HUBS3511 · Human Reproduction And Pregnancy
The Menstrual Cycle
The female reproductive system is a feedback loop that runs on a timer. The brain (hypothalamus + pituitary) drives the ovary; the ovary talks back with steroids. For most of the cycle that feedback is negative — it keeps the system damped and stable, exactly like the male axis. But once per cycle a single, deliberate sign-flip to positive feedback fires the LH surge that triggers ovulation. This is the most-examined system in HUBS3511. You must read the four-hormone graph (FSH, LH, oestrogen, progesterone) and say which peaks when; explain the positive-feedback switch (high, sustained oestrogen flips the sign); tie each hormone to its source and action; walk the three ovarian phases (follicular → ovulation → luteal) against the time-aligned uterine phases (menstrual → proliferative → secretory); and say what happens without fertilisation (corpus luteum → albicans → falling progesterone → menses).
What this chapter covers
- 01A. The three nodes of the HPG axis: hypothalamus, pituitary, ovary
- 02A.1 The HPG axis and its negative-feedback default; inhibin → FSH only
- 03GnRH must be pulsatile (continuous GnRH downregulates the pituitary)
- 04B. The pre-ovulatory positive-feedback switch and the LH surge
- 05C. The four-hormone graph and the three ovarian phases
- 06C.1 Walking follicular → ovulation → luteal (and without fertilisation)
- 07C.2 The uterine (endometrial) cycle as the read-out
- 08The hormone source → action lines
Worked example: what triggers the LH surge?
- +1Default state: for most of the cycle, low-to-moderate oestrogen exerts negative feedback on the hypothalamus and pituitary, keeping GnRH/LH suppressed.
- +1Rising signal: as the dominant follicle matures, its granulosa cells secrete ever more oestrogen, so the level rises and is sustained.
- +1The sign-flip: once oestrogen is high AND sustained for ~2–3 days, the same hormone flips to positive feedback on the hypothalamus/pituitary.
- +1The surge: GnRH and LH are now amplified → a sharp LH spike (with a smaller FSH rise).
- +1The result: the LH surge triggers ovulation — the Graafian follicle ruptures and the secondary oocyte (arrested at metaphase II) is released.
Key terms
- HPG axis
- The hypothalamic–pituitary–gonadal axis: a three-tier command chain. Hypothalamus → GnRH → pituitary → FSH/LH → ovary → oestrogen/progesterone/inhibin fed back up.
- Positive-feedback switch
- The once-per-cycle sign-flip in which high, sustained oestrogen stops inhibiting and starts stimulating the hypothalamus/pituitary, producing the LH surge. It is the single most-tested concept in the course.
- LH surge
- The sharp, narrow spike of luteinising hormone at ~day 14 that triggers ovulation, completes oocyte meiosis I and luteinises the ruptured follicle into the corpus luteum.
- Luteal phase
- Days ~15–28 of the cycle, driven by progesterone from the corpus luteum. Its length is near-constant (~14 days); cycle-length variation comes mostly from the follicular phase.
- Inhibin
- A peptide from granulosa cells (and Sertoli cells in males) that selectively suppresses FSH only — not LH. It helps select a single dominant follicle.
The Menstrual Cycle FAQ
Does a fall in oestrogen trigger ovulation?
No — this is the number-one trap of the course. Oestrogen peaks just before the surge: it is high, sustained oestrogen that flips to positive feedback and triggers the LH surge. Low/moderate oestrogen is inhibitory (negative feedback).
Why does inhibin only affect FSH?
Inhibin selectively suppresses FSH at the pituitary; it does not lower LH. Steroids (oestrogen, progesterone) suppress both. Writing “inhibin lowers LH” is the classic distractor.
Why must GnRH be pulsatile?
GnRH works only in pulses. Continuous GnRH paradoxically downregulates the pituitary — which is exactly how GnRH-agonist drugs suppress the axis (for example in IVF). Same molecule, opposite effect depending on the rhythm.
When does progesterone peak, and what ends the cycle?
Progesterone peaks in the luteal phase (~day 21), not at ovulation. The event that ends the cycle is falling progesterone as the corpus luteum dies (becoming the corpus albicans), which causes the spiral arteries to spasm and the functional layer to shed as menses.
Exam move
This chapter is the biggest single block of short-answer marks, so over-invest here. Be able to sketch the four-hormone graph from a blank axis and place every peak: FSH modest-early then a small mid bump; oestrogen climbing to a peak just before the surge; LH a flat line then a sharp day-14 spike; progesterone near-zero then a broad luteal dome at ~day 21. Drill the one positive-feedback switch until you can state it in one sentence (“high + sustained oestrogen flips to positive feedback → LH surge → ovulation”), and keep the negative-feedback default and the inhibin→FSH-only rule separate from it. Then align the ovarian and uterine cycles on the same day axis, and memorise the hormone source→action lines verbatim — examiners want them back word for word.
