Maturation of the Rat Cumulus Oophorus: A Scanning Electron Microscopic Study

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Abstract
The process of mucification of the preovulatory cumulus oophorus in the rat was examined by the scanning electron microscope (SEM). Induction of mucification in vitro by either LH or FSH was also studied. Mucification in vivo was studied in follicular cumuli isolated at different times during proestrus. The morphology of these cumuli was compared with that of postovulatory cumuli isolated from the oviductal ampullae. For the in vitro studies, follicular cumuli were isolated at early proestrus and incubated for various time periods in presence of ovine or rat preparations of either LH or FSH. The beginning of mucus deposition in follicular cumuli was observed at about 5 h before the expected ovulation. Cumuli isolated 2 h before ovulation were totally embedded in the extracellular matrix. In vitro, first stages of mucification were observed in cumuli incubated for 3 h in medium containing LH. Extracellular hyaluronidase-sensitive material completely covered the cumuli by 20 h of culture in presence of either LH or FSH. In the absence of the gonadotropins, mucification was not observed. Parietal granulosa tissue showed inability to mucify in the presence of LH in vitro. No accumulation of extracellular material could be observed in cumuli cultured for 24 h in hormone free medium and then exposed to LH. It was concluded that the hyaluronidase sensitive matrix is accumulated between the cumulus cells in the follicle before ovulation. This intercellular mucus is secreted specifically by the cumulus cells as a direct response to gonadotropic stimulation. The cultured cumulus cells lose their ability to respond to LH.