Mechanism Of FERTILIZATION IN HUMANS ! Biological Sciencess !!

FERTILIZATION IN HUMANS

Only a single ovum is released in human females from one of the two ovaries towards the middle of ovarian/menstrual cycle. It passes into fallopian tube and rests inside ampulla for some time. Human male produces 300 - 400 million sperms per ejaculation. They are deposited in vagina during coitus. The process of deposition of sperms in the female genital tract is called
insemination. A number of them are demobilised or eaten.

Fertilization involves the following steps

(i) Approximation of Sperm and Ovum.

Sperms can remain motile for 24-48 hours.
They swim at the rate of 1. 5 - 3.0 mm/min. They are able to reach the ampulla part
of female genital tract partly by their own swimming and partly by contraction of
uterus and fallopian tubes. One sperm comes to lie against the ovum and undergo
fertilizin (from ovum) and antifertilizin (from sperm) compatibility reaction (Lillie,
1919) in the region of animal pole.
(ii) Acrosome Reaction. It prepares the sperm to fertilize ovum. The phenomenon is called capacitation. In contact with corona radiata, the acrosome covering lyses to release acrosome. With the help of sperm lysins (chiefly hyaluronidase), acrosome
dissolves corona radiata and zona pellucida in front of it.

(iii) Egg Reaction. 

A small protuberance or fertilization cone (cone of reception), develops from the surface of ovum in the region of animal pole.

(iv) Penetration of Sperm. 

Sperm head establishes contact with lateral surface of fertilization cone. Plasma membranes of the two dissolve. Contents of head (nucleus), neck and middle piece of sperm enter ooplasm. Tail is left outside. Fertilization cone subsides. Depolarisation of egg membrane kills other sperms. Plasma membrane of the egg is now modilled with the help of mucopolysaccharide cortical granules into
fertilization membrane. A perivitelline space is created between it and zona pellucida.

(v) Activation of Ovum. 

Ovum (previously in secondary oocyte stage) undergoes meiosis II and extrudes a secondary polar body. It is now the actual ovum or female gamete.

(vi) Fusion of Sperm and Egg Nuclei. 

The envelopes of the sperm and egg nuclei
degenerate and their chromosomes intermingle to form ‘synkaryon’. The act is called karyogamy or syngamy. The proximal centriole brought by sperm help~ form the spindle for the division of synkaryon (cleavage nucleus). Fertilized egg is also called zygote

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