INTRAFOLLICULAR CONTROL OF OOCYTE MATURATION IN THE PIG

The mammalian oocyte becomes arrested at the diplotene stage of the first meiotic division during prenatal or early postnatal life. It remains arrested in meiosis until shortly before ovulation when the surge of gonadotropin induces resumption and completion of meiosis to the metaphase II stage. When oocytes are harvested from medium-sized or large follicles of pig and other species and cultured, they resume meiosis spontaneously indicating that the follicles exert an inhibitory influence on meiosis. To analyze the control of meiosis by follicular components, culture of isolated pig oocytes in the presence of follicular cells or follicular fluid (FF1) has been used as a model in this laboratory. An oocyte maturation inhibitor (OMI) has been isolated and partially purified by ultrafiltration and gel chromatography of FF1 and shown to be a polypetide with a molecular weight in the order of 2000 daltons. Physiological characterization has shown that the effect of OMI in vitro is reversible and that it can be overcome by luteinizing hormone (LH). The action of OMI requires the presence of cumulus cells surrounding the oocyte since it was found that denuded oocytes, stripped of cumulus cells, do not respond to OMI. Furthermore, when cumulus-enclosed oocytes were cultured, OMI inhibited the differentiation of the cumulus cells in terms of morphology and progesterone secretion in a dose-related manner. The inhibition of cumulus differentiation by OMI was reversible and could be overcome by LH. The results indicate that the effect of partially purified OMI upon meiosis may be mediated by the cumulus cells. © 1979 Tissue Culture Association.