What are the physicochemical events that take place during the process of fertilization?

Physical and Chemical events taking place during fertilization:

(i) Acrosomal Reaction: The capacitated sperms undergo acrosomal reaction and release various chemicals present in the acrosome. These chemicals are collectively termed as sperm lysins. Some of them are
i) hyaluronidase - acts on the ground substances of follicle cells
ii) corona penetrating enzyme - dissolves corona radiata
iii) zona lysine - helps to digest the zona pellucida

Due to acrosomal reaction, plasma membrane of the sperm fuses with the plasma membrane of the secondary oocyte and sperm contents enter the oocyte. This binding induces depolarization of the oocyte plasma membrane. Depolarization prevents polyspermy and ensures monospermy.

(ii) Cortical Reaction: After the fusion of sperm and plasma membranes of oocyte, the secondary oocyte shows a cortical reaction. 

These granules fuse with the plasma membrane of the oocyte and release their contents. These enzymes harden the zona pellucida which prevents entry of additional sperms and prevents polyspermy.

(iii) Sperm Entry: The secondary oocyte forms a projection which receives the sperm. The distal centriole of the sperm divides and forms two centrioles to generate the mitotic spindle formation for cell division. 

(iv) Karyogamy: Entry of sperm stimulates the secondary oocyte to com­plete the suspended second meiotic division. It produces a haploid mature ovum and a second polar body. The head of the sperm which contains the nucleus separates from the middle piece and the tail and becomes the male pronucleus. The second polar body and the sperm tail degenerate. The male and female pronuclei move towards each other leading to degeneration of nuclear membranes. Mixing up of the chromosomes of a sperm and an ovum is known as karyogamy. The fertilized ovum is called zygote. 

(v) Activation of Egg: Sperm entry stimulates metabolism in the zygote which greatly increases the rates of cellular respiration and protein synthesis. 

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