The LH surge triggers ovulation approximately 36 hours later. Which biochemical event in the dominant follicle is most directly responsible for the resumption of meiosis (completion of meiosis I) in the oocyte?

• A LH directly binds oocyte LH receptors, triggering Ca2+ transients that activate MPF
• B LH surge increases gap junction permeability between cumulus cells and oocyte, allowing direct entry of Ca2+
• C LH surge raises cAMP in granulosa cells → activates EGF-like peptides (amphiregulin, epiregulin) → EGF receptor signalling in cumulus cells → reduces cAMP in oocyte (via PDE3A) → relieves meiotic arrest ✓
• D LH stimulates progesterone synthesis, which directly diffuses into the oocyte and activates maturation-promoting factor (MPF)

Explanation

Before the LH surge, oocyte meiotic arrest at prophase I is maintained by high cAMP (generated by Gs-coupled GPR3/12 receptors and adenylyl cyclase in the oocyte) and cGMP (from granulosa/cumulus cells entering via gap junctions, inhibiting PDE3A in the oocyte). The LH surge activates EGF-like peptides (amphiregulin, epiregulin) in granulosa cells via cAMP-PKA. These bind EGF receptors on cumulus cells, which close gap junctions (reducing cGMP supply to oocyte) and activate PDE3A in the oocyte via a kinase cascade, degrading oocyte cAMP. The drop in cAMP relieves PKA inhibition of CDC25B, dephosphorylating and activating CDK1 (MPF), driving meiosis I resumption.

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

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