Question
Explain the hormonal control of the menstrual cycle, describing the interplay between FSH, LH, estrogen, and progesterone through all four phases.
Solution — Step by Step
flowchart TD
A[Hypothalamus - GnRH] --> B[Anterior Pituitary]
B -->|FSH| C[Follicle Development]
C -->|Estrogen rises| D[Endometrium rebuilds]
D -->|High Estrogen| E[LH Surge - Positive Feedback]
E -->|Ovulation Day 14| F[Corpus Luteum Forms]
F -->|Progesterone + Estrogen| G[Endometrium maintained]
G -->|No fertilization| H[Corpus Luteum degenerates]
H -->|Progesterone drops| I[Menstruation - Day 1]
I -->|Low hormones| A
G -->|Negative Feedback| BLow levels of estrogen and progesterone (because the corpus luteum from the previous cycle has degenerated). Without progesterone support, the endometrial lining breaks down and is shed as menstrual flow. The low hormone levels release the hypothalamus from negative feedback, so GnRH secretion resumes, stimulating FSH release.
Rising FSH stimulates several ovarian follicles to develop. The growing follicles secrete increasing amounts of estrogen. Estrogen rebuilds the endometrium (proliferative phase). At moderate levels, estrogen exerts negative feedback on FSH and LH, preventing premature ovulation. One dominant follicle (Graafian follicle) emerges.
When estrogen from the dominant follicle reaches a critical high threshold, it switches from negative to positive feedback on the pituitary, triggering a sharp LH surge (and a smaller FSH surge). This LH surge causes the Graafian follicle to rupture, releasing the secondary oocyte — this is ovulation.
The ruptured follicle transforms into the corpus luteum, which secretes high levels of progesterone (and some estrogen). Progesterone makes the endometrium thick, glandular, and rich in blood vessels (secretory phase) — preparing it for implantation. High progesterone and estrogen exert negative feedback on GnRH, FSH, and LH, preventing new follicle development.
If no fertilization occurs, the corpus luteum degenerates (becomes corpus albicans) around day 24-28. Progesterone and estrogen levels plummet. Without progesterone, the endometrium cannot be maintained — menstruation begins. The drop in hormones releases the hypothalamus from inhibition, and a new cycle starts. If fertilization occurs, the embryo secretes hCG, which maintains the corpus luteum and its progesterone production.
Why This Works
The menstrual cycle is a precisely orchestrated hormonal feedback loop. The key insight is that estrogen has a dual role: at low-moderate levels it inhibits LH (negative feedback), but at sustained high levels it stimulates LH (positive feedback). This switch triggers ovulation at exactly the right time.
Common Mistake
The trickiest concept: estrogen causes BOTH negative and positive feedback on LH, depending on concentration and duration. Low estrogen = negative feedback (suppresses LH). High sustained estrogen from a mature Graafian follicle = positive feedback (triggers LH surge). Students who write “estrogen always inhibits LH” will lose marks.