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:
“Perhaps our own woods and fields,—in the best wooded towns, where we need not quarrel about the huckleberries,—with the primitive swamps scattered here and there in their midst, but not prevailing over them, are the perfection of parks and groves, gardens, arbors, paths, vistas, and landscapes. They are the natural consequence of what art and refinement we as a people have.... Or, I would rather say, such were our groves twenty years ago.”
—Henry David Thoreau (1817–1862)
“No gray hairs streak my soul,
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twentytwoyearold.”
—Vladimir Mayakovsky (1893–1930)