Q.Explain the process of formation of placenta in a human female after the implantation of the blastocyst in the endometrium of the uterus.
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Start your 14-day free trial to unlock the full solution →The placenta forms from both foetal (chorionic villi) and maternal (decidua basalis) tissues after implantation, creating a vascular interface for nutrient, gas, and waste exchange between mother and foetus.
The placenta is not a single organ that appears fully formed — it develops gradually as the blastocyst embeds itself into the uterine lining. The key idea is that two separate tissues, one from the embryo and one from the mother, interlock and reorganise to create a specialised exchange surface. Without this, the growing embryo would have no way to get oxygen or nutrients beyond the earliest stages.
Let’s walk through the process step by step.
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Implantation triggers the transformation of the endometrium.
Once the blastocyst attaches to the endometrium (around day 6–7 after fertilisation), the uterine lining responds by becoming the decidua. The part of the decidua directly under the blastocyst is called the decidua basalis — this will be the maternal side of the placenta. The rest of the decidua (decidua capsularis over the embryo, and decidua parietalis elsewhere) plays a supporting role but does not form the placenta.
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The trophoblast differentiates into two layers.
The outer cell layer of the blastocyst, the trophoblast, now begins to invade the decidua. It splits into:
- Cytotrophoblast — an inner layer of distinct, cuboidal cells that continue to divide.
- Syncytiotrophoblast — an outer, multinucleated mass formed by fusion of cytotrophoblast cells. This layer is invasive; it erodes into the maternal tissues and blood vessels, creating spaces (lacunae) that fill with maternal blood.
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Chorionic villi begin to form.
Finger-like projections of the trophoblast, called chorionic villi, push into the decidua basalis. Each villus has a core of mesoderm (from the extraembryonic mesoderm) that contains foetal blood vessels, surrounded by cytotrophoblast and then syncytiotrophoblast. These villi are the functional units of the placenta — they bring foetal blood close to maternal blood without mixing them.
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The decidua basalis remodels to supply maternal blood.
The syncytiotrophoblast erodes the walls of maternal spiral arteries in the decidua basalis. This allows maternal blood to flow into the intervillous spaces — the gaps between the chorionic villi. The maternal blood bathes the villi directly, while foetal blood runs inside the villi. Exchange of gases, nutrients, and wastes occurs across the thin barrier (the placental membrane). …
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