Gastrulation Process - Formation of Primitive Streak
Gastrulation in all amniotes involving eutherian mammals is related to a characteristic structure termed as the primitive streak formed on the epiblast surface during the first 10-18 hours of incubation at 38.5°C temperature. It makes as a result of convergence of epiblast cells to the dorsal midline of the blastoderm. The starting of primitive streak formation is first pointed out by a thickening in the central posterior region of area pellucida immediately after the creation of hypoblast. The thickening narrows and elongates growing anteriorly in the centre of area pellucida. While fully formed the primitive streak is a narrow structure with a groove (Primitive groove) in its floor together its length flanked by a fold (or ridge) on either side. It extends anteriorly upon about three fourth the length of area pellucida in which it ends in a deep pit called Hensen's Node with thick borders.
During the formation of primitive streak the shape of area pellucida slowly changes from circular to pea shaped with the broad side anterior and narrow side posterior. This change is because of convergence of cells toward dorsal midline starting at posterior end and progressing anteriorly but stopping where the Hensen's Node is formed. With reference to the fate map it should be noted that the posterior end of the primitive streak would be in the centre of the posterior edge of the area of presumptive extra-embryonic mesoderm, and its anterior end (Hensen's Node) would be situated in the presumptive endodermal area posterior to notochordal area. Primitive streak marks the median anterior-posterior axis of the embryo and establishes bilateral symmetry.