Theory: Human Reproduction / Endocrinology
1. Menstrual Cycle Hormonal Events
Follicular phase (days 1-13): FSH rises → follicle development → oestrogen increases. As oestrogen rises: negative feedback (low levels). Late follicular: sustained high oestrogen → switches to POSITIVE feedback → LH surge (day 12-14). Luteal phase (days 15-28): LH → corpus luteum → progesterone + oestrogen. High progesterone: negative feedback on LH/FSH. Day 24-26: corpus luteum degenerates → hormone fall → menstruation (day 28/1).
2. Key Hormones and Their Sources
GnRH (Gonadotrophin Releasing Hormone): hypothalamus → stimulates pituitary. FSH (Follicle Stimulating Hormone): anterior pituitary → follicle development, oestrogen production. LH (Luteinising Hormone): anterior pituitary → ovulation, corpus luteum formation, progesterone production. Oestrogen: Graafian follicle (theca/granulosa cells) → endometrial proliferation, positive feedback for LH surge. Progesterone: corpus luteum → maintains endometrium, prevents new ovulation. hCG: trophoblast of early embryo → maintains corpus luteum in early pregnancy.
3. Ovarian Cycle vs Uterine (Endometrial) Cycle
Ovarian cycle: Follicular phase (FSH-driven follicle maturation) + Ovulation (LH surge) + Luteal phase (corpus luteum). Uterine cycle: Menstruation (days 1-5) + Proliferative phase (oestrogen rebuilds endometrium, days 6-14) + Secretory phase (progesterone makes endometrium receptive for implantation, days 15-28). These two cycles are hormonally linked and coordinated.
4. Clinical Applications
OCP (Oral Contraceptive Pills): combined oestrogen + progestogen maintain constant high hormone levels → negative feedback → suppress LH surge → no ovulation. Clomiphene citrate (fertility drug): anti-oestrogen → blocks negative feedback → FSH/LH increase → ovulation induction in anovulatory women. Ovulation predictor kits: detect LH surge in urine → predict ovulation 24-36 hours ahead. hCG injection: mimics LH surge → used to trigger ovulation in ART (IVF/IUI).
Frequently Asked Questions
1. What is the LH surge? ⌄
The LH surge is a rapid, large increase in LH secretion from the anterior pituitary that occurs around day 13-14 of the 28-day menstrual cycle. It is triggered by the high oestrogen levels from the mature Graafian follicle (positive feedback). The LH surge causes: ovulation (rupture of Graafian follicle, release of secondary oocyte), formation of corpus luteum from remaining follicle cells.
2. What is the feedback relationship between oestrogen and LH? ⌄
Oestrogen has DUAL feedback effects depending on level and duration: Low oestrogen (early-mid follicular phase): NEGATIVE feedback on hypothalamus (reduces GnRH) and pituitary (reduces LH/FSH release) → keeps LH suppressed. High oestrogen (sustained high levels, late follicular phase, just before ovulation): POSITIVE feedback → triggers massive LH surge → ovulation. This dual feedback switch (from negative to positive) at high oestrogen is unique and critical for the menstrual cycle.
3. What is the menstrual cycle in brief? ⌄
Days 1-5: Menstruation (uterine lining shed). Days 1-13: Follicular phase (FSH stimulates follicle development, oestrogen rises). Day 13-14: LH surge → ovulation. Days 15-28: Luteal phase (corpus luteum produces progesterone + oestrogen, prepares uterus for implantation). If no fertilisation: corpus luteum degenerates → hormone drop → menstruation.
4. What is the corpus luteum? ⌄
Corpus luteum (yellow body): formed from the ruptured Graafian follicle after ovulation, stimulated by LH. Produces: progesterone (main), oestrogen, inhibin. Progesterone: maintains endometrium for implantation, inhibits further FSH/LH release (negative feedback). If fertilisation occurs: hCG from trophoblast maintains corpus luteum. If no fertilisation: corpus luteum degenerates (luteolysis) at day 24-26 → hormone fall → menstruation.
5. What is positive feedback in endocrinology? ⌄
Positive feedback: the response amplifies the original signal (more of A → more of A). Rare in physiology (most regulation is negative feedback for homeostasis). Examples: LH surge: high oestrogen → more LH → more oestrogen → even more LH (until ovulation). Parturition (childbirth): oxytocin → uterine contraction → more oxytocin → more contraction → baby delivered. Blood clotting cascade: each step amplifies the next.