Expression domains of neuroD (nrd) in the early postembryonic zebrafish brain.

NeuroD acts as one of the earliest transcription factors promoting differentiation in newborn neurons immediately after determination by neurogenin1 (ngn1). Here, we show the expression of neuroD in the 2 days postfertilization (dpf) zebrafish brain by in situ hybridization on sections. Generally, neuroD expressing cells lie one to several cell rows away from the ventricular surface, i.e., they seem to flank proliferative cells. However, most cells of the gray matter lie even more peripheral and are unstained. Highly specific local patterns of neuroD expressing cells are observed along the entire rostrocaudal zebrafish brain axis at that age with characteristic neuroD expression free zones (e.g., preoptic region and hypothalamus). Interestingly, these regions where neuroD expression is absent display a higher proliferative activity compared to neuroD-positive areas. To date it is an open question, whether this negative correlation of neuroD-expression and proliferation has a causal background.