Segregation of rhombomeres by differential chemoaffinity.

The developing hindbrain is transiently subdivided into structural repeat units, rhombomeres, whose formation is matched by both differential regulatory gene expression and a metameric pattern of early neuronal differentiation and axogenesis. Individual rhombomeres are polyclonal cell lineage restriction units; once defined by transverse interrhombomere interfaces, cells are confined within the territory of a single rhombomere. In order to assess the relevance of this restriction to hindbrain development. It is necessary to understand the underlying mechanism. One possibility is that cells of adjacent rhombomeres acquire differential affinities or adhesive properties. To explore this possibility, we isolated rhombomere cells, mixed them together in short-term aggregation cultures, and assessed the composition of the resulting aggregates. We found that rhombomeres do differ in their affinity: cells from even-numbered rhombomeres sort out from cells of odd-numbered rhombomeres. They also segregate from cells of other even-numbered rhombomeres but to a much lesser extent. This selective cell affinity operates from the time of rhombomere formation until late stages in development. The region-specific segregation was abolished when Ca(2+)-dependent adhesion molecules were inactivated but not when Ca(2+)-independent adhesion molecules were inactivated. These findings suggest that distinct cell affinity restricts cell mixing between adjacent rhombomeres and may be involved in establishing the series of discrete compartments, thereby maintaining anteroposterior positional information during hindbrain development. These results support a general role for cell adhesion molecules in subdividing CNS territories.