Single neuroectodermal cells delaminate from the surface epithelium, in a fixed pattern, and move into the interior of the embryo to form neural precursor cells called neuroblasts. Thus the earliest steps in patterning the CNS are the formation and specification of neuroblasts. Proneural genes promote neuroblast formation, whereas the neurogenic genes inhibit neuroblast formation. Neuroblast identity is determined in the neuroectoderm.
Neuroblasts delaminate in five waves spanning approximately three hours.
The pair-rule genes wingless , hedgehog , gooseberry , and engrailed are expressed in stripes of neuroectoderm that subdivide the AP axis. For example, gooseberry is expressed in row 5 neuroectoderm. For information on the expression of segmentation genes and neuroblast identity genes in specific neuroblasts, see Chris Doe's Hyper-Neuroblast map . There is a hierarchical cascade of transcriptional repression. Vnd represses ind expression to establish the ventral boundary of ind transcription, and ind represses msh to establish the ventral boundary of msh transcription.