Formation of the neural crest
Truly the cells of the neural crest (stage 9) represent a fourth embryonic germinal layer that exhibits a partially segmented organization and is involved in the genesis of the peripheral nervous system (neurons as well as glial cells of the sympathetic, parasympathetic and sensory systems).
During the closure of the neural tube the nerve cells multiply in the transition region between the neuroectoderm and ectoderm as the result of an interaction between these two tissues. At the same time, these neurons detach themselves from the association and migrate into the depths
The cells of the neural crest exhibit a noteworthy ability for migration and a phenotypical diversity. Numerous and various types of cells originate from them
The migration of neural crest cells is in a close relationship to the disappearance of the N-CAM and cadherin formed from the neural tube as well as the appearing of membrane integrin. The molecules of the TGF-β family appear to trigger the migration of the neural crest cells in that they modify their adhesion properties at the level of the extracellular matrix (fibronectin, laminin, collagen). Indeed, these cells bind themselves via specific membrane receptors (integrin) at molecules of the extracellular matrix (fibronectin and laminin) in order to move. The end of the cell migration occurs together with a renewed expression of cadherin and N-CAM that mediate the cells' adhesion.
Studies have shown the significance of the molecules of the BMPs, FGFs, Wnts families and retinoic acid in the induction of the neural crest. For its part SHH should prevent the spread of the neural crest cells by activating integrin β1.