Primitive Streak - Components

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).