The placenta forms through a complex process that begins soon after fertilization and involves the development of specialized cells called trophoblasts.
Here's a breakdown of the process:
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Fertilization and Early Development: After the sperm fertilizes the egg, a zygote is formed. The zygote undergoes rapid cell division, eventually forming a blastocyst.
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Implantation: Around 6-12 days after fertilization, the blastocyst implants into the uterine wall (endometrium).
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Trophoblast Differentiation: The outer layer of the blastocyst, called the trophoblast, differentiates into two layers:
- Cytotrophoblast: The inner layer of the trophoblast.
- Syncytiotrophoblast: The outer layer of the trophoblast. This layer is formed by the fusion of cytotrophoblast cells.
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Lacunae Formation: The syncytiotrophoblast invades the uterine wall, creating spaces called lacunae. These lacunae become filled with maternal blood.
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Chorionic Villi Development: Projections called chorionic villi extend from the cytotrophoblast into the lacunae. These villi are the functional units of the placenta.
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Maternal Blood Supply: Maternal blood circulates within the lacunae, bathing the chorionic villi.
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Fetal Blood Vessels: Fetal blood vessels develop within the chorionic villi. These vessels are connected to the fetal circulation through the umbilical cord.
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Placental Barrier Formation: The chorionic villi, along with the syncytiotrophoblast, form the placental barrier. This barrier allows for the exchange of nutrients, gases (oxygen and carbon dioxide), and waste products between the maternal and fetal blood. Importantly, the placental barrier prevents significant mixing of maternal and fetal blood, although some minor mixing may occur.
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Continued Growth and Development: The placenta continues to grow and develop throughout the pregnancy, increasing the surface area for exchange.
In summary, the placenta arises from the trophoblast cells of the blastocyst that implant in the uterine wall. These cells differentiate to form layers facilitating the exchange of nutrients and waste between maternal and fetal circulations while maintaining a selective barrier.
