In order to determine whether newly born cells in the dentate gyrus of the adult rat express the neuronal marker, neuron-specific enolase, or the glial marker, glial fibrillary acidic protein, we performed combined immunohistochemistry and autoradiography on brains from adult rats perfused at various times ranging from 1 h to four weeks following [3H]thymidine administration. Light-microscopic examination revealed a negligible number of [3H]thymidine-labeled cells showing neuron-specific enolase immunoreactivity during mitosis. However, by two weeks after [3H]thymidine administration, a significant increase in the density of [3H]thymidine-labeled neuron-specific enolase-immunoreactive cells was detected. Three weeks following [3H]thymidine injection the majority of [3H]thymidine-labeled cells (> 70%) were immunoreactive for the neuronal marker. At the four-week time-point, [3H]thymidine-labeled neuronspecific enolase-immunoreactive cells were indistinguishable from neighboring granule cells. In contrast, glial fibrillary acidic protein immunoreactivity was observed in a small but significant number of [3H]thymidine cells at the 1-h time-point and the proportion of labeled cells that were immunoreactive for this cell marker did not increase with time. [3H]Thymidine-labeled cells that were immunoreactive for glial fibrillary acidic protein typically showed morphologic characteristics of radial glia at all time-points. At the 1-h time-point, the majority of [3H]thymidine-labeled cells were observed in the hilus (> 60%) with the remainder being located in the granule cell layer. However, with a four-week survival-time most [3H]thymidine-labeled cells (> 85%) were located in the granule cell layer. The majority of newly born cells in the adult dentate gyrus differentiate into neurons. Some of these cells are born locally in the granule cell layer while others arise from the hilus and migrate to the granule cell layer. The presence of [3H]thymidine-labeled presumptive radial glia observed in the adult dentate gyrus supports the contention that many of the newly born neurons migrate. These results set the stage for future studies aimed at determining the factors that regulate these processes.
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