1 . Fertilization

Fertilization is a cell-cell recognition process that occurs between two distinct cells: a small asymmetric and motile sperm cell and a large and nonmotile egg. The stages of fertilization can be divided into four processes: 1) sperm preparation, 2) sperm-egg recognition and binding, 3) sperm-egg fusion and 4) fusion of sperm and egg pronuclei and activation of the zygote. The specific structures of the sperm and egg that are important for fertilization will be discussed and experiments that led to the identification of the egg receptor for the sperm and the sperm receptor for the egg will be described. Membrane fusion of sperm and eggs is an incompletely understood process, but the discovery of proteins known as ADAM proteins on the sperm surface has suggested new mechanisms to explain sperm-egg fusion. Finally, we will consider how fertilized eggs prevent additional sperm from fusing (a condition known as polyspermy) and how the fertilized egg is activated to begin development.