CELL FATE SPECIFICATION AND SYMMETRICAL ASYMMETRICAL DIVISIONS IN THEDEVELOPING CEREBRAL-CORTEX/

Two different modes of cell division are adopted by progenitor cells t o generate the neurons and glia of the cerebral cortex: they either di vide symmetrically to generate other progenitors or a pair of postmito tic cells or divide asymmetrically to generate both a progenitor and a postmitotic cell. In this study we used a lineage marker, the BAG ret rovirus, in embryonic day 16 rats in combination with bromodeoxyuridin e (BrdU) to identify patterns of cell generation in the cerebral corte x, and investigated the relationship between the phenotype of cells an d the history of their lineages. The location, phenotype and birth ord er of clonally related cells were studied in the subsequent 3 weeks. O nly pyramidal neurons and/or astrocytes formed discrete clusters in wh ich several generations of family members were present, whereas nonpyr amidal neurons were found exclusively in pairs or as single cells. Ana lysis of BrdU levels in these cells showed that nonpyramidal neurons w ere originally part of larger clones and were found dispersed in the n eocortex because of tangential migration of their progenitors, dispers ion of postmitotic cells, or death of clonal relatives. These results suggest that both symmetrical and asymmetrical division can be adopted by progenitor cells to generate cortical neurons and glial cells and that cell extrinsic events contribute to the isolation of nonpyramidal neurons.