Primitive streak

The primitive streak forms a longitudinal midline structure in the narrower posterior (caudal) region of the developing embryo on its dorsal side.

[4] The primitive streak establishes bilateral symmetry, determines the site of gastrulation, and initiates germ layer formation.

[2] The primitive streak forms a longitudinal midline structure in the narrower caudal (posterior) region of the developing embryo on its dorsal side.

[13] Cells overlaying Koller's sickle in the posterior end of the chick embryo move towards the midline, meet and change direction towards the center of the epiblast.

[8][14] As these cells move and concentrate at the posterior end of the embryo, the streak undergoes a single- to multi-layered epithelial sheet transition that makes it a macroscopically visible structure.

[8] 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.

[20][21][22][23][19] Similarly, Vg1 (a transforming growth factor beta family member (TGF-β)) misexpression and grafts of the posterior marginal zone[16] in chicks can also induce ectopic streaks, but only within the marginal zone of the embryo,[22] indicating a specific characteristic of this region in its ability to induce streak formation.

[24] In addition, mutating the intracellular negative regulator of Wnt signaling, Axin,[25] and misexpression of the chick cWnt8C[26] produces multiple streaks in mouse embryos.

[19] Eventually, the hypoblast gets displaced anteriorly by the moving endoblast, allowing streak formation at the posterior end.

[19] Similarly to the hypoblast in chick, the AVE in the mouse secretes two antagonists of Nodal signaling, Cerberus-like, Cerl, and Lefty1.

[19][28] In mouse, Cer-/-; Lefty1-/- double mutants develop multiple streaks[17] as indicated by ectopic expression of Brachyury and can be partially rescued by the removal of one copy of the Nodal gene.

[17] In the mouse, the AVE restricts streak formation through the redundant functions of Cer1 and Lefty1, which negatively regulate Nodal signaling.

[17] The role of the mouse's AVE in ensuring the formation of a single primitive streak is evolutionarily conserved in the hypoblast of the chick.

[17][19] Another important pathway in modulating formation of the primitive streak is FGF, which is thought to work together with Nodal to regulate this process.

Cellular differentiation arising from the development of the primitive streak during gastrulation following implantation
Primitive streak in relation to later developed notochord shown on black crested gibbon embryo
An intricate network of signaling pathways regulate the formation of the primitive streak.