Are steroids obligatory mediators of luteinizing hormone/human chorionic gonadotropin-triggered resumption of meiosis in mammals?

Steroids mediate the gonadotropic stimulus of oocyte maturation in fish and amphibians. Such a role of steroids in mammals has not been confirmed until recently. A series of studies presented data suggesting that steroids might be involved in meiosis of mouse oocytes. Here we examined this suggestion using in vitro cultures of rat and mouse follicle-enclosed oocytes (FEOs) and cumulus-enclosed oocytes (CEOs). In FEOs that mature only in response to gonadotropins or other stimuli, we tested the ability of steroids to trigger meiosis and whether addition of steroid receptor antagonists blocks LH/human chorionic gonadotropin stimulation of meiosis. In CEOs that mature spontaneously, we tested whether steroid antagonists block maturation and whether steroids overcome the inhibition of maturation by hypoxanthine (Hx), a mild inhibitor of meiotic resumption. The progesterone antagonists mifepristone (RU 486) and Organon 31710 as well as the estrogen antagonist faslodex did not prevent LH-triggered maturation of rat or mouse FEOs or the spontaneous maturation of CEOs. In accordance, the progesterone agonist promegestone (R5020) and estradiol did not stimulate the resumption of meiosis in rat and mouse FEOs, and both did not overcome the Hx inhibition of meiosis in rat and mouse CEOs. Flutamide, an androgen antagonist, did block meiosis in rat FEOs, but this action could not be affected by adding dihydrotestosterone, suggesting that it was not androgen receptor mediated. Flutamide did not affect spontaneous maturation of rat CEOs, and dihydrotestosterone could not stimulate meiosis inhibited by Hx. Thus, in contrast to lower vertebrates, in mammals, steroids do not seem to serve as an obligatory signal by which the somatic cells of the follicle transfer the gonadotropic stimulation of meiosis to the oocyte.