Fertilization is defined as the process of union of two germ cells, egg and sperm, whereby the somatic chromosome number is restored and the development of a new individual exhibiting characteristics of the species is initiated. If fertilization fails to take place, both egg and sperm degenerate relatively rapidly in the female reproductive tract, since the two highly differentiated cells cannot survive long on their own.
Among mammals, the process of union of germ cells includes several ordered steps ( It begins in the oviduct with binding of free-swimming sperm to the ovulated egg extracellular coat, the zona pellucida (ZP) (Figure 1 and Figure 2), and ends a short time later with fusion of egg and sperm plasma membranes to form a single “activated” cell, the zygote. Along the way, several recognizable events take place, including the sperm acrosome reaction (a form of cellular exocytosis), penetration of the egg ZP by sperm, and the egg cortical reaction and zona reaction. The latter results in alteration of the ZP such that free-swimming sperm are unable to bind to fertilized eggs. Each of these events in the fertilization pathway has been studied in some detail.