Sperm-Egg Coat Binding

Induction of the acrosome reaction is a key event that occurs in sea urchins, starfish, ascidians and mammals. However, it is not clear that in the amphibian Xenopus sperm undergo the same type of acrosome reaction, because thus far it has not been possible to detect a morphologically identifiable acrosome vesicle in these sperm. The acrosome reaction in echinoderms and in mammals has been well studied. During the acrosome reaction the acrosomal vesicle fuses with the plasma membrane and the acrosomal components are externalized. Several proteolytic activities thought to be involved in penetration through the extracellular matrix of the egg are deteeted in the acrosomal contents. In addition, exposure of egg coat binding molecules on the surface of the sperm occurs (see below). 

Figure 4. Scheme showing sperm-egg interaction in the abalone. 1. The sperm binds to the egg VE by the plasma membrane at the tip of the AV (AG), (F, flagellum M, mitochondrion N, nucleus). 2. The sperm acrosome reacts releasing lysin and the 18K protein from its anterior tip. 3. Lysin disrupts the fibers of the VE and the 18K coats the extending acrosome process as it extends. 4. The sperm passes through the hole in the VE and the membrane covering the tip of the acrosomal process fuses with the egg (from Vacquier and Lee, 1993). 

Miller, D.J., Macek, M.B., and Shur, B.D. (1992). Complementarity between sperm surface P-1,4-galactosyltransferase and egg-coat ZP3 mediates sperm-egg binding. Nature 357 589-593. 

During fertilization, sperm must first bind in a species-specific manner to the eggs thick extracellular coat, the zona peUucida or vitelline envelope and then undergo a form of... 

Fertilization is the fusion of the sperm and ovum. The sperm head binds to the plasma membrane of the egg (oolemma), and the entire spermatozoon enters the cytoplasm of the ovum. Only capacitated spermatozoa with intact acrosomes can enter and pass through the cumulus oophorus. The acrosome is a membrane-bound, cap-like structure covering the anterior portion of the sperm nucleus. The acrosomal reaction is the release of materials that lyse the glycoprotein coat (zona pellucida) surrounding the ovum. This is necessary for fertilization to take place. Before undergoing the acrosomal reaction, sperm go through a type of hypermotility called hyperactivation. The... 

Focarelli R, Rosati F (1995) The 220-kDa vitelline coat glycoprotein mediates sperm binding in the polarized egg of Unio elongatulus through O-linked oligosaccharides. Dev Biol 171 606-614... 

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