These results indicate that FCS inhibits the zona reaction without preventing CG exocytosis. The same results have also been previously reported for mouse oocytes. If mouse oocytes were matured in a serum-free medium, zona hardening occurred and sperm penetration was prevented. However, when FCS or follicular fluid was included in the medium, zona hardening was inhibited, and the fertilizability of oocytes was maintained. Du-cibella et al. first demonstrated that premature CG exocytosis was the reason for zona hardening of mouse oocytes matured in a serum-free medium, because the limited CG exocytosis modified ZP proteins; thus sperm could not bind to ZP and penetrate oocytes. buy ortho tri-cyclen online
By contrast, if the medium was supplemented with FCS, FCS did not prevent premature CG exocytosis but did inhibit ZP conversion. Also, in mice, DMSO can induce CG exocytosis and zona hardening, and prevent sperm penetration of oocytes. However, when FCS was included in the medium, it also prevented DMSO-induced ZP hardening and ZP2 conversion without preventing CG exocytosis. Another indirect piece of evidence that FCS inhibits the zona reaction is provided by the study of IVF in bovine oocytes. Bovine oocytes matured and inseminated in vitro do not show high polyspermy because of normal cortical and zona reactions. However, if FCS was included in the fertilization medium, polyspermic penetration increased from less than 30% in medium without FCS to more than 80%.