# E1.4 Hormonal Control of Reproductive Cycle

F.6 HKDSE Biology · Topic E1.4 · Lesson 3

Four hormones run the menstrual cycle through feedback, and the same hormones are used in contraceptive pills and infertility treatment.

## Figure

Hormone levels in a 28-day menstrual cycle (schematic, two panels with separate scales): the oestrogen peak causes the LH surge, and the LH surge triggers ovulation.

## Key points

- **FSH (促卵泡激素)**, from the pituitary gland, stimulates the development of follicles and stimulates them to secrete oestrogen.
- **LH (促黃體激素)**, from the pituitary gland, triggers ovulation; the ruptured follicle then develops into a **yellow body** (corpus luteum).
- **Oestrogen** causes thickening of the uterine lining; **progesterone** from the yellow body maintains its thickness.
- **Significance**: only one ovum is released in each cycle, and the uterine lining is prepared for implantation.
- **Pregnancy**: **HCG (人絨毛膜促性腺激素)** from the membranes around the embryo prevents the degeneration of the yellow body; later the placenta takes over.
- **Combined pills** (synthetic oestrogen and progesterone) inhibit FSH and LH secretion: follicle development and ovulation stop.
- **Mini pills** mainly thicken the cervical mucus; **emergency pills** delay or prevent ovulation and are taken within 72 hours of sexual intercourse.
- **Infertile (不育)**: no pregnancy after one year of regular and unprotected sexual intercourse.
- **Fertility drugs and IVF**: FSH and LH make several follicles mature, so **multiple births (多胎)** are more likely.

## Must know

| Hormone level (when) | Effect | Result |
|-------|------|------|
| **Oestrogen** rising but still low (before ovulation) | inhibits FSH secretion by the pituitary gland (**negative feedback**) | usually only one follicle matures |
| **Oestrogen** at its peak (just before ovulation) | stimulates a sudden release of FSH and LH (**positive feedback**) | the LH surge triggers ovulation |
| **Progesterone and oestrogen** high (yellow body, after ovulation) | inhibit FSH and LH secretion (**negative feedback**) | no more follicles mature; low LH, so the yellow body degenerates |
| **Progesterone and oestrogen** falling (no fertilization) | the uterine lining is no longer maintained; the inhibition of the pituitary gland is removed | menstruation; FSH rises and a new cycle begins |
| **HCG** from the membranes around the embryo (pregnancy) | prevents the degeneration of the yellow body | oestrogen and progesterone stay high: no menstruation, no ovulation |

In a data question, answer with reference to the graph given, not from memory.

## Common mistakes

- ✗ "The oestrogen peak triggers ovulation." ✓ The **LH surge** triggers ovulation; the oestrogen peak causes the LH surge (positive feedback).
- ✗ "Oestrogen always inhibits FSH secretion." ✓ Oestrogen has opposite effects: a rising but still low level inhibits FSH, while its peak level stimulates FSH and LH secretion.
- ✗ "Ovulation is on day 14", whatever the graph shows. ✓ Ovulation is not always on day 14: it happens just after the LH peak (LH peak on day 15 → ovulation on day 16).
- ✗ "HCG is secreted by the pituitary gland or the ovaries." ✓ HCG is made by the membranes around the embryo.
- ✗ "The high FSH level makes the yellow body degenerate." ✓ The yellow body needs LH: it degenerates when the LH level falls. FSH rises only afterwards, a result and not the cause.

## Quick check

1. Name the two hormones from the pituitary gland that control the menstrual cycle. Which one triggers ovulation?
2. In a woman's graph the LH level peaks on day 17. On which day did ovulation most likely occur? Give a reason.
3. If no fertilization occurs, why does the yellow body degenerate?
4. How do combined pills prevent pregnancy?
5. A woman's progesterone level stays high after day 28. What does this indicate, and which hormone keeps the yellow body working?

## Answers

1. FSH and LH; the surge of LH triggers ovulation.
2. Day 18: ovulation happens just after the LH peak, because the surge of LH triggers ovulation.
3. High levels of progesterone and oestrogen inhibit FSH and LH secretion by the pituitary gland; due to the low level of LH, the yellow body degenerates.
4. Synthetic oestrogen and progesterone inhibit FSH and LH secretion; follicle development and ovulation stop, so no ova are released for fertilization.
5. She is pregnant; HCG from the membranes around the embryo prevents the degeneration of the yellow body, which continues to secrete progesterone.

## Did you know?

Before modern home pregnancy tests, which also detect HCG in urine, a woman's urine was injected into an African clawed frog: if the frog laid eggs within hours, the woman was pregnant.