Initiated by signals from the underlying hypoblast, formation of the primitive streak is predicated on epiblast cell migration, mediated by Nodal, from the lateral-posterior regions of the epiblast to the center midline.
22.
This difference in cellular localization causes the ICM cells exposed to the fluid cavity to adopt a primitive endoderm ( or hypoblast ) fate, while the remaining cells adopt a primitive ectoderm ( or epiblast ) fate.
23.
The epiblast in this region moves towards the primitive streak, dives down into it, and forms a new layer, called the endoderm, pushing the hypoblast out of the way ( this goes on to form the amnion . ) The epiblast keeps moving and forms a second layer, the mesoderm.
24.
The transcript " cHex ", which is a product of " Hex ", has been detected in Koller's sickle during chick embryogenesis . " cHex " is also involved with the formation of the hypoblast, the endoderm in an anterior arc that overlaps the cardiogenic region, pharyngeal endoderm immediately adjacent to the forming myocardium, in the endocardium, and in the liver and thyroid gland primordia.
