Which hormone is primarily responsible for preparing the endometrium for implantation during the secretory phase of the uterine cycle?
Estrogen
Follicle-Stimulating Hormone (FSH)
Luteinizing Hormone (LH)
Progesterone
The Correct Answer is D
The secretory phase follows ovulation and spans days 15–28 of a typical 28-day menstrual cycle. It is driven by progesterone secreted by the corpus luteum. Progesterone transforms the previously estrogen-primed proliferative endometrium into a thick, vascular, and glandular lining capable of supporting implantation. Endometrial glands become coiled and secrete glycogen-rich fluid to nourish a potential embryo. Progesterone levels typically peak at 5–20 ng/mL in the mid-luteal phase. If implantation does not occur, progesterone falls sharply, leading to menstruation.
Rationale for correct answer
D. Progesterone is the main hormone that prepares the endometrium during the secretory phase. It induces glandular secretion, stromal edema, and vascularization to support early pregnancy if fertilization occurs.
Rationale for incorrect answers
A. Estrogen plays a primary role in the proliferative phase, promoting regeneration of the endometrium but not in its secretory transformation. It works before ovulation, not after.
B. FSH stimulates follicular development and estrogen secretion during the follicular phase, but it does not act on the endometrium directly and is not dominant in the luteal/secretory phase.
C. LH triggers ovulation and supports the formation of the corpus luteum, which secretes progesterone, but LH itself does not directly act on the endometrium during the secretory phase.
Take home points
- Progesterone transforms the endometrium into a receptive state during the secretory phase.
- Estrogen prepares the endometrium in the proliferative phase, not the secretory.
- FSH and LH regulate ovarian activity but have no direct endometrial action in the luteal phase.
- Secretory changes include gland coiling, glycogen secretion, and stromal edema.
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Related Questions
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.
Correct Answer is C
Explanation
Estrogen function in the menstrual cycle
Estrogen is a key steroid hormone secreted primarily by the granulosa cells of the developing follicle. It plays a central role in the follicular phase of the menstrual cycle by stimulating endometrial proliferation, enhancing FSH receptor expression in follicles, and exerting negative feedback on FSH and positive feedback on LH as ovulation approaches. Normal estradiol (E2) levels range from 30–400 pg/mL, peaking around 200–300 pg/mL just before ovulation. Estrogen also supports cervical mucus thinning to facilitate sperm transport and primes the endometrium for possible implantation.
Rationale for correct answer
C. Estrogen stimulates mitotic activity and proliferation of the endometrial functional layer during the follicular phase. This prepares the uterine lining for potential implantation post-ovulation.
Rationale for incorrect answers
A. Ovulation is triggered by a surge in LH, not directly by estrogen. Estrogen contributes indirectly by reaching a threshold that induces the LH surge, but it does not trigger ovulation by itself.
B. Maintenance of the corpus luteum is supported by LH and, if pregnancy occurs, human chorionic gonadotropin (hCG). Estrogen does not sustain corpus luteum function.
D. Estrogen initially inhibits FSH secretion via negative feedback, but this is not its primary role. Its main function is endometrial proliferation and preparing the body for ovulation.
Take home points
- Estrogen stimulates the endometrial lining to proliferate during the follicular phase.
- LH, not estrogen, is the primary trigger for ovulation.
- Corpus luteum maintenance depends on LH or hCG, not estrogen.
- Estrogen has a dual feedback effect on FSH depending on its concentration and timing.
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