The process of fertilization, which induces embryo development, has two major components. The fusion of the sperm with the egg and the union of the genetic material contained in the gamete's nuclei. Penetration occurs in several steps. The sperm's secretion of hyaluronidase dissolves the egg's protective layer of follicle cells. Next, the acrosome, a membrane-bounded vesicle at the tip of the sperm's head, fuses with the membrane of the sperm cell. The acrosome forms a tube that penetrates the coating of the egg cell and fuses with one of the egg cell's microvilli. The nucleus of the sperm then passes through the resulting channel. The union of the sperm and egg membranes causes the egg to form an impenetrable fertilization membrane around itself, in order to prevent additional sperm from entering, and causes an increased rate of metabolism within the egg. Thus, penetration activates the development of the embryo even before fusion of genetic materials has occurred.
The union of the genetic material contained in the sperm and egg nuclei constitutes true fertilization. In order for fusion to occur, both sets of genetic material must undergo de condensation. The re condensed zygote nucleus can then carry out the DNA replication necessary for the miotic divisions of embryonic development.
