Menstrual Cycle: MDCAT Biology notes
Menstrual cycle MDCAT notes: GnRH, FSH, LH, oestrogen and progesterone, follicular and luteal phases, ovulation day, corpus luteum, menarche and menopause.
Overview
The menstrual cycle is the set of changes in the ovary and uterus that repeats about every 28 days. It includes ovulation and the breakdown and loss of the uterine lining. It starts at puberty with menarche, the first menstruation, which happens in girls but has no equivalent in boys. It ends in later life with menopause, when ova production stops.
Hormonal control
- The hypothalamus secretes GnRH (gonadotrophin-releasing hormone), which controls the pituitary.
- The anterior pituitary releases the gonadotrophins FSH and LH. These are the pituitary hormones that regulate the cycle.
- FSH stimulates follicle development and makes follicle cells secrete oestrogen.
- In the FSc textbook account, the fall in FSH together with the rise in oestrogen causes the pituitary to release LH. Rising oestrogen also inhibits further FSH.
- LH triggers ovulation and turns the empty follicle into the corpus luteum.
- Progesterone from the corpus luteum thickens and maintains the endometrium. It also suppresses FSH, LH and further ovulation.
Phases of a 28-day cycle
| Days | Phase | Ovary | Uterus (endometrium) |
|---|---|---|---|
| 1–5 | Menstrual | New follicles start growing | Lining breaks down and is shed |
| 6–13 | Proliferative (follicular, pre-ovulatory) | A follicle matures into a Graafian follicle and secretes oestrogen | Lining regenerates and thickens |
| About 14 | Ovulation | LH surge (days 11–14); the secondary oocyte is released | — |
| 15–28 | Secretory (luteal) | The corpus luteum secretes progesterone and some oestrogen | Glands secrete, and the lining becomes ready for implantation |
The most fertile days of a 28-day cycle are around days 11–14. LH is at its maximum in the blood at ovulation.
Follicles and corpus luteum
- Follicle cells surround the developing oocyte and nourish it.
- The primary oocyte completes meiosis I to form a secondary oocyte and a polar body.
- Many follicles never mature. Their degeneration is called follicular atresia.
- After ovulation, the remains of the Graafian follicle form the corpus luteum, which secretes progesterone. The Graafian follicle itself secretes oestrogen.
- If fertilization does not happen, the corpus luteum degenerates. Progesterone then falls and the endometrium is shed, which is menstruation. If pregnancy begins, the placenta later takes over making progesterone.
Oestrogen and progesterone together maintain the endometrium and make it receptive to the embryo.
Oxytocin
Oxytocin comes from the posterior pituitary. It acts on the myometrium to cause labour contractions. Labour is a classic positive feedback loop: contractions trigger more oxytocin, which causes stronger contractions. Oxytocin also causes milk ejection and has a role in social bonding.
Worked example
Question: A woman with a regular 28-day cycle begins menstruating on 1 March. When is ovulation most likely?
Answer: Ovulation falls on about day 14 of the cycle, so around 14 March.
Common MDCAT traps
- FSH produces follicle growth and oestrogen. LH produces ovulation and the corpus luteum.
- The corpus luteum makes progesterone, not LH or FSH.
- The hypothalamus controls the pituitary, not the ovary or uterus.
- Atresia means follicle degeneration, not maturation.
- The proliferative phase is the same as the follicular (pre-ovulatory) phase.
Quick revision
- Ovulation happens on about day 14.
- Progesterone suppresses ovulation.
- The endometrium degenerates if there is no fertilization.
- GnRH comes from the hypothalamus.
- Menopause means ova production stops.