Fertilization starts with the contact of the sperm on the oocyte and terminates with the genetic material fusion, contained in these gametes. This contact occurs specifically with the binding protein of the zona pellucida (in red on the oocyte) and the binding protein of the sperm (in red on the sperm). The contact starts the acrosome reaction (acrosome in brown on the sperm head), it enables the penetration of the sperm by dissolving the zona pellucida and opening a tight entrance. This reaction is species specific. The sperm reaches the cytoplasmic membrane of the oocyte (in white in the oocyte). When the sperm contacts the oocyte plasma membrane (in white), it triggers the cortical reaction: the cortical granules (brown points in the oocyte) will release enzymes to shield the oocyte against polyspermy, modifying the zona pellucida (brown ring growing) and deactivating its binding protein. This process is called zona reaction , it blocks multiple sperm penetration or polyspermy. In addition, the contact of the gametes membranes activates the oocyte restarting the meiotic process, suspended at the metaphase II. The oocyte nucleus or female pro nucleus and the male pro nucleus (both in blue) migrate to the centre, and the decondensation of the chromatin occurs. After the genetic material had fusioned, the egg cell or zygote will prepare to the first cleavage.

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Biology	Animal	Fertilization