Changes in the Distribution of Mouse Oocyte Cortical Granules and Ability to Undergo the Cortical Reaction during Gonadotropin-stimulated Meiotic Maturation and Aging in Vivo1

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Abstract
Mouse oocyte cortical granule (CG) activation and distribution were investigated during in vivo meiotic maturation to determine the onset of competence of undergo the cortical reaction, which is considered responsible for the block to polyspermy. In the present study, the resumption of oocyte maturation was stimulated by hCG administration. Competence to undergo the cortical reaction (assessed with calcium ionophore A23187) was undetectable (0% loss) in germinal vesicle-stage oocytes 0.5 h after hCG administration. When germinal vesicle breakdown and metaphase I had taken place (3 and 7 h post hCG, respectively), approximately 30% CG loss was observed. Maximal (A23187-inducible) levels of CG loss, 67% and 72%, were present at 10 and 13 h, respectively, during metaphase II. Cortical granule distribution changed dramatically during metaphase I, polar body formation, metaphase II, and post-ovulatory aging in vivo. A stable metaphase II distribution was present from 13 to 18 h. After 24 and 32 h, 28% and 83% of the eggs, respectively, exhibited major alterations in the cortical distribution of CGs, some of which did not appear to be susceptible to release by A23187. These data support the hypothesis that just before ovulation the egg cortex completes the development of its normal structure and physiological competence, which are maintained for only a brief of time afterward. The implications are discussed for normal fertilization and polyspermy in mammals, including humans.