The primitive streak is a structure that forms in the blastula during the early stages of avian, reptilian and mammalian embryonic development.
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 first 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 antero-posterior body axis, becoming the first symmetry-breaking event in the embryo, and marks the beginning of gastrulation. This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position, where they will differentiate into the three germ layers (endoderm, mesoderm, ectoderm) that will give rise to all the tissues of the adult organism.
Read more about Primitive Streak: Components, Cellular Movements, Formation, Ethical Implications, Additional Images
Famous quotes containing the words primitive and/or streak:
“How far men go for the material of their houses! The inhabitants of the most civilized cities, in all ages, send into far, primitive forests, beyond the bounds of their civilization, where the moose and bear and savage dwell, for their pine boards for ordinary use. And, on the other hand, the savage soon receives from cities iron arrow-points, hatchets, and guns, to point his savageness with.”
—Henry David Thoreau (1817–1862)
“No gray hairs streak my soul,
no grandfatherly fondness there!
I shake the world with the might of my voice,
and walk—handsome,
twentytwoyearold.”
—Vladimir Mayakovsky (1893–1930)