Describe the Chemical events fertilization of Mammalian egg.

Explanation

Chemical events during fertilisation include 1. Acrosomal Reaction. After ovulation, the secondary oocyte reaches the Fallopian tube (oviduct). The capacitated sperms undergo acrosomal reaction and release various chemical contained in the acrosome. These chemicals are collectively called sperm lysins. Important sperm lysins are (i) hyaluronidase that acts on the ground substances of follicle cells, (ii) corona penetrating enzyme that dissolves corona radiata and (iii) zona lysine or acrosin that helps to digest the zona pellucida. Optimum pH, `Ca^(++), Mg^(++)` ions concentration and temperature are essential for acrosomal reaction. `Ca^(++)` plays major role in acrosomal reaction. In the absence of `Ca^(++)` , fertilization does not occur. Due to acrosomal reaction, plasma membrane of the sperm fuses with the plasma membrane of the secondary oocyte so that the sperm contents enter the oocyte. Binding of the sperm to the secondary oocyte induces depolarization of the oocyte plasma membrane. Depolarization prevents polyspermy (entry of more than one sperm into the oocyte). It ensures monospermy (entry of one sperm into the oocyte). (ii) Cortical Reaction. Just after the fusion of sperm and plasmamembranes of oocyte, the secondary oocyte shows a cortical reaction. The cortical granules are present beneath the plasma membrane of the secondary oocyte. These granules fuse with the plasma membrane of the oocyte and release their contents including cortical enzymes between the plasma membrane and the zona pellucida. These enzymes harden the zona pellucida which also prevents entry of additional sperms (polyspermy).