Fertilisation and Implantation

Ovum surrounded by few sperms

The process of fusion of a sperm with an ovum is called fertilization.

Fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube. The result of this union is the production of a zygote cell, or fertilized egg, initiating prenatal development.

Ø During coitus, the semen is released into the vagina, passes through the cervix of the uterus and reaches the ampullary-isthmic junction of the fallopian tube.

Ø The ovum is also released into the junction for fertilisation to occur.

Ø The process of fusion of the sperm and the ovum is known as fertilisation.

Ø During fertilisation, the sperm induces changes in the zona pellucida and blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum.

Ø The enzymatic secretions of the acrosomes help the sperm enter the cytoplasm of the ovum.

Ø This causes the completion of meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum (ootid) and a secondary polar body.

Ø Then, the haploid sperm nucleus fuses with the haploid nucleus of the ovum to form a diploid zygote.

Ø Mitosis starts as the zygote moves through the isthmus of the oviduct (cleavage) and forms 2, 4, 8, 16 daughter cells called blastomeres.

Ø The 8−16 cell embryo is called a morula, which continues to divide to form the blastocyst. The morula moves further into the uterus.

Ø The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass.

Ø The trophoblast gets attached to the uterine endometrium, and the process is called implantation. This leads to pregnancy.

Ø The inner cell mass gets differentiated to form the embryo.