Question

Explain the process of fertilisation and implantation in humans.

Answer 1

The process of fusion of the sperm and the ovum is called fertilisation. It consists of the following events:

• During fertilisation, the sperm induces changes in the zona pellucida of the ovum and blocks the entry of other sperms. This ensures that only one sperm fertilises the ovum.
• The enzymatic secretions of the acrosome help the sperm to enter the cytoplasm of the ovum.
• The meiotic division of the ovum is then resumed and completed, resulting in the production of an ootid and a secondary polar body.
• The fusion of the nuclei of the sperm and the ovum then leads to the formation of a zygote.

The process of implantation consists of the following events:
1. Fertilisation results in the formation of a diploid zygote.
2. As the zygote moves through the uterus, mitotic divisions occur in the zygote that form 2, 4, 8, 16 daughter cells called blastomeres.
3. The embryo with 8 to 16 blastomeres is known as the morula.
4. The morula undergoes further cleavage and develops into a blastocyst. The cells of the blastocyst get arranged into an outer layer called trophoblast and inner cells called inner cell mass.
5. The trophoblast layer of the blastocyst gets attached to the endometrium, while the inner cell mass gets differentiated to form an embryo.
6. After attachment, the uterine cells divide rapidly. As a result, the blastocyst gets embedded in the endometrium of the uterus, thereby leading to pregnancy.