Specification of dorsoventral polarity in the embryonic chick mesencephalon and its presumptive role in midbrain morphogenesis.

The chick midbrain is subdivided into functionally distinct ventral and dorsal domains, tegmentum and optic tectum. In the mature tectum, neurons are organized in layers, while they form discrete nuclei in the tegmentum. Dorsoventral (DV) specification of the early midbrain should thus play a crucial role for the organization of the neuronal circuitry in optic tectum and tegmentum. To investigate regional commitment and establishment of cellular differences along the midbrain DV axis, we examined the commitment of gene expression patterns in isolated ventral and dorsal tissue in vivo and in vitro, and studied their cell mixing properties. Use of explant cultures, and grafting of dorsal midbrain into a ventral environment or vice versa, revealed a gradual increase in the autonomy of region-specific gene regulation between stages 12 and 18 (embryonic day 2 to 3). This process becomes independent of the activity of midline organizers, such as floor and roof plate, by stage 16. Once the DV axis polarity is fixed, cells from dorsal and ventral midbrain adopt differential adhesive properties. Thus between stages 18 to 23 (embryonic day 3 and 4), cells of dorsal and ventral origin start to separate from each other, at a time-point when the majority of midbrain cells is not yet differentiated. Hence, our results suggest that progressive specification of the midbrain DV axis is accompanied by progressively reduced cell mixing between dorsal and ventral precursors, leading to a partial regionalization of midbrain tissue into autonomous units of precursor cell populations.