Identification of mitotic oligodendrocytes in semithin sections of the developing mouse corpus callosum and hippocampal commissure.

Mitotic oligodendrocytes in all phases of mitosis and with processes attached to myelin sheaths were demonstrated in semithin sections of mouse corpus callosum and hippocampal commissure at 17 days postnatum. Mitotic oligodendrocytes make up to one fifth of the mitotic cells present. The majority (54%) of mitotic cells are pale staining glial cell precursors, while the remainder are probably dark glioblasts (23%) and microglial cells (4%). Mitotic oligodendrocytes with both light and medium staining cytoplasm were found in similar stages of mitosis. It is suggested that mitotic oligodendrocytes play only a small part in the large increase in oligodendrocyte number accompanying myelination.