During the mid-cycle LH surge that triggers ovulation, a transient rise in progesterone secretion from granulosa cells precedes ovulation. What physiological role does this pre-ovulatory progesterone rise play in the positive feedback surge mechanism?

• A Pre-ovulatory progesterone amplifies the LH surge by acting synergistically with oestradiol at the level of the GnRH pulse generator to increase GnRH pulse amplitude, thereby enhancing pituitary LH secretion ✓
• B Pre-ovulatory progesterone acts directly on anterior pituitary gonadotrophs to induce LH receptor up-regulation, increasing pituitary sensitivity to GnRH
• C Pre-ovulatory progesterone is a negative feedback signal that terminates the LH surge by suppressing GnRH pulsatility once ovulation is imminent
• D Pre-ovulatory progesterone stimulates prostaglandin E2 synthesis in the granulosa cells to directly rupture the follicle wall, independent of the LH surge

Explanation

In the late follicular phase, rising oestradiol switches from negative to positive feedback on GnRH/LH. The small pre-ovulatory progesterone rise (from LH-stimulated granulosa cells) amplifies this oestradiol-driven positive feedback: progesterone, acting centrally on GnRH neurons (which express progesterone receptors), enhances GnRH pulse amplitude and duration, augmenting the LH surge. This explains why blockade of progesterone action can blunt or delay the LH surge in animal models. After ovulation, rising luteal progesterone then switches to negative feedback (suppressing GnRH pulsatility), but this is distinct from the pre-ovulatory amplifying role. PGE2 is involved in follicle rupture but as a downstream mediator of the LH surge, not as a progesterone-mediated direct rupture mechanism.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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