Question

Trace the events from fertilization till implantation of blastocyst in human female. Also mention the site where fertilization takes place.

Hint:

Remember the cell counts: Morula is 8--16 cells. Blastocyst is the stage that actually implants.

Answer 1

Step 1: Understanding the Concept:
Fertilization is the fusion of a haploid sperm with a haploid ovum to form a diploid zygote.
This process initiates a series of developmental stages as the embryo moves towards the uterus for attachment.
Step 2: Detailed Explanation:
1. Site of Fertilization: Fertilization usually occurs in the ampullary-isthmic junction of the fallopian tube (oviduct).
2. Zygote Formation: The fusion of gametes results in a single-celled diploid zygote.
3. Cleavage: As the zygote moves through the isthmus towards the uterus, it undergoes rapid mitotic divisions called cleavage. It forms 2, 4, 8, and then 16 daughter cells called blastomeres.
4. Morula: The embryo with 8 to 16 blastomeres is called a morula.
5. Blastocyst Formation: The morula continues to divide and transforms into a blastocyst as it enters the uterus. The blastomeres in the blastocyst are arranged into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.
6. Implantation: The trophoblast layer gets attached to the endometrium, and the inner cell mass gets differentiated as the embryo. After attachment, the uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation.
Step 3: Final Answer:
The site is the ampullary-isthmic junction. The zygote undergoes cleavage to form a morula, then a blastocyst, which finally implants into the uterine endometrium.