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Male reproductive anatomy centers on testes (spermatogenesis, testosterone production), epididymis (sperm maturation), vas deferens, and accessory glands (seminal vesicles, prostate, bulbourethral glands) that contribute to semen. Female reproductive anatomy includes ovaries (oogenesis, estrogen and progesterone production), fallopian tubes, uterus, cervix, and vagina. The hypothalamic-pituitary-gonadal (HPG) axis regulates both sexes: GnRH drives FSH and LH release, which regulate gonadal function. The female menstrual cycle integrates follicular phase, ovulation (triggered by the LH surge — a rare positive feedback loop), and luteal phase, with cyclical changes in the uterine endometrium. Fertilization, implantation, and pregnancy involve dramatic hormonal shifts that override the normal cycle.
Graph estrogen, progesterone, LH, and FSH levels across the ~28-day menstrual cycle on the same timeline. Identify the key events (follicular development, ovulation, corpus luteum formation/regression) and link each to the hormonal changes driving them.
From your study of endocrine glands and hormones, you know that the body's regulatory axes follow a hierarchical pattern: a hypothalamic releasing hormone drives pituitary hormone release, which drives a target gland, which feeds back negatively to suppress the hypothalamus and pituitary. The hypothalamic-pituitary-gonadal (HPG) axis follows exactly this logic. GnRH (gonadotropin-releasing hormone), secreted in pulses from hypothalamic neurons, drives the anterior pituitary to release FSH (follicle-stimulating hormone) and LH (luteinizing hormone). These gonadotropins act on the gonads to produce both gametes and sex steroids, which then feed back to suppress GnRH and gonadotropin secretion — negative feedback that keeps the system in steady state most of the time. What makes the female reproductive system remarkable is that this negative feedback is briefly overridden at a critical moment in the cycle.
In the male, the HPG axis runs as a steady-state negative feedback loop. Leydig cells in the testes produce testosterone in response to LH; testosterone feeds back to suppress GnRH and LH. Sertoli cells in the seminiferous tubules support spermatogenesis in response to FSH and locally high testosterone. Sperm produced in the testes are immature and non-motile; they mature and acquire motility during their two-week passage through the epididymis — a coiled tubule along the posterior testis. At ejaculation, sperm travel through the vas deferens and mix with secretions from the seminal vesicles (fructose for energy), prostate (alkaline fluid that neutralizes vaginal acidity), and bulbourethral glands (pre-ejaculatory mucus). Testosterone has effects far beyond the testes: it drives muscle growth, bone density, libido, and secondary sexual characteristics, all mediated by androgen receptors expressed throughout the body.
In the female, the HPG axis operates cyclically. During the follicular phase, rising FSH recruits a cohort of primordial follicles. One dominant follicle emerges, secreting increasing amounts of estrogen, which initially exerts negative feedback — suppressing FSH to prevent further follicle recruitment. As ovulation approaches, estrogen rises above a threshold (sustained at ~200 pg/mL for roughly 36 hours), and the pituitary response *switches*. Using your understanding of positive feedback mechanisms: the same signal that was being suppressed now amplifies its own production, triggering the LH surge — a massive spike in LH that induces rupture of the dominant follicle and release of the secondary oocyte. This switch is mediated by estrogen acting on different pituitary cell populations and is a genuinely rare positive feedback loop in physiology.
After ovulation, the ruptured follicle becomes the corpus luteum, which secretes both progesterone and estrogen during the luteal phase. Progesterone prepares the uterine endometrium for implantation: it thickens the lining, increases vascularization, promotes secretory gland development, and suppresses uterine contractions. If fertilization does not occur, the corpus luteum regresses after approximately 14 days, progesterone and estrogen fall, the endometrium sheds (menstruation), and the next cycle begins. If implantation occurs, the embryo secretes hCG (human chorionic gonadotropin), which acts like LH to rescue the corpus luteum, maintaining progesterone production until the placenta takes over at around week 10. This is why a positive pregnancy test — which detects hCG — confirms successful implantation: the embryo is actively signaling its presence to prevent menstruation.