Biology · Human Reproduction · NEET
The blastocyst reaches the uterus after moving from the oviduct. Its outer layer, the trophoblast, gets attached to the endometrium (inner lining of the uterus). After attachment, the uterine cells divide fast and cover the blastocyst. So the blastocyst becomes embedded in the endometrium. This whole event is called implantation, and it leads to pregnancy.
The trophoblast attaches to the endometrium. The blastocyst has two parts: an outer layer called the trophoblast and an inner group of cells called the inner cell mass. The trophoblast does the attaching. The inner cell mass later becomes the embryo. NEET often asks which layer attaches, so remember: trophoblast attaches, inner cell mass forms the baby.
Implantation takes place in the endometrium of the uterus. Fertilisation happens earlier in the ampulla of the oviduct, but the blastocyst travels down to the uterus and embeds there. Do not confuse the two places: fertilisation = ampulla (oviduct); implantation = endometrium (uterus).
The corpus luteum secretes large amounts of progesterone, which keeps the endometrium thick and ready. Such a prepared endometrium is necessary for implantation of the fertilised ovum. If there is no fertilisation, the corpus luteum degenerates, progesterone falls, the endometrium breaks down, and menstruation happens instead. So a healthy endometrium is a must for the blastocyst to embed.
No. Fertilisation is the fusion of sperm and ovum to form a zygote, and it happens in the ampulla of the oviduct. Implantation is much later — the blastocyst embeds into the uterus wall. Between them come cleavage, morula and blastocyst formation. Mixing these up is a common NEET mistake.
Assertion A: Endometrium is necessary for implantation of blastocyst. Reason R: In the absence of fertilization, the corpus luteum degenerates that causes disintegration of endometrium. In the light of the above statements, choose the correct answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Implantation is when the blastocyst attaches to the uterus wall and gets buried in the endometrium. Its trophoblast layer sticks first, then uterine cells cover it. This leads to pregnancy.
The trophoblast, the outer layer of the blastocyst, attaches to the endometrium. The inner cell mass does not attach; it later forms the embryo.
In the endometrium, the inner lining of the uterus. The blastocyst becomes embedded in the endometrium of the uterus.
After implantation, finger-like projections called chorionic villi appear on the trophoblast. These join with uterine tissue to form the placenta, the link between the foetus and the mother.
After implantation, pregnancy begins. All events of the menstrual cycle stop and there is no menstruation, because the corpus luteum and later the placenta keep progesterone high to maintain the endometrium.
