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Second Week of Development
The second week of development is a critical phase in embryogenesis, characterized by the establishment of the bilaminar germ disc and the formation of the extraembryonic structures. During this period, the blastocyst continues to differentiate, leading to significant developments in the embryo's early structure.
At the beginning of the second week, the blastocyst implants more deeply into the uterine lining. The trophoblast differentiates into two layers: the cytotrophoblast and the syncytiotrophoblast. The syncytiotrophoblast is responsible for invading the uterine wall and establishing the early connection with maternal blood vessels, which is essential for nutrient exchange.
Simultaneously, the inner cell mass of the blastocyst differentiates into two distinct layers, forming the bilaminar germ disc. These two layers are the epiblast and the hypoblast. The epiblast, the outer layer, will eventually give rise to all three germ layers (ectoderm, mesoderm, and endoderm) during gastrulation. The hypoblast, the inner layer, contributes to the formation of the yolk sac.
As the bilaminar germ disc forms, two cavities develop. The amniotic cavity forms within the epiblast, surrounded by amnioblasts, which will eventually form the amnion. The yolk sac forms from the hypoblast and provides early nutrition to the developing embryo. These cavities play essential roles in the protection and nourishment of the embryo during early development.
The formation of the chorionic cavity, or extraembryonic coelom, occurs as spaces within the extraembryonic mesoderm merge. This cavity surrounds the amniotic cavity and the yolk sac, with the developing embryo suspended within it. The chorion, which consists of the trophoblast and the extraembryonic mesoderm, will contribute to the formation of the placenta.
By the end of the second week, the primary structures necessary for further development are established. The embryo is now securely implanted in the uterine lining, and the formation of the primitive uteroplacental circulation begins, setting the stage for the critical processes that will occur in the following weeks.
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