Question

In which of the following developmental stage of the embryo does the implantation take place?

• A. A
  
• B. B
• C. C
• D. D

Answer 1

The correct option is D D

Picture A, B, C and D are representing the different stages of embryonic development.
Fertilization of male and female gametes leads to the formation of zygote. After the formation of the zygote, the diploid zygote starts dividing by the process of cleavage as it moves in the fallopian tube towards the uterus.
First, the zygote divides into two equal halves, making two blastomeres. Similarly, they divide into four, eight and sixteen cells.
This 8-16 celled stage of the developing embryo is called a morula. Gradually, the cells formed migrate to form trophoblast on the periphery and a clump of cells inside called the inner cell mass.
The different cells derived from the cleavage upto to the blastula stage are called blastomeres. These divisions occur within the same cytoplasm of the zygote and zona pellucida is still intact. Now the embryo needs to get out of the zona pellucida to implant in the cells of the endometrium and this process is called zona hatching.
After this, the blastocyst gets implanted in the endometrium.
Picture D shows the blastocyst stage which finally gets attached to the cells of the endometrium of the uterus.