Independent spatial waves of biochemical differentiation along the surface of chicken brain as revealed by the sequential expression of acetylcholinesterase.

AChE-positive cells suddenly amass in a superficial layer of the neuroepithelium; this layer finally covers, in a sheat-like manner, the entire surface of the embryonic chicken brain. This feature is functionally not understood; however, it appears shortly after the neurons become postmitotic, and the lateral extensions of this layer can easily be traced using histochemistry on serial brain sections. The layer can therefore be exploited to delineate spatially the waves of onset of biochemical tissue differentiation. We have studied whole brains between stages 11 and 30 and provide the first complete spatial schemes of brain differentiation based on computer-reconstructed, two- and three-dimensional maps. The brain does not differentiate in one smooth coherent wave, but instead five separate primary AChE-activation zones are detected: the first originating at stage 11 ("rhombencephalic wave"), the second at the same time ("midbrain wave"), the third at stage 15 ("tectal wave"). A fourth zone develops later, at stage 18, from the bottom part of the telencephalon to the top. Retinal development also starts at stage 18. In a given area, it appears that AChE-development shortly precedes that of the formation of major fiber tracts. AChE might therefore represent a prerequisite for fiber growth and pathfinding.