FERTILIZATION IN HUMANS:

FERTILIZATION IN HUMANS: The sperms present in the semen travel a long distance, from the uterus to the Fallopian tube. The sperms swim in the fluid medium of  the female genital track which is also partly helped by uterus.

Sperms swims at the rate of 1.5 to 3 mm per minute. A single ejaculation of sperms contains about 200 to 400 million of sperms in some 3ml. of fluid secretion. The sperms remains active for 24 to 48 hours after that it exhausts its energy and dies. 

Increased levels of GnRH stimulates the anterior pitiitary gland which then secretes the FSH and LH .FSH stimulates sertoli cells to secretes some factors which helps in spermatogenesis.

The motile sperms swim rapidly the cervix, enter the uterus and reaches the ampullary - isthamic junction of the oviduct, where the fertilization takes place.

A sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane to block the entry of the additional sperms.

The enzyme of the acrosome of sperm help to dissolve zonna pellucida and plasma membrane of the ovum and sperm head is allowed to enter into the cytoplasm of the ovum. This includes completion of Meiosis II to form secondary oocytes and II polar body.

The activated spermatozoa undergo acrosomal reaction and release various chemicals, like hyaluronidase that acts on the ground substance of follicle cells, corona penetrating enzyme that dissolves corona radiate, and zona lysine or acrosin that help to digest the  zona pellucida.All these chemicals are contained in the acrosome, located at the tip of sperm head and are collectively termed  sperm lysins. Fertilization of egg with only one spermatozoan is known as monospermy.

Numerous physical and chemical changes occur in response to egg- sperm binding. First egg become activated and undergoes depolarization of its membrane. Second the egg exhibits cortical reaction and shows zona reaction, which makes egg impervious to any other sperm.

As the fertilizing sperm enters the ovum it usually looses its flagellum. The sperm entry stimulates the egg to complete its second meiotic division.

Ultimately diploid zygote is formed by the fusion of a sperm with an ovum.

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