Gamete interactions during mammalian fertilization.
1994
Wassarman P.M.
Complementary adhesion molecules are located on the surface of mouse eggs and sperm. These molecules support species-specific interactions between sperm and eggs that lead to gamete fusion (fertilization). Modification of these molecules shortly after gamete fusion assists in prevention of polyspermic fertilization. mZP3, an 83,000 MW glycoprotein located in the egg extracellular coat, or zona pellucida, serves as primary sperm receptor. Gamete adhesion in mice is carbohydrate-mediated, since sperm recognize and bind to certain mZP3 serine/threonine- (O-) linked oligosaccharides. As a consequence of binding to mZP3, sperm undergo the acrosome reaction (exocytosis), which enables them to penetrate the zona pellucida and fertilize the egg. Initiation of the acrosome reaction involves components of a signal transduction pathway analogous to that found in somatic cells. sp56, a 56,000 MW protein located in plasma membrane surrounding acrosome-intact mouse sperm heads, is a putative primary egg-binding protein. It is suggested that sp56 recognizes and binds to certain mZP3 O-linked oligosaccharides. Acrosome-reacted sperm remain bound to eggs by interacting with mZP2, a 120,000 MW zona pellucida glycoprotein. Thus, mZP2 serves as secondary sperm receptor. Perhaps a sperm protease associated with inner acrosomal membrane, possibly (pro)acrosin, serves as secondary egg-binding protein. mZP3 and mZP2 are inactivated as sperm receptors following sperm-egg fusion. In this manner, these egg and sperm surface molecules regulate fertilization in mice. Homologous molecules apparently regulate fertilization in other mammals, including human beings.
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