How neuronal migration contributes to the morphogenesis of the CNS: insights from the zebrafish

We used transgenic zebrafish expressing GFP or YFP in subpopulations of neurons to study the migration, homing process and axon extension of groups of CNS neurons in different regions of the zebrafish brain. We found that extensive migration takes place at all levels of the CNS and gives rise to nuclei or cell populations with specific identities. Here, we describe 4 previously unknown or only partially characterized migratory events taking place in the zebrafish telencephalon and rhombic lip, using 3 different transgenic lines, and identify the phenotypes of the cells undertaking these migrations. The migration of a subgroup of mitral cell precursors from the dorsocaudal telencephalon to the olfactory bulb, visualized in the tg(tbr1:YFP) transgenic line, is coupled with morphogenetic transformation of the dorsal telencephalon. The tg(1.4dlx5a-6a:GFP) transgenic line provides a means to analyze the migration of GABAergic interneurons from the ventral to the dorsal telencephalon, thus extending the occurrence of this migration to another vertebrate. The tg(Xeom:GFP) transgenic line provides the first demonstration of the dorsoventral migration of glutamatergic septal neurons, present in mammals and now described in fish, thus reconciling the contrasting evidence of dorsal patterning genes (tbr1, eomes) expressed in a ventral cell population. Furthermore, migration studies in the tg(1.4dlx5a-6a:GFP) and tg(Xeom:GFP) lines help determine the origin of 2 important cell populations in the fish cerebellum: projection neurons and Purkinje cells. These examples reinforce the concept that migratory events contribute to the distribution of cell types with diverse identities through the CNS and that zebrafish transgenic lines represent excellent tools to study these events.