Detailed immunohistology of Pax6 protein and tyrosine hydroxylase in the early zebrafish brain suggests role of Pax6 gene in development of dopaminergic diencephalic neurons.

Spatiotemporal developmental dynamics of Pax6 protein containing (i.e., Pax6) cells were investigated immunohistochemically in embryonic and postembryonic zebrafish brain sections (especially at 2 and 5 day), allowing for a neuroanatomically detailed resolution previously only reported for the mouse. Besides strikingly close correspondences of early Pax6 domains - including many spatiotemporal changes - in mouse and zebrafish brains, some critical differences were noted. There is no pallial (i.e., cortical) Pax6 expression domain in the ventricular proliferative layer in the zebrafish as in the mouse. The main pallial Pax6 domain in the zebrafish is comparable to the migrating stream of Pax6 cells at the pallial-subpallial boundary. This indicates that some developmental functions of Pax6 (i.e., inhibition of subpallial cell migration into pallium by Pax6 migrating stream) might be shared with the mouse and maybe all vertebrates, while others (i.e., control of intrapallial neuronal radial migration via Pax6 expressing radial glia cells) may be special for mammals. Another prominent feature in the early zebrafish forebrain is that the alar plate ventral thalamic Pax6 domain extends far caudolaterally into the periphery of the basal plate posterior tuberculum and hypothalamic inferior lobe. This indicates that the alar plate ventral thalamus invades the forebrain basal plate and contributes to the development of basal forebrain structures. The close spatiotemporal association of Pax6 cells and TH cells of the ventral thalamus indicates a local role of Pax6 in the development of ventral thalamic (as recently demonstrated in the mouse) and, maybe, posterior tubercular TH cells. However, our confocal microscopical analysis of zebrafish brain sections double-immunostained for Pax6 and TH did not reveal cells double-labeled for these two proteins in this location, but rather indicates an inductive interaction of Pax6 cells onto TH cells.