What happens to the corpus luteum if fertilization does not occur?
It continues to secrete progesterone
It transforms into the corpus albicans
It triggers ovulation
It secretes estrogen
The Correct Answer is B
The corpus luteum is formed from the ruptured ovarian follicle after ovulation and functions as a temporary endocrine gland, primarily secreting progesterone (5–20 ng/mL mid-luteal) and smaller amounts of estrogen. It maintains the endometrium in preparation for implantation. If fertilization and implantation do not occur, the absence of human chorionic gonadotropin (hCG) causes the corpus luteum to degenerate around day 24–26 of a typical 28-day cycle. It then becomes a fibrous scar tissue called the corpus albicans, leading to a drop in progesterone and estrogen levels, and triggering menstruation.
Rationale for correct answer
B. If fertilization does not occur, the corpus luteum degenerates due to lack of hCG support and transforms into the corpus albicans. This structural regression leads to hormonal withdrawal and menstruation.
Rationale for incorrect answers
A. Progesterone secretion continues only if fertilization occurs and hCG is produced by the trophoblast. Without hCG, the corpus luteum involutes and stops secreting hormones.
C. The corpus luteum forms after ovulation; it does not trigger it. Ovulation is triggered by the LH surge prior to corpus luteum formation.
D. The corpus luteum secretes some estrogen, but its primary function is progesterone production. After degeneration, it no longer produces either hormone in significant amounts.
Take home points
- The corpus luteum regresses into the corpus albicans without fertilization.
- Hormonal support from hCG is necessary to sustain the corpus luteum.
- Progesterone levels drop, triggering menstruation.
- The corpus luteum does not cause ovulation; it follows it.
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Correct Answer is D
Explanation
The menstrual cycle is divided into the menstrual, follicular, ovulatory, and luteal phases, regulated by hormonal fluctuations of estrogen, progesterone, FSH, and LH. The menstrual phase begins on day 1 of the cycle and lasts about 3–7 days, during which the functional layer of the endometrium is shed due to a sudden drop in progesterone (from normal mid-luteal values of 5–20 ng/mL to <1 ng/mL) after corpus luteum regression. FSH (normal: 3–20 IU/L) begins to rise to recruit new follicles. Blood loss ranges from 30–80 mL, and prostaglandins contribute to uterine contractions and cramping.
Rationale for correct answer
D. The menstrual phase involves the shedding of the functional layer of the endometrium due to the withdrawal of progesterone and estrogen. It marks the beginning of a new cycle.
Rationale for incorrect answers
A. The follicular phase follows menstruation and involves regeneration of the endometrium under the influence of rising estrogen levels from developing follicles.
B. Ovulation is the release of an ovum due to the LH surge, around day 14. The endometrium is not shed during this phase; it continues to thicken in preparation for implantation.
C. The luteal phase occurs after ovulation and is marked by progesterone secretion from the corpus luteum to maintain the endometrium. Shedding occurs only if implantation does not happen.
Take home points
- The menstrual phase is when endometrial shedding occurs due to hormonal withdrawal.
- Estrogen and progesterone drop sharply, triggering menstruation.
- The follicular phase is for endometrial regeneration, not shedding.
- Luteal phase maintains the endometrium; shedding starts only if it ends.
Correct Answer is C
Explanation
Estrogen, specifically estradiol (E2), plays a dual role in the hypothalamic-pituitary-ovarian axis through both negative and positive feedback mechanisms. During the late follicular phase, when estradiol levels remain consistently high—typically >200 pg/mL for at least 50 hours—the feedback on the hypothalamus and anterior pituitary shifts from negative to positive. This stimulates a GnRH surge, which triggers a massive LH release (LH surge), leading to ovulation. The LH surge also initiates the transformation of the ruptured follicle into the corpus luteum.
Rationale for correct answer
C. Estrogen at sustained high levels switches to positive feedback, stimulating GnRH and causing the LH surge. This is the critical hormonal trigger for ovulation around day 14 of a typical 28-day cycle.
Rationale for incorrect answers
A. Progesterone exerts only negative feedback on GnRH, FSH, and LH throughout the cycle, especially during the luteal phase. It does not cause the LH surge and actually prevents further ovulation.
B. FSH is regulated by negative feedback via estrogen and inhibin. It does not trigger the LH surge and is not part of the positive feedback loop leading to ovulation.
D. Inhibin is secreted by granulosa cells and specifically suppresses FSH release via negative feedback. It plays no role in triggering the LH surge or stimulating GnRH.
Take home points
- High sustained estrogen levels trigger positive feedback on GnRH and LH.
- The LH surge causes ovulation and luteinization of the follicle.
- Progesterone and inhibin exert negative feedback and suppress further ovulation.
- FSH supports follicle growth but is not involved in the LH surge trigger.
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