Progesterone and 17 alpha-OH-progesterone in concentrations similar to that of preovulatory follicular fluid is without effect on resumption of meiosis in mouse cumulus enclosed oocytes cultured in the presence of hypoxanthine.

Some intermediates in the cholesterol biosynthesis between lanosterol and cholesterol are capable of inducing resumption of meiosis in cultured mouse oocytes without the presence of gonadotropins. The mechanism by which these so-called Meiosis Activating Sterols (MAS) activate the meiotic process is unknown, and it is uncertain whether they participate in the physiological control of resumption of meiosis. Recently, it has been shown that accumulation of MAS occurs in a liver cell line and in rat testis tissue cultured in the presence of micromolar concentrations of progesterone and 17 alpha-OH-progesterone. Such high concentrations of progesterone and 17 alpha-OH-progesterone only occur in fluid of preovulatory follicles. In connection with the mid-cycle surge of gonadotropins, this may represent one mechanism whereby follicular accumulation of MAS takes place. In the present study, the effect of 10 micro M progesterone and 10 micro M 17 alpha-OH-progesterone on resumption of meiosis was evaluated using mouse cumulus enclosed oocytes (CEO) cultured in the presence of 4mM hypoxanthine. By the end of the 24-h culture period, the frequency by which oocytes had resumed meiosis was assessed by the determination of germinal vesicle breakdown (GVBD). Neither progesterone nor 17 alpha-OH-progesterone or a combination showed any effect on GVBD. In addition, progesterone and 17 alpha-OH-progesterone in combination with a sub-optimal dose of FSH (4 IU/l) did not affect GVBD. In conclusion, accumulation of MAS to an extent that allows resumption of meiosis to occur in CEO is unlikely to be induced by progesterone and 17 alpha-OH-progesterone or a combination.