Events after Sperm and Egg Fusion
The entry of the sperm into the egg activates the egg. The activation of egg results in two types of responses.
(1) The early responses and
(2) The late responses.
The early response for the entry of the sperm into the egg is the prevention of polyspermy i.e. the entry of or fusion of more than one sperm into the egg. Polyspermy may result in several abnormalities such as polyploidy, abnormal mechanism of chromosomal separation during cell division and ultimate death of the embryo. Organisms have evolved different types of mechanisms for prevention of entry of more than one sperm into egg. Generally any one of the three different strategies may be involved in the prevention of polyspermy. For instance in fishes the sperm can enter into the egg only through the narrow opening the micropyle, the rest of the egg being covered by the impermeable chorion. In sea urchins and mammals there is a restriction on the number of sperm that are able to penetrate the extracellular coats and fuse with the egg. In mammals the sperm have to migrate the long female reproductive tract to reach the egg and further, the structural changes in zona pellucida block polyspermy. Certain animals such as salamanders do permit the entry several sperm into the egg but only the nucleus of one of the sperm can fuse with the egg to form the zygote nucleus; other sperm nuclei degenerate.