Primitive streak

The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the broad end towards the anterior, and the narrower region projected to the posterior. The primitive streak forms a longitudinal midline structure in the narrower posterior (caudal) region of the developing embryo on its dorsal side. At first formation the primitive streak extends for half the length of the embryo. In the human embryo this appears by stage 6, about 17 days. The primitive streak establishes bilateral symmetry, determines the site of gastrulation, and initiates germ layer formation. To form the primitive streak mesenchymal stem cells are arranged along the prospective midline, establishing the second embryonic axis, and the site 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. Gastrulation 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 epiblast, a single epithelial layer of the bilaminar embryonic disc, is the source of all embryonic material in amniotes, and some of its cells will give rise to the primitive streak. In amphibians the equivalent structure is the blastopore. The primitive streak forms a longitudinal midline structure in the narrower caudal (posterior) region of the developing embryo on its dorsal side. At first formation the primitive streak extends for half the length of the embryo. In the human embryo this appears by Carnegie stage 6, about 17 days. Towards the cranial (anterior) end of the disc the primitive streak expands into an area known as the primitive node which is the organizer for gastrulation.