Questions: Reproductive Hormonal Cycles and Gametogenesis

Question 1

A medical student claims: 'Estrogen always suppresses LH through negative feedback, which is why birth control pills containing estrogen prevent ovulation.' What is incomplete about this explanation?

• A. It is entirely correct — estrogen provides only negative feedback on LH throughout the cycle
• B. While low sustained estrogen suppresses LH, a sustained high-estrogen signal switches to positive feedback, triggering the LH surge — the pill works by maintaining stable hormone levels that prevent this positive-feedback threshold from being reached
• C. The pill works by suppressing FSH, not LH, so estrogen's effect on LH is irrelevant to contraception
• D. Estrogen provides only positive feedback on LH; the pill prevents ovulation through a different mechanism unrelated to estrogen-LH interactions

Answer: (B)

Estrogen's feedback on LH is context-dependent and is the key event of the female cycle. At low-to-moderate levels (early follicular phase), estrogen provides negative feedback, suppressing both FSH and LH. But when estrogen rises above a threshold and is sustained for ~36 hours (as the dominant follicle reaches peak maturity), the pituitary switches to positive feedback mode, producing the massive LH surge that triggers ovulation. Combined oral contraceptives maintain stable, moderate estrogen and progesterone levels that prevent this buildup, keeping the system in a permanently 'non-surge' state. The negative-feedback-only account misses the sign reversal that makes the cycle self-oscillating.

Question 2

During the follicular phase, why does the dominant follicle survive while other recruited follicles undergo atresia?

• A. The dominant follicle secretes an inhibitory signal that directly triggers apoptosis in competing follicles
• B. As estradiol from the growing follicle rises, FSH is suppressed via negative feedback; only the most FSH-sensitive follicle continues developing while less-sensitive follicles starve of FSH
• C. LH selects the dominant follicle by binding selectively to a unique receptor subtype expressed only on that follicle
• D. Progesterone from the corpus luteum inhibits all follicles except the dominant one

Answer: (B)

The follicle selection mechanism is elegant negative feedback. FSH recruits a cohort of follicles, and the growing follicles produce estradiol. As estradiol rises, it suppresses FSH via negative feedback — FSH levels fall. The most FSH-sensitive follicle (the one with the most FSH receptors or the greatest exposure to FSH) can continue developing even at the lower FSH levels; the rest, requiring more FSH support, undergo atresia. The dominant follicle essentially out-competes rivals by creating the FSH-suppressing signal that kills them. Progesterone is not involved — it is produced by the corpus luteum only after ovulation.

Question 3

Female fertility declines with age partly because women are born with their entire lifetime supply of primary oocytes already formed, and these oocytes accumulate damage over decades, increasing the risk of chromosomal non-disjunction.

• T. True
• F. False

Answer: True

This is a critical asymmetry between male and female gametogenesis. Female oocytes are arrested in prophase I beginning before birth and remain in this state for decades — up to 40–50 years in some cases. During this prolonged arrest, the cohesion proteins holding homologous chromosomes together gradually degrade, making proper segregation during meiosis I increasingly error-prone. This is the primary reason chromosomal trisomies (including Down syndrome) increase sharply with maternal age. Male spermatogonia continuously divide and produce new cells, so sperm do not accumulate decades of arrested-state damage in the same way.

Question 4

The LH surge that triggers ovulation is caused by rising progesterone from the growing dominant follicle, which switches the pituitary from negative to positive feedback mode.

• T. True
• F. False

Answer: False

It is estradiol — not progesterone — that switches the pituitary to positive feedback and triggers the LH surge. The growing dominant follicle produces estradiol; when this rises above a threshold and is sustained for ~36 hours, the hypothalamus and pituitary respond with a massive LH surge. Progesterone is produced by the corpus luteum, which forms from the ruptured follicle *after* ovulation has already occurred. Post-ovulation progesterone then enforces negative feedback, suppressing LH and FSH and stabilizing the luteal phase. Swapping estradiol for progesterone as the cause of the LH surge reverses the temporal sequence of events.

Question 5

Why is the menstrual cycle described as a self-contained oscillation, and what single event causes it to reset and repeat rather than reaching a stable hormonal equilibrium?

Model answer: The cycle is self-oscillating because it contains a built-in instability: the positive-feedback LH surge. In the follicular phase, rising estradiol from the dominant follicle suppresses FSH (negative feedback, stabilizing) but accumulates toward the positive-feedback threshold. Once reached, the LH surge triggers ovulation and luteinization — converting the follicle into the corpus luteum, which now produces progesterone. Progesterone enforces negative feedback, suppressing the next cohort of follicles. If pregnancy does not occur (no hCG to rescue the corpus luteum), the corpus luteum degenerates, progesterone falls, the endometrium sheds, and FSH rises again to recruit the next cohort. The absence of fertilization is the reset event — it removes the progesterone brake and allows the follicular phase to begin again.

This oscillatory design contrasts with the male system, which has no equivalent sign reversal and therefore runs continuously rather than cyclically. The single sign switch in estrogen feedback — from negative to positive — is the biological mechanism that makes the female reproductive system a limit cycle rather than a stable equilibrium.