Reproductive System
The reproductive system encompasses organs, ducts, glands, and the HPG axis governing gametogenesis, fertilisation, gestation, and parturition. Its hormonal products — oestrogens, androgens, progestins — exert pervasive effects on bone (OPG/RANKL axis), the cardiovascular system (oestrogenic atheroprotection via eNOS and LDL-R), the immune system (sex-dimorphic autoimmunity), and the CNS (neuroprotection, mood, thermoregulation, libido). Both sexes use the same architecture (GnRH → LH/FSH → gonadal steroids) with sex-specific feedback dynamics, gametogenic rates, and anatomical implementations.
Overview
The reproductive system is unique in two ways. First, it is the only system whose primary function is explicitly interindividual rather than individual survival — its products govern the continuation of the species. Second, it is the only system with distinct male and female anatomical configurations converging on a shared endocrine regulatory architecture.
The systemic effects of sex steroids extend far beyond reproduction. Oestradiol is atheroprotective (↑eNOS → ↑NO → vasodilation, ↑LDL receptor, ↑HDL-C, ↓ICAM-1/VCAM-1 → ↓monocyte adhesion). Its loss at menopause explains the equalisation of male-female cardiovascular risk post-menopause. Testosterone drives erythropoiesis (↑EPO → ↑haematocrit — explaining normal sex differences in haemoglobin). Both steroids are essential for bone density via the OPG/RANKL axis. Understanding these systemic effects is prerequisite to understanding ageing, HRT decisions, androgen deprivation therapy toxicity, and oral contraceptive risks.
HPG Axis — Shared Architecture
KISSPEPTIN NEURONS (hypothalamus)
├── Arcuate nucleus: pulse generator (both sexes)
└── AVPV nucleus: LH surge generator (females only)
KISS1 → Kp-10/Kp-13/Kp-54 → GPR54 on GnRH neurons
Suppressed in childhood by MKRN3 (maternally imprinted)
→ puberty onset when MKRN3 falls + kisspeptin rises
│
▼
GnRH (decapeptide, arcuate + preoptic neurons)
→ pulsatile release into hypophyseal portal blood
→ anterior pituitary gonadotrophs
(Pulse ~60–120 min males; varies by cycle phase females)
│
▼
LH → Leydig cells (males): testosterone synthesis
(CYP11A1 → pregnenolone → CYP17A1 → DHEA →
androstenedione → 17β-HSD3 → testosterone)
LH → theca cells (females): androstenedione
granulosa corpus luteum: progesterone (luteal phase)
FSH → Sertoli cells (males): spermatogenesis support
FSH → granulosa cells (females): aromatase → oestradiol
│ Negative feedback
└─ Testosterone/oestradiol + inhibin B → suppress
GnRH pulse frequency + LH/FSH amplitude
│ Positive feedback (females ONLY)
└─ E2 >200 pg/mL for >36h → AVPV kisspeptin →
GnRH surge → 10× LH surge → ovulation (day ~14)
Key Components
| Structure | Key biology |
|---|---|
| Testes | Seminiferous tubules (~250 m/testis): spermatogonial stem cells → spermatogenesis (74 days, ~1,500 spermatozoa/s). Blood-testis barrier (Sertoli cell tight junctions: claudin-11, occludin). Leydig cells between tubules: LH → testosterone synthesis (~7 mg/day) |
| Epididymis | ~6 m coiled; 12–21 days transit; sperm acquire progressive motility (CatSper Ca²⁺ channel, CRISP proteins); storage in cauda |
| Seminal vesicles | 70% of ejaculate volume: fructose (sperm carbon source), prostaglandins, semenogelin I/II (coagulation) |
| Prostate | 30% of ejaculate volume: PSA (kallikrein-3 → liquefies semenogelin), zinc (bacteriostatic), citric acid. DHT-driven growth (5α-reductase → DHT → AR → EGF/IGF-1/FGF-7) |
| Ovaries | ~400,000 primordial follicles at puberty; ~400 ovulate lifetime; folliculogenesis under FSH + LH; ovarian cycle: follicular phase → ovulation (LH surge) → luteal phase (corpus luteum → progesterone) |
| Uterus | Endometrium: proliferative (E2) → secretory (P4) → menstruation (P4/E2 withdrawal → PGF2α → spiral artery vasoconstriction → shedding). Myometrium: quiescent (P4 ↓gap junctions); parturition: ↑oxytocin receptors + PGE2/PGF2α |
| Fallopian tubes | Ciliated columnar epithelium + peristaltic smooth muscle; fertilisation in ampulla; cilia frequency modulated by E2 (↑)/P4 (↓); ectopic pregnancy if transport fails (tubal PID damage) |
Oestrogen — Systemic Actions
Oestradiol (E2) binds ERα (ESR1) and ERβ (ESR2) — nuclear receptors dimerising on EREs; also rapid non-genomic signalling via membrane-associated ERα/GPER. The breadth of oestrogen action explains both its atheroprotective role and its contribution to breast cancer risk and thrombosis:
| System | Oestrogen effect |
|---|---|
| Bone | ↑OPG → ↓RANKL → ↓osteoclastogenesis; ↑type I collagen. Deficiency → postmenopausal osteoporosis (rapid bone loss in first 5 years after menopause) |
| Cardiovascular | ↑eNOS → NO → vasodilation; ↑LDL-R → ↓LDL-C; ↑ApoA1 → ↑HDL-C; ↓ICAM-1/VCAM-1 → anti-atherogenic; coronary vasodilation. Loss at menopause → CVD risk equalises with males. |
| CNS | Neuroprotection (↑BDNF, ↑BCL-2); ↑serotonin synthesis/reuptake inhibition → mood; VMH thermostat disruption at menopause → vasomotor symptoms (hot flushes); memory, executive function |
| Immune | ↑Th1/Th17, ↑B cell survival, ↑autoantibody → female autoimmune predominance (SLE, RA, MS ~3:1 F:M); ↑IFN-γ production; ↑NK cytotoxicity |
| Coagulation | ↑FVII, FVIII, fibrinogen, vWF; ↓protein S → net pro-thrombotic. Pharmacological doses (OCP/HRT) → 3–5× ↑VTE risk |
| Liver | ↑SHBG (↓free androgens), ↑angiotensinogen (→ ↑BP on OCP), ↑TBG (↑T4 requirement in hypothyroid women on OCP) |
Regulatory Mechanisms
Testosterone metabolism
5α-reductase (SRD5A1/2; skin, prostate, liver) → DHT (2–3× higher AR affinity → dominant in prostate, skin/scalp → androgenetic alopecia). Aromatase (CYP19A1; adipose, brain, bone) → oestradiol (in males: essential for negative feedback on HPG, bone density, libido; in females: follicular oestradiol).
Menstrual cycle dynamics
28-day cycle: follicular phase (FSH → dominant follicle → ↑E2 → endometrial proliferation) → ovulation (E2 >200 pg/mL → LH surge) → luteal phase (CL → P4 peak ~20 ng/mL day 21 → secretory endometrium) → luteolysis → ↓P4/E2 → PGF2α → menses. Implantation: hCG from trophoblast rescues CL → sustains P4.
Pathology
Polycystic Ovary Syndrome (PCOS)
Most common endocrine disorder in reproductive-age females (~10–15%). Rotterdam criteria (2 of 3): oligo/anovulation, clinical/biochemical hyperandrogenism, PCOM on ultrasound. Pathophysiology: ↑GnRH pulse frequency → ↑LH:FSH → ↑theca androgen (androstenedione, T) → partial aromatisation → anovulation. Insulin resistance (70%) → ↑insulin → ↑theca LHR sensitivity → further ↑androgens. Treatment: OCP (↑SHBG → ↓free androgens), metformin (↓IR → ↓androgen drive), clomiphene/letrozole (ovulation induction), weight loss, spironolactone (AR antagonism → ↓hirsutism).
Endometriosis
Ectopic endometrial glands and stroma (ovaries, peritoneum, uterosacral ligaments). Oestrogen-dependent: lesions express aromatase → local E2 production. PGE2 → ↑aromatase (vicious cycle). Dysmenorrhoea, dyspareunia, chronic pelvic pain, infertility (30–50% of infertile women). Diagnosis: laparoscopy (gold standard). Treatment: NSAIDs (↓PGF2α + ↓aromatase), combined OCP, progestins (dienogest), GnRH agonists (leuprolide → medical menopause + add-back HRT), GnRH antagonists (elagolix), surgery (laparoscopic excision/ablation).
Prostate Cancer (PCa)
Most common non-skin cancer in males. Androgen-dependent: AR + TMPRSS2:ERG fusion (AR-driven promoter → ERG oncogene), PTEN loss → ↑PI3K/AKT. Localised: active surveillance (low-risk), RP, or RT. Metastatic hormone-sensitive (mCSPC): ADT (GnRH agonist/antagonist) + abiraterone (CYP17A1 inhibitor) or enzalutamide/apalutamide (AR antagonists). Castration-resistant (mCRPC): abiraterone, enzalutamide, taxanes (docetaxel/cabazitaxel), PARP inhibitors (olaparib for BRCA1/2), ¹⁷⁷Lu-PSMA-617 (radioligand therapy).
Benign Prostatic Hyperplasia (BPH)
DHT (5α-reductase → AR → ↑EGF/IGF-1/FGF-7) drives periurethral transition zone proliferation. Affects >50% of men >60 years. LUTS: obstructive (poor stream, hesitancy) + irritative (urgency, frequency). Treatment: α1-blockers (tamsulosin → prostate smooth muscle relaxation), 5α-reductase inhibitors (finasteride/dutasteride → ↓DHT → gland shrinkage), combination superior; surgery (TURP, HoLEP) for refractory disease.
Turner Syndrome (45,X0) and Klinefelter Syndrome (47,XXY)
Turner: X monosomy → streak gonads → oestrogen deficiency → ↑FSH/LH (hypergonadotropic hypogonadism) → absent puberty without HRT; short stature (SHOX haploinsufficiency); coarctation of aorta (15%), bicuspid aortic valve (30%). Treatment: oestrogen + progesterone replacement; GH therapy. Klinefelter: most common sex chromosome aneuploidy (~1:550 males); supernumerary X → progressive seminiferous tubule fibrosis → azoospermia; Leydig cells partially preserved → low-normal testosterone + ↑LH (compensated hypogonadism) + gynaecomastia. Treatment: testosterone replacement; fertility via TESE + ICSI possible even with Klinefelter.
Ovarian and Cervical Cancer
High-grade serous ovarian carcinoma (HGSOC, ~70%): arises from fallopian tube fimbriae (STIC precursor); TP53 mutation universal; BRCA1/2 mutations ~15% hereditary; HRD tumours respond to PARP inhibitors (olaparib, niraparib) + carboplatin/paclitaxel + bevacizumab. Cervical cancer: HPV-16 (SCC) + HPV-18 (adenocarcinoma) responsible for ~70%; E6 degrades p53; E7 degrades pRb → uncontrolled cell cycle. Prevention: Gardasil-9 (nonavalent VLP vaccine) >90% efficacy against targeted types. Screening: cervical cytology + HPV co-testing.
Cross-Atlas Connections
References
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021. elsevier.com
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022. NCBI Bookshelf
- Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus Workshop Group. Revised 2003 consensus on diagnostic criteria and long-term health risks related to polycystic ovary syndrome. Fertil Steril. 2004;81(1):19-25. doi:10.1016/j.fertnstert.2003.10.004
- Swerdlow AJ, et al. Mortality in patients with Klinefelter syndrome in Britain. J Clin Endocrinol Metab. 2005;90(12):6516-6522. doi:10.1210/jc.2005-1287
- de Bono JS, et al. Abiraterone and Increased Survival in Metastatic Prostate Cancer. N Engl J Med. 2011;364(21):1995-2005. doi:10.1056/NEJMoa1014618