CUMULUS CELLS SECRETE A MEIOSIS-INDUCING SUBSTANCE STIMULATION WITH FORSKOLIN AND DIBUTYRIC CYCLIC ADENOSINE-MONOPHOSPHATE

The role of the cumulus cells in initiating the resumption of meiosis after exposure to forskolin and dbcAMP was studied in the mouse. The r esumption of meiosis was monitored by the percentage of germinal vesic le breakdown (GVBD) and polar body formation (PB). The cumulus-enclose d oocytes (CEO) and denuded oocytes (DO) were cultured with and withou t hypoxanthine (HX) in the culture medium. Three types of experiments were performed: (1) Effect of forskolin on spontaneous resumption of m eiosis, i.e. cultures without HX, and two experiments in which HX is p resent throughout the culture: (2) Effect of transient exposure to for skolin or dibutyric-cyclic adenosinemonophosphate (dbcAMP) on GVBD pri or to continued culture without forskolin or dbcAMP (oocyte priming). (3) Priming of CEO with forskolin for 2 hr, separation of cumulus cell s and oocytes, followed by coculture of rejoined cumulus cells and ooc ytes, or coculture of the cumulus cells and new, unprimed DO. (1) Fors kolin inhibited a spontaneous resumption of meiosis in a dose-dependen t manner during the first 5 hr of culturing. After 22 hr all controls and CEO resumed meiosis, whereas only half of the DO did. (2) At least 1 hr of priming the CEO with forskolin is needed to induce GVBD and P B formation, but forskolin inhibited the resumption of meiosis when pr esent for 24 hr. Similar results were obtained with a high concentrati on of dbcAMP. (3) A separation and rejoining of oocytes and cumulus ce lls after priming induced the resumption of meiosis in a significantly greater number of oocytes than in the control oocytes which were not primed. The GVBD of unstimulated DO also increased significantly when cocultured with cumulus cells from primed CEO. The percentage of GVBD in unprimed DO and in DO isolated from primed CEO was the same. We sug gest that within 1-2 hr, forskolin and cAMP stimulate cumulus cells to produce a diffusible meiosis-inducing substance which overcomes HX-in hibition and induces oocyte maturation, including both GVBD and PB for mation. The CEO must be primed for more than 2 hr before the resumptio n of meiosis in DO isolated from such CEO is induced. Oocyte-cumulus c onnections are crucial as far as initiating the production of a meiosi s-inducing substance is concerned. Oocyte-cumulus connections are not needed for transferring this substance to the oocyte. (C) 1994 Wiley-L iss, Inc.