MORPHOGENETIC REORGANIZATION OF THE BRAIN DURING EMBRYOGENESIS IN THEGRASSHOPPER

We have studied the morphogenetic reorganization that occurs in the gr asshopper brain during embryogenesis. We find that morphogenetic movem ents occur at three organizational levels during brain development. Fi rst, the entire developing brain changes its orientation with respect to the segmental chain of ventral ganglia. A 90 degrees shift in the a ttitude of the brain neuraxis occurs during embryogenesis due to a gra dual upward movement of the cerebral structures in the head. Second, t he clusters of proliferating neuroblasts and progeny that generate the neuroarchitecture of the mature brain move relative to one another an d to nonneural structures such as the stomodeum. This is especially pr onounced for the pars intercerebralis and for the tritocerebrum, as sh own by annulin and engrailed immunoreactivity. Third, individual neuro blasts within a given proliferative cluster undergo positional reorgan ization during embryogenesis. Identified neuroblasts of the tritocereb rum and the pars intercerebralis are displaced within the brain. We co nclude that the transformation of the simple sheet-like structure of t he early embryonic brain into the highly differentiated structure of t he mature brain involves a series of morphogenetic movements that occu r in virtually all parts of the brain. (C) 1995 Wiley-Liss, Inc.