The hypothalamic-pituitary-gonadal (HPG) axis controls reproduction through pulsatile GnRH secretion, which stimulates LH and FSH release. In females, positive feedback from rising estrogen triggers ovulation, then negative feedback from progesterone maintains the luteal phase. In males, constant hormonal stimulation maintains continuous spermatogenesis. Sex steroids coordinate gonadal function with secondary sexual characteristics and reproductive behavior.
Plot hormone levels across the menstrual cycle and correlate with follicular development and ovulation. Understand why early positive feedback on estrogen production differs from late negative feedback. Compare continuous male hormone patterns with cyclical female patterns.
From your prerequisite on endocrine glands and hormones, you know the general architecture of hormonal feedback: a releasing hormone stimulates a pituitary hormone, which stimulates a target gland, whose product feeds back to suppress the original signal. The HPG axis — hypothalamus → pituitary → gonads — follows this template, but with a crucial twist in females: midcycle, the feedback switches sign from negative to positive, and this reversal is the trigger for ovulation. Understanding the female cycle means tracking when and why that switch occurs.
The female cycle unfolds as a hormonal story driven by pulsatile GnRH released from the hypothalamus approximately every 90 minutes. These pulses stimulate pituitary release of FSH (follicle-stimulating hormone), which recruits a cohort of follicles in the ovary. The growing follicles produce estradiol, which initially feeds back negatively — suppressing FSH and ensuring that only the dominant follicle, the most FSH-sensitive, survives while the rest undergo atresia. But as the dominant follicle grows and estradiol rises above a threshold sustained for roughly 36 hours, the pituitary's response switches: instead of suppression, estradiol triggers a massive LH surge through positive feedback. This surge causes the dominant follicle to rupture and release the oocyte — ovulation. The ruptured follicle then reorganizes into the corpus luteum, which produces progesterone. Progesterone now enforces negative feedback on both LH and FSH, preventing new follicular development and stabilizing the luteal phase. If no pregnancy occurs (no hCG to maintain it), the corpus luteum degenerates, progesterone and estradiol fall, the endometrium sheds (menstruation), and rising FSH begins the next cycle. The entire cycle is a self-contained oscillation driven by a single sign reversal at the moment of peak estrogen.
The male system omits this oscillatory switch. GnRH pulses drive continuous and relatively stable LH and FSH secretion. LH stimulates Leydig cells in the testes to produce testosterone, which feeds back negatively to suppress LH and GnRH. FSH stimulates Sertoli cells, which support spermatogenesis — a continuous production line generating approximately 100 million sperm per day. Spermatogenesis takes about 74 days from spermatogonial stem cell to mature sperm; Sertoli cells provide nutrients, hormonal signals, and the blood-testis barrier that protects haploid cells from immune attack. The contrast between female cyclicity and male continuity reflects fundamentally different reproductive strategies, yet both are governed by the same three-tier axis. A final asymmetry in gametogenesis itself: females are born with their entire lifetime supply of primary oocytes, arrested in prophase I since before birth, while males continuously produce new spermatogonia from puberty onward. This means female fertility declines with age as oocyte quality deteriorates (chromosomal non-disjunction becomes more frequent), while male gametogenesis is more renewable but takes weeks to respond to hormonal disruption.
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