Modifications of the mouse zona pellucida during oocyte maturation: Inhibitory effects of follicular fluid, fetuin, and α2HS-glycoprotein

Abstract

Mouse oocyte maturation in vivo or in vitro is accompanied by a precocious release of cortical granules. When oocytes are matured in vitro in the absence of serum, cortical granule exocytosis results in modifications of the zona pellucida (ZP) that are known to constitute the zona pellucida block to polyspermy. Fetuin, a glycoprotein found in newborn calf serum, can inhibit these maturation-associated changes in the zona pellucida. In this report we demonstrate that the human fetuin homologue, α2HS-glycoprotein, is present in human follicular fluid. Human follicular fluid, or purified human α2HS- glycoprotein, inhibits the maturation-associated conversion of mouse ZP2 to ZP21 in a concentration-dependent manner. Although the concentration of α2HS-glycoprotein in human follicular fluid is high enough to account for the inhibitory effect of human follicular fluid on this maturation-associated conversion, human follicular fluid immunodepleted of α2HS-glycoprotein retains its full inhibitory activity. Thus, other inhibitory molecules are likely to be present in follicular fluid and contribute to the inhibition of the maturation-associated conversion of ZP2.