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
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... Near the narrow, posterior end, an opaque streak, called the primitive streak, makes its appearance and extends along the middle of the disk for about one-half of its length at ... A shallow groove, the primitive groove, appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the blastopore, with the yolk sac ... The primitive streak is produced by a thickening of the axial part of the ectoderm, the cells of which multiply, grow downward, and blend with those of the ...
... In front of the primitive streak, two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line formed by the streak ... one on either side of the anterior end of the primitive streak ... After the coalescence of the neural folds over the anterior end of the primitive streak, the blastopore no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory ...
Famous quotes containing the words streak and/or primitive:
“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)
“Anthropologists are a connecting link between poets and scientists; though their field-work among primitive peoples has often made them forget the language of science.”
—Robert Graves (1895–1985)