Investigation of glial cells in semithin sections. III. Transformation of subependymal cells into glial cells, as shown by radioautography after 3 H‐thymidine injection into the lateral ventricle of the brain of young rats

Strong labeling of the cells in the subependymal layer was produced by stereotaxic injection of 5 μCi of 3 H‐thymidine into the left lateral ventricle of the brain of one and a quarter month old rats weighing about 100 gm. These animals were sacrificed by glutaraldehyde perfusion from two hours to 21 days later. Blocks of corpus callosum with adjacent subependymal and ependymal layers were excised from the injected and non‐injected sides, and embedded in Epon; 0.5 μ thick sections were radioautographed and stained with toluidine blue. In the subependymal region, on both injected and non‐injected sides, there was an immediate uptake of label by many cells followed by an increase and later a decrease in the percent cells labeled. In the corpus callosum while at first the percent labeling of glial cells was rather low, it did increase slowly with time and, after seven days, exceeded that in the subependymal region. These results were interpreted as indicating that cells arising in the subependymal layer had migrated into the corpus callosum. Up to four days after injection, most of the label in corpus callosum was present in immature‐looking cells resembling the cells of the subependymal layer and referred to as free subependymal cells. With time, the percent labeling decreased in these cells while increasing in some of the glial cells. A labeling peak was observed for light oligodendrocytes at four to seven days and for dark oligodendrocytes at 21 days, whereas labeling of medium shade oligodendrocytes occurred at intermediate times. The succession of labeling peaks indicated a sequence of development from free subependymal cells through light and medium shade to dark oligodendrocytes. Few astrocytes carried label at any time; those which did seemed to have arisen from the transformation of labeled free subependymal cells. Microglia were unlabeled at two hours, but their percent labeling was high at 4–14 days. While the labeling of other glial cells reflected their physiological behavior, the labeling of microglia was a consequence of the trauma produced by the injection 0f tracer into the ventricle. In conclusion, cells coming from the subependymal layer appear to migrate into the corpus callosum where, in 100 gm rats, many of them transform into oligodendrocytes and a few into astrocytes.