A major function of hypoblast is to initiate gastrulation through formation of the primitive streak in the overlying epiblast through which epiblast cells migrate to form prospective endoderm and mesoderm. Thus, the initial position of the hypoblast determines the body axis. At the onset of streak formation, the blastoderm is 5-6 mm in diameter. The primitive streak begins to extend forward from the posterior of the area pellucida and ingressing epiblast cells, which mostly have an endodermal fate, migrate anteriorly and centrifugally displacing the hypoblast. Extension of the streak (between HH stages 3 and 4) is by recruitment of anterior cells rather than by cell movement from the existing streak. Hypo-blast cells driven to the anterior of the area pellucida will form the primary germ cells. As gastrulation proceeds, the area pellucida changes from being round to becoming pear-shaped with the expanded end anterior.
Lateral epiblast cells converge toward the streak, invaginate, extend, and diverge ventrally. Those cells of the epiblast that do not involute are fated to form the ectoderm and neuroectoderm, and divide to compensate for the loss of ingressing cells. At intermediate streak stages (HH stage 3, 12 h postlaying), prospective mesoderm begins to ingress through caudally, whereas endoderm is still ingressing through rostral streak. At this stage, the anterior end of the streak broadens to form Hensen's node, a structure roughly equivalent to the organizer of Xenopus embryos and the Shield of zebrafish. As the streak reaches its longest extent (HH stage 4, 16-20 h), prospective mesoderm cells fated to contribute to lateral plate, somites (posterior streak levels), heart (midstreak), and notochord (head process; derived form the deep node) are migrating through the streak.
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