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 n euromere migrate outward to the mantle zone. Here, neuroblasts gradual ly accumulate and aggregate either into sheets (''laminae'') or into r oundish structures (''nuclei''). As brain architecture matures, sets o f 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 embry o, the cell adhesion molecule R-cadherin is expressed in several strip es and patches in the forebrain neuroepithelium. This expression patte rn reflects, at least in part, the neuromeric organization of the fore brain. For example, in both the ventral and dorsal thalamus, R-cadheri n expression has a sharp border at the respective caudal neuromere bou ndary. Moreover, focusing on the mid-hypothalamic region, we demonstra te that a subset of postmitotic neuroblasts in the ventricular zone ex press R-cadherin during their migration to the mantle zone, where they aggregate into particular nuclei. In the mantle zone, R-cadherin-expr essing neuroblasts accumulate in parallel with neuroblasts expressing another cadherin, N-cadherin. The two types of cells segregate from ea ch other to form adjacent nuclei. Some of the R- and the N-cadherin-po sitive nuclei form parts of particular functional circuits in the matu re brain. In conclusion, our results suggest that cadherins play a rol e in the formation of brain nuclei and in the developmental transforma tion from neuromeric to functional organization in the vertebrate fore brain.