bone maturation.
Male accessory glands:
- Seminal vesicles: fructose-rich alkaline fluid (~majority of semen volume); prostaglandins.
- Prostate: zinc-containing, slightly acidic fluid with PSA and enzymes that liquefy semen.
- Bulbourethral (Cowper) glands: pre-ejaculatory mucus lubrication.
Emission and ejaculation integrate sympathetic (emission, internal urethral sphincter) and somatic (bulbospongiosus) pathways; parasympathetic drives erection (NO → cGMP). Phosphodiesterase-5 inhibitors prolong cGMP for erectile dysfunction—classic Step 1 linkage.
Female Reproductive Physiology
Ovarian follicle development
Primordial follicles (oocyte arrested in dictyotene of meiosis I since fetal life) are recruited under FSH. Granulosa cells proliferate; theca cells acquire LH receptors and produce androgens; granulosa aromatase converts thecal androgens to estradiol (two-cell, two-gonadotropin model). A dominant follicle emerges; others undergo atresia. At ovulation, the oocyte completes meiosis I, extrudes the first polar body, and arrests in metaphase of meiosis II until fertilization.
Corpus luteum forms from the ruptured follicle: granulosa-lutein and theca-lutein cells secrete progesterone (and some estrogen). Without hCG (pregnancy), the corpus luteum regresses after ~14 days → menses. With pregnancy, hCG rescues the corpus luteum until the placenta takes over progesterone production.
Menstrual cycle phases and hormone peaks
| Phase | Days (idealized 28-d) | Dominant hormone pattern | Ovary | Endometrium |
|---|---|---|---|---|
| Follicular / proliferative | ~1–13 | Rising FSH early; rising estradiol; LH low until late | Follicle growth, dominant selection | Estrogen-driven proliferation; glands straight |
| Ovulation | ~14 | LH surge (triggered by sustained high E2); smaller FSH peak | Follicle rupture; oocyte to metaphase II | Transition |
| Luteal / secretory | ~15–28 | High progesterone (+ some E2); FSH/LH suppressed | Corpus luteum | Secretory glands, spiral arteries, edema; prepared for implantation |
| Menses | 1–5 of next cycle | Steroid withdrawal if no pregnancy | Corpus albicans | Ischemia/slough of functionalis |
Timeline pearls for questions:
- Peak estradiol is late follicular, just before the LH surge.
- Peak LH is midcycle (ovulation trigger).
- Peak progesterone is mid-luteal.
- Basal body temperature rises ~0.5 °C after ovulation (progesterone effect).
- Cervical mucus becomes thin, watery, and stretchy (spinnbarkeit) under high estrogen near ovulation; thickens under progesterone.
Estrogen and progesterone actions
Estrogens (mainly estradiol): breast duct development; endometrial proliferation; female fat distribution; increased hepatic SHBG and clotting factor synthesis (thrombosis risk with pharmacologic estrogen); bone protection (anti-resorptive tone); feedback on GnRH/gonadotropins (negative most of cycle; positive at midcycle); up-regulation of uterine oxytocin receptors near term (with other factors).
Progesterone: endometrial secretory conversion and stabilization; thick cervical mucus; raises body temperature; smooth-muscle relaxation (ureters, GI—pregnancy symptoms); breast lobuloalveolar development (with estrogen/prolactin); maintains pregnancy; decreases uterine excitability relative to estrogen-dominant states.
Combined oral contraceptives exploit constant negative feedback: suppress LH surge (block ovulation), thicken mucus, and thin endometrium. Progestin-only methods emphasize mucus and endometrial effects and variable ovulation suppression.
Puberty and Tanner Physiology
Puberty begins with CNS disinhibition of the GnRH pulse generator (kisspeptin pathways are high-yield molecular context). Sequence is relatively stereotyped:
Females (typical order): thelarche (breast budding, Tanner B2) → peak height velocity → menarche. Males: testicular enlargement (volume ≥4 mL, Tanner G2) is the first sign → penile growth → peak height velocity later than in girls.
Tanner staging scores breast (B1–B5), genital (G1–G5), and pubic hair (P1–P5). Exam items may give a Tanner description and ask which hormone axis is active or whether precocious puberty is gonadotropin-dependent (central) vs independent (peripheral steroid source).
Adrenarche (DHEA-S rise, pubic/axillary hair, acne) is adrenal and can dissociate from gonadarche.
Menopause
Ovarian follicular reserve depletes → inhibin and estradiol fall → loss of negative feedback → FSH rises markedly (LH also rises; FSH is the classic laboratory marker). Symptoms: vasomotor instability (hot flashes), vaginal atrophy, accelerated bone loss, possible mood/sleep changes. Unopposed estrogen (no progesterone) in anovulatory peri/postmenopause or obesity-related aromatization increases risk of endometrial hyperplasia/carcinoma—bridge to section 16.3.
Premature ovarian insufficiency presents similarly with high FSH at younger age; distinguish from hypogonadotropic causes (low/normal FSH).
Fertilization Basics
Capacitated sperm penetrate corona radiata and zona pellucida (acrosome reaction). Fusion triggers cortical reaction (polyspermy block) and completion of meiosis II by the oocyte (second polar body). Pronuclei fuse → zygote. Cleavage → morula → blastocyst; implantation around day 6–7 with trophoblast invasion and hCG production (pregnancy tests). Ectopic risk rises when tubal transit is impaired (PID scarring)—pathophysiology in 16.3.
Integration checklist for CBSE stems: name the deficient cell (Leydig vs Sertoli; theca vs granulosa); state whether feedback is negative or midcycle-positive; convert T→DHT or T→E2 with the right enzyme; place the patient on the cycle day by which hormone peaks; interpret high FSH as primary gonadal failure until proven otherwise.
A 28-year-old man has azoospermia with normal virilization and normal serum testosterone. LH is normal; FSH is elevated. Which cell type is most likely primarily impaired?
Which hormonal pattern best matches the mid-luteal phase of a normal ovulatory cycle?
An XY newborn has ambiguous external genitalia, normal testes, and no uterus. At puberty, virilization occurs without breast development. Which enzyme deficiency best explains this pattern?