Autoradiographic investigations of glial proliferation in the brain of adult mice. The DNA synthesis phase of neuroglia and endothelial cells

The proliferation of glial cells outside the subependymal layer of the lateral ventricle as well as of endothelial cells was studied autoradiographically in the brain of the adult and untreated mouse. The double labeling method with 3 H‐ and 14 C‐thymidine was applied in order to show experimentally the existence of a DNA synthesis phase (S phase) and to measure its duration. Adult mice received a first injection of 14 C‐thymidine, two or four hours later a second injection of 3 H‐thymidine and were sacrificed one hour after the last injection by perfusion fixation. Double layer autoradiographs were made from serial sections of the region from the corpus callosum/commissura anterior up to the corpus callosum/commissura fornicis ventralis in order to register purely 3 H‐, doubly 3 H‐ and 14 C‐, and purely 14 C‐labeled nuclei. From the ratio of all 3 H‐labeled cells with and without 14 C to the purely 3 H‐labeled cells a DNA synthesis phase of 9.4 ± 0.5 hours for glial cells and one of 11.0 ± 2.2 hours for endothelial cells was obtained. Based on the first appearance of labeled mitoses and labeled pairs of glial cells after injections of labeled thymidine the G 2 phase was estimated to be < three hours and G 2 + M about five hours. The duration of the measured S phase as well as the appearance of labeled mitoses about three hours after application of labeled thymidine are very similar to these cycle parameters in many other somatic cells in different kinds of animals. This has led to the conclusion that a well‐defined DNA synthesis phase with doubling of the DNA content and a successive mitosis also exists in glial and endothelial cells of the adult mouse brain.