To reach the plasma membrane of an oocyte, a spermhas to traverse the zona pellucida (ZP), a thick, transparent, noncellular envelope that surrounds the oocyte.The zona pellucida contains three glycoproteins of interest, ZP1–3. Of particular note is ZP3, an O-linked glycoprotein that functions as a receptor for the sperm. Aprotein on the sperm surface, possibly galactosyl transferase, interacts specifically with oligosaccharide chains ofZP3; in at least certain species (eg, the mouse), this interaction, by transmembrane signaling, induces the acrosomal reaction, in which enzymes such as proteases andhyaluronidase and other contents of the acrosome of thesperm are released. Liberation of these enzymes helpsthe sperm to pass through the zona pellucida and reachthe plasma membrane (PM) of the oocyte. In hamsters,it has been shown that another glycoprotein, PH-30, isimportant in both the binding of the PM of the sperm tothe PM of the oocyte and also in the subsequent fusionof the two membranes. These interactions enable thesperm to enter and thus fertilize the oocyte. It may bepossible to inhibit fertilization by developing drugs orantibodies that interfere with the normal functions ofZP3 and PH-30 and which would thus act as contraceptive agents.
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