Fusion of Sperm and the Egg
After the penetration of the extracellular layers by sperm there occurs the fusion of the - sperm plasma membrane with that of the egg. In mammals the fusion begins at the - equatorial region of the sperm head. The plasma membrane of the two cells become continuous to form a cytoplasmic bridge through which the sperm nucleus enters the egg cytoplasm. Usually the entire sperm including the nucleus, centrioles, mitochondria, plasma membrane and even the flagellar axoneme enters the egg cytoplasm. Once the sperm enters the egg there is the formation of the fertilization cone. The fertilization cone is an extension of the egg cytoplasm around the entering sperm head. Microfilaments in the fertilization cone virtually draw the sperm into the egg. Inhibitors of microfilament formation such as cytochalasin B inhibit the formation of fertilization cone and sperm entry into the egg. Figure shows the events occurring during fertilization in sea urchin egg.
Figure: Events that occur during the sperm - egg fusion in sea urchin