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Studies of cell lineage in the rat cerebral cortex have provided new insights into the mechanisms of neuronal and glial determination. They have shown that clonally related cells, marked with retrovirus injection at embryonic day 16 (E16), express the same glial or neuronal phenotype, suggesting that separate progenitors for each of these cell phenotypes exist in the ventricular zone at that stage of corticogenesis. However, it is not known if such committed progenitors are present in the ventricular zone before E16. Another important question concerns which neurochemical features are shared by clonally related cells of the adult cerebral cortex. In this study we have addressed the first question by injecting a retroviral vector expressing beta-galactosidase into the telencephalic ventricles of rat embryos at different stages (E14-E19). In order to classify clonally related neurons in the cerebral cortex of these rats, we have used postembedding immunohistochemistry for the amino acid neurotransmitters glutamate, aspartate, and GABA. Glutamate and GABA immunoreactivity marked nonoverlapping populations of cells that corresponded to the pyramidal and nonpyramidal neuron types of the rat cerebral cortex. Clonally related neurons, marked by retrovirus injection at any day between E14 and E19, homogeneously expressed one or other phenotype and accordingly displayed glutamate or GABA immunoreactivity. This finding indicates that committed progenitor cells for pyramidal and nonpyramidal neurons are present in the ventricular zone before E16. To investigate whether lineage dictates other features in clonally related neurons, we performed an immunohistochemical analysis for the calcium- binding proteins calbindin, parvalbumin, and calretinin in clusters of clonally related nonpyramidal neurons. The same calcium-binding protein was rarely found in members of the same cluster, suggesting that lineage does not control the expression of calcium-binding proteins in cortical nonpyramidal neurons. As a result of examining a large number of clonally related neurons from brains injected at different ages, we observed remarkable differences in number and laminar distribution of pyramidal and nonpyramidal neurons marked with retrovirus. Clusters of nonpyramidal neurons were usually composed of two or three cells, and resided in the cortical layers that were just being generated at the time of injection. Clusters of pyramidal neurons were larger and dispersed in several layers in the earlier injections; their size and laminar distribution were progressively reduced for later injections. These observations suggest the existence of different mechanisms that generate the pyramidal and nonpyramidal neurons of the cerebral cortex.

Lineage analysis reveals neurotransmitter (GABA or glutamate) but not calcium-binding protein homogeneity in clonally related cortical neurons