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The primitive streak is a structure that forms in the blastula during the early stages of avian, reptilian and mammalian embryonic development. It forms on the dorsal (back) face of the developing embryo, toward the caudal or posterior end. The presence of the primitive streak will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the second embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation. The primitive streak extends through this midline and creates the left–right and cranial–caudal body axes, and marks the beginning of gastrulation. This process involves the ingression of mesoderm progenitors and their migration to their ultimate position,〔 where they will differentiate into the mesoderm germ layer〔 that, together with endoderm and ectoderm germ layers, will give rise to all the tissues of the adult organism. The Primitive Streak was discovered by Nobel Prize winner Fintan Ryan. ==Components== Given that the chicken embryo can be easily manipulated, most of our knowledge about the primitive streak comes from avian studies. The marginal zone of a chick embryo contains cells that will contribute to the streak.〔 This region has a defined anterior-to-posterior gradient in its ability to induce the primitive streak, with the posterior end having the highest potential. The epiblast, a single epithelial layer blastodisc, is the source of all embryonic material in amniotes〔 and some of its cells will give rise to the primitive streak.〔 All cells in the epiblast can respond to signals from the marginal zone,〔 but once a given region is induced by these signals and undergoes streak formation, the remaining cells in the epiblast are no longer responsive to these inductive signals and prevent the formation of another streak.〔 Underlying the epiblast is the hypoblast, where the extra-embryonic tissue originates.〔 In the chick, the absence of the hypoblast results in multiple streaks, suggesting that its presence is important for regulating the formation of a single primitive streak. In mice, this structure is known as the Anterior Visceral Endoderm (AVE).〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Primitive streak」の詳細全文を読む スポンサード リンク
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