SEGREGATION OF RHOMBOMERES BY DIFFERENTIAL CHEMOAFFINITY

The developing hindbrain is transiently subdivided into structural rep eat units, rhombomeres, whose formation is matched by both differentia l 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 interrhomb omere interfaces, cells are confined within the territory of a single rhombomere. in order to assess the relevance of this restriction to hi ndbrain development, it is necessary to understand the underlying mech anism. One possibility is that cells of adjacent: rhombomeres acquire differential affinities or adhesive properties. To explore this possib ility, we isolated rhombomere cells, mixed them together in short-term aggregation cultures, and assessed the composition of the resulting a ggregates. We found that rhombomeres do differ in their affinity: cell s from even-numbered rhombomeres sort out from cells of odd-numbered r hombomeres. They also segregate from cells of other even-numbered rhom bomeres but to a much lesser extent. This selective cell affinity oper ates from the time of rhombomere formation until late stages in develo pment. The region-specific segregation was abolished when Ca2+-depende nt adhesion molecules were inactivated but not when Ca2+-independent a dhesion molecules were inactivated. These findings suggest that distin ct cell affinity restricts cell mixing between adjacent rhombomeres an d may be Involved in establishing the series of discrete compartments, thereby maintaining anteroposterior positional information during hin dbrain development. These results support a general role for cell adhe sion molecules in subdividing CNS territories.