Meiosis-activating sterol protects oocytes from precocious chromosome segregation.

BACKGROUND Follicular fluid meiosis-activating sterol (FF-MAS) overcomes hypoxanthine (HX)-mediated meiotic arrest in mammalian oocytes. METHODS In order to determine whether chromosome segregation was normal in oocytes matured in FF-MAS, the development, chromosomal constitution and chromosome alignment was analysed in spontaneously matured as well as HX-arrested mouse oocytes cultured in the absence or presence of FF-MAS. RESULTS FF-MAS-induced meiotic maturation was significantly less effective compared with spontaneous maturation in supporting cytokinesis ( approximately 40 and approximately 90% polar body formation respectively). The majority of oocytes stimulated by FF-MAS to overcome the HX block developed to metaphase II (MII), but 23.4% of meiosis II oocytes were diploid. Chromosomes were well aligned on the spindle, and hyperploidy was low in spontaneously matured oocytes and HX-arrested oocytes cultured with or without FF-MAS. Unexpectedly, almost 40% of spontaneously matured MII oocytes contained chromatids/monads. Precocious loss of chromatid cohesion was significantly reduced in spontaneously matured as well as HX-arrested oocytes cultured in the presence of FF-MAS but not lanosterol. CONCLUSIONS FF-MAS induces full nuclear maturation to MII, and chromosomes segregate with high fidelity. However, in delayed FF-MAS-stimulated meiotic maturation, anaphase I may occur in the absence of cytokinesis. FF-MAS appears to protect mammalian oocytes from precocious chromatid segregation.