R-cadherin expression during nucleus formation in chicken forebrain neuromeres.

The primordial neuroepithelium of the vertebrate forebrain consists of transverse and longitudinal morphogenetic compartments ("neuromeres"). During development, neurons born in the ventricular zone of each neuromere migrate outward to the mantle zone. Here, neuroblasts gradually accumulate and aggregate either into sheets ("laminae") or into roundish structures ("nuclei"). As brain architecture matures, sets of nuclei and laminae derived from several neuromeres become connected by fiber tracts to form functional circuits. We show by immunostaining and in situ hybridization techniques that, in the E3-E5 chicken embryo, the cell adhesion molecule R-cadherin is expressed in several stripes and patches in the forebrain neuroepithelium. This expression pattern reflects, at least in part, the neuromeric organization of the forebrain. For example, in both the ventral and dorsal thalamus, R-cadherin expression has a sharp border at the respective caudal neuromere boundary. Moreover, focusing on the mid-hypothalamic region, we demonstrate that a subset of postmitotic neuroblasts in the ventricular zone express R-cadherin during their migration to the mantle zone, where they aggregate into particular nuclei. In the mantle zone, R-cadherin-expressing neuroblasts accumulate in parallel with neuroblasts expressing another cadherin, N-cadherin. The two types of cells segregate from each other to form adjacent nuclei. Some of the R- and the N-cadherin-positive nuclei form parts of particular functional circuits in the mature brain. In conclusion, our results suggest that cadherins play a role in the formation of brain nuclei and in the developmental transformation from neuromeric to functional organization in the vertebrate forebrain.