Dissociation of Preovulatory Maturational Events in Rat Oocytes and Cumuli in the Presence of Dibutyryl Cyclic AMP

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Abstract
Mammalian oocytes enter the first meiotic division during pre‐natal life. Shortly thereafter the oocytes become arrested at the diplotene (dictyate) stage of the first meiotic division and they remain arrested until just prior to ovulation. The ovulatory LH‐surge leads to resumption of meiosis and the oocytes of preovulatory follicles undergo nuclear progression from dictyate to metaphase II (oocyte maturation). Although it is generally agreed that LH is responsible for the resumption of meiosis, the cellular mechanism for this hormonal effect remains to be elucidated. The immature oocyte in the follicle is surrounded by specialized granulosa cells comprising the cumulus oophorus. Morphological (Zamboni 1970) as well as biochemical (Biggers et al. 1967, Gwatkin and Andersen 1976) evidences suggest a role of the cumulus cells in the regulation of both oocyte growth and maturation. Following the ovulatory LH‐surge the cumulus surrounding the maturing oocyte is radically changed into a dispersed structure of cells embedded in a mucopolysaccharide matrix (e.g. Austin 1961). In a recent report isolated preovulatory follicles of PMSG‐injected immature rats were studied in an incubation system with chemically defined medium (Hillensjöet al. 1976). When LH was added to the medium the follicle‐enclosed oocyte resumed meiosis and the cumulus was transformed into a dispersed structure which was sensitive to hyaluronidase. The aim of this study was to see whether dibutyryl cyclic AMP (dbcAMP) would mimic the effects of LH on the follicle‐enclosed oocyte itself and on the cumulus oophorus.