Question

Describe the sequence of events from zygote till the implantation of human blastocyst in the uterus.

Answer 1

1. After fertilisation in the ampullary–isthmic region of the oviduct, the zygote begins rapid mitotic cleavage as it moves towards the uterus.
2. Cleavage produces progressively smaller cells called blastomeres. Their compact solid mass is the morula.
3. Further cell rearrangement creates a fluid-filled cavity, the blastocoel, producing the blastocyst. It has an outer trophoblast and an inner cell mass at one pole. The inner cell mass forms the embryo; the trophoblast helps with implantation and contributes to the foetal part of the placenta.
4. The zona pellucida disappears, allowing the blastocyst to expand.
5. About 7–8 days after fertilisation, the blastocyst attaches to and becomes embedded in the thickened endometrium. This process is called implantation.