| Abstract: | The mammalian cerebral cortex, although a structure of greatcomplexity, is characterized by a high degree of organizationwhere the proportions, spatial relationships, and propertiesof the various cell types are rigidly controlled. The mechanismsresponsible for the creation of such a rigid distribution ofcell types are not known. Lineage studies in adult rats havesuggested that each of the cortical progenitor cells liningthe telencephalic ventricles during embryonic development givesrise to progeny of the same phenotype (homogeneous clones).However, the possibility that homogeneous clones are the resultof complex processes affecting both the final number and thephenotype of clonally related cells during development has notbeen investigated. In the present study, we followed the developmentof cortical cell lineages labeled with retroviral injectionsat embryonic day (E) 16 in rats of 7, 14, or 21 d of age usingelectron microscopy and immunocytochemistry for the neurotransmittersglutamate and GABA. We found that a significant number of corticalclones at postnatal day (P) 7 and P14, and fewer at P21, showedmixed pyramidal/nonpyramidal cell composition. We sometimesobserved that "mixed" neuronal clones contained cells immunoreactivefor both glutamate and GABA. In the general population of corticalcells, "bireactive" neurons represented 3.7% of all neuronsat P7, 1.8% at P14, and 0.6% in adult rats. Although the changein the composition of neuronal clones between the third weekof postnatal life and adulthood may be due to changes in thephenotype of some developing neurons, we would like to suggestthat it is probably due to selective neuronal cell death. |
