Spatial and temporal pattern of postnatal proliferation of glial cells in the parietal cortex of the rat.

Rats were injected with [3H]thymidine on different postnatal days (PD 0 approximately PD 16) and sacrificed after 6 h. Parietal cortices were embedded in epoxy resin and then semithin sections for autoradiography and ultrathin ones for electron microscopy were made alternately. On the day of birth (PD 0), about 70% of total labeled cells within the cortical gray, which were proved to be glial cells, were observed in the inner half of the cortical width. On PD 4, however, the intracortical distribution of labeled cells was reversed, i.e. about 70% of labeled cells were found in the outer half. Thereafter, on PD 6 and PD 8, no significant difference of the number of the labeled cells could be noted between the outer and the inner halves of the cortical gray. The pattern of glial proliferation in the parietal cortex of early postnatal rat thus showed an inside-out tendency, although not so distinctive as that of prenatal neurogenesis in the neocortex. Electron microscopy of the labeled cells revealed that on earliest days, they showed the condensation of nuclear chromatin and abundant free ribosomes, both suggesting the immaturity of cells, while, on later days, the nuclear chromatin became dispersed and free ribosomes decreased, indicating the progress of cellular differentiation as glial cells.