sexes have been a matter of intense investigation. Early experiments suggested that a diffusible, sex-unspecifi c meiosis-inducing substance is produced by gonads of both sexes but induces meiotic initiation only in female germ cells. It was suggested that meiosis in the prespermatogonia could be blocked by the antagonistic action of meiosis-preventing substance, secreted by testicular cords ( 1, 2 ). In 1995, two structurally related sterols, termed meiosis-activating sterols (MAS), with the ability to trigger the resumption of meiosis in mouse oocytes cultured in vitro, were identifi ed ( 3 ). Follicular fl uid meiosis-activating sterol (FF-MAS), isolated from human follicular fl uid, and testis meiosis-activating sterol (T-MAS), isolated from bull testes, were the fi rst reported sterol precursors with potential biological activity and the fi rst compounds with meiosis-activating potency. Based on the sex-and species-unspecifi c action of MAS, a hypothesis about the important role of MAS in reproduction emerged ( 3, 4 ). However, follow-up in vitro experiments have provided evidence that MAS might not be absolutely essential for gonadotropin-mediated resumption of meiosis ( 5 ). In contrast, some studies have demonstrated that MAS and related analogs promote nuclear and cytoplasmic maturation of oocyte in vitro ( 6 ). Therefore, despite the existence of a relatively large body of in vitro studies indicating an important regulatory role of MAS in the onset of meiosis in oocytes, in vivo proof has not yet been provided. a Young Scientist Fellowship to R. K.;
Abstract