Question Bank [2022] — BIOLOGY STD 12 Science — Question

Implantation usually begins on day 7 after fertilization and by end of the 10th day, the embryo is completely buried inside the endometrium. The embryo usually implants in the region of the fundus of the uterus. Implantation occurs in the following manner: In the process, the embryo attaches itself by its embryonic pole, close to the endometrium. The trophoblast cells of the animal pole adhere to the uterine wall. Rapid division of the trophoblast cells lying against the embryonal knob takes place. This results in the formation of two distinct layers syncytiotrophoblast and cytotrophoblast.

1. The outer layer, syncytiotrophoblast is syncytium i.e. a layer of protoplasm with many nuclei. It gives out processes that extensively invade the endometrium. The lytic enzymes secreted by the trophoblasts, rupture the endometrial cells thereby making a burrow, into which the embryo begins to get implanted.
2. The inner layer of cells is called cytotrophoblast (cells with defined membrane) since the cells retain their cell boundaries.