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Bilaminar blastocyst
Bilaminar blastocyst or Bilaminar disc refers to the epiblast and the hypoblast, evolved from the embryoblast.〔(The Third Week Of Life: )〕〔(Bilaminar Disc )〕 These two layers are sandwiched between two balloons: the primitive yolk sac and the amniotic cavity.
The inner cell mass, the embryoblast, begins to transform into two distinct epithelial layers just before implantation occurs. The epiblast is the outer layer that consists of columnar cells.The inner layer is called the hypoblast, or primitive endoderm, which is composed of cuboidal cells. As the two layers become evident, a basement membrane presents itself between the layers. The final two layers of the embryoblast are known collectively as the bilaminar embryonic disc as well as the bilaminar blastocyst or bilaminar blastoderm. This bilaminar blastocyst also defines the primitive dorsal ventral axis.〔 Blastocyst implantation will occur during the second week of fetal development in the endometrium of the uterus;〔"Bilaminar Embryonic Disc." Atlas of Human Embryology. Chronolab A.G. Switzerland, n.d. Web. 27 Nov. 2012. .〕 the epiblast is dorsal and the hypoblast is ventral.〔Schoenwolf, Gary C., and William J. Larsen. Larsen's Human Embryology. 4th ed. Philadelphia: Churchill Livingstone/Elsevier, 2009. Print.〕
==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 cells, 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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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