15. Fertilisation

 

Therefore, fertilisation must take place within \(24\) \(hours\). Male discharges semen into the female's vagina close to the cervix during sexual intercourse or copulation. From the vagina, the sperm swims up the uterus and then enter the oviducts. Initial transportation of sperms is by the uterine and fallopian tube contractions.

 

Later, the sperms swim using their tail (due to its motility) in the fluid medium to enter the ovum to fuse with it. Numerous sperms surround the ovum, but only one sperm succeeds in fertilising the ovum. As soon as a sperm enters the ovum, chemical reactions obstruct the further entry of any sperm.

 

 

There is a fusion of male and female gamete resulting in the formation of a "zygote".

The zygote is the fertilised ovum. Before fertilisation, the sperm needs to undergo a process called capacitation.

The secretions of the female tract and the acidic environment remove the coating deposited on the sperm's surface. Thus, the acrosome becomes more permeable, and the sperm thus becomes active to penetrate the egg.

The acrosome of the sperm thus can secrete the lytic enzymes. The enzymes help the sperm to break down and penetrate the layers of the cells around the ovum. Thus, the sperm comes in contact with the ovum.

 

After the fusion of sperm and the ovum, the oocytes display cortical reactions. The cortical granules present in the oocyte fuse with the plasma membrane to release cortical enzymes.

 

Cortical enzymes harden the surface of the ovum. Thus, it hinders the entry of other sperms. The process ensures that fertilisation occurs with only one sperm.

The ovum and the sperm fuse together to form a zygote. Once the ovum fuses with the sperm to form a zygote, the corpus luteum formed from the Graafian follicles secretes progesterone and helps in the development of zygote.