[Morphological analysis of resumption of meiosis observed in oocytes of mouse vesicular follicles].

Resumption of meiosis observed in vesicular follicles of immature 32-34 day-old mice was classified morphologically, and the morphological mechanism to induce resumption of meiosis was examined. Resumption of meiosis was observed in 22.1% of vesicular follicles, and the dominant stage of the meiotic division was the first or second metaphase. Cumulus-enclosed and cumulus-free oocytes were separated from ovaries by dissection of ovaries. All of the cumulus-enclosed oocytes contained germinal vesicle, however 57.9% of cumulus-free oocytes did not, suggesting that the disappearance of cumulus cells around oocytes precedes the induction of resumption of meiosis. About 80-90% of the oocytes with germinal vesicle formed first polar body after 15 hrs of incubation. After incubation of oocytes showing resumption of meiosis, but without the first polar body, 36% of the oocytes extruded the first polar body indicating that resumption of meiosis is associated with progression to the second metaphase. Morphological characteristics of the surrounding structures of oocytes showing resumption of meiosis was classified into three categories; extensive proliferation of granulosa cells, degeneration of cumulus cells and their disappearance around oocytes, and detachment of cumulus cell-oocyte complexes from granulosa cell layer. It was suggested that cell-to-cell interaction among cumulus cells, oocytes and granulosa cells was impaired at the time of resumption of meiosis. Granulosa cells secrete an inhibitor of the resumption of meiosis. The resumption of meiosis after the surge of luteinizing hormone follows cumulus dispersion which make oocytes independent from the influence of granulosa cells by the disruption of communication.(ABSTRACT TRUNCATED AT 250 WORDS)