Bilaminar Blastocyst - Formation of The Blastocyst

Formation of The Blastocyst

The zygote undergoes cleavage as it journeys from the oviduct to the uterus. As it transforms from 2 to 4 to 8 to 16 cells, it becomes a Morula. During these divisions, the zygote remains the same size, only the amount of cells increases. The morula differentiates into an outer and inner group of cells: the peripheral outer cell layer, the trophoblast, and the central inner cell mass, the embryoblast. The trophoblast goes on to become the fetal portion of the placenta and related extraembryonic membranes. The epiblast and hypoblast arise from the embryoblast and later give rise to the embryo proper and its affiliated extraembryonic membranes. Once the zygote has differentiated into 30 cels, it starts to form a fluid-filled central cavity called the blastocyst cavity (blastocoele). This cavity is essential because as the embryo continues to divide, the outer layer of cells grows very crowded and they have a tough time gaining adequate nutrients from surrounding fluid. Therefore, the blastocyst cavity serves as a nutrient center and the fluid is able to reach and feed cells so that they can continue growing and dividing. The embryo is called a blastocyst at about the 6th day of development once it has reached nearly 100 cells. Once this has happened, the embryo begins its journey through the uterus to start implanting in the endometrium.

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