Morphology and distribution of postnatally generated glial cells in the somatosensory cortex of the rat: an autoradiographic and electron microscopic study.

Postnatal cytogenesis in rat somatosensory cortex was examined by the technique combining light microscopic autoradiography with electron microscopy. Tritiated thymidine was injected intraperitoneally to rats on different days (0-21 days). All animals were sacrificed on the twenty-fifth day after birth. Coronal slices including somatosensory cortex were embedded in epoxy resin. Semithin sections for autoradiography and ultrathin ones for electron microscopy were made alternately. The labeled cortical cells were found mainly in the cases injected with [3H]thymidine during the first and the second weeks. Examination of laminar distribution of the labeled cells revealed that the cells in deeper laminae were labeled on earlier postnatal days than those in more superficial laminae. The labeled cells were examined with electron microscope to identify their nature. By this, it was revealed that ultrastructural morphology of the labeled cells were that of glial cells (either astrocyte or oligodendrocyte). Time and space pattern of this neocortical postnatal gliogenesis shows the tendency of the inside-out sequence, though the pattern is not as distinctive as that of prenatal neocortical neurogenesis. The relationships between the pattern of gliogenesis and the maturation of cortical neurons is suggested.