In nature, fertilization is a highly specialized example of cell-to-cell interaction. The introduction of ICSI in assisted conception treatment bypasses the majority of events that were previously thought to be essential for the fertilization process, opening a whole new area of questions and molecular events which are yet to be elucidated (Hewitson et al., 1999). The discussion here is limited to 'natural' fertilization without reference to ICSI.
In order to trigger metabolic activation of the oocyte, the spermatozoon itself must encounter and respond to signals originating from the oocyte and its investments. Sperm-oocyte interaction is a complex multi-step process that starts with the specific recognition of complementary receptors on the surfaces of the two gametes and terminates with syngamy, the union of the maternal and paternal chromosomes. In addition to delivering the paternal genome, the spermatozoon triggers the quiescent female gamete into metabolic activity, releasing the meiotic block so that early embryogenesis may be sustained.
The central event of the normal fertilization process is fusion of the plasma membranes of the two cells. Both activation of the spermatozoon and activation of the oocyte are regulated by changes in intracellular messengers such as Ca2+, H+, cAMP, cyclic adenosine diphosphate ribose (cADPr) and IP3. The initial stages of fertilization depend principally on two structures: the acrosome of the spermatozoon and the ZP of the oocyte. There are three major events in sperm-oocyte interaction:
1. Attachment of the spermatozoon to the ZP.
2. The spermatozoon undergoes the acrosome reaction, as a result of which digestive enzymes are released and the inner acrosomal membrane is exposed.
3. This highly fusogenic sperm membrane makes contact with the oolemma and the two membranes fuse together.
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