Electron microscopic identification of three classes of oligodendrocytes and a preliminary study of their proliferative activity in the corpus callosum of young rats

Abstract

In recent electron microscope studies of the corpus callosum in 60–80 gm male rats, some of the cells were identified as microglia by staining with the weak silver carbonate method of del Rio‐Hortega, and others as astrocytes by staining with the gold chloride sublimate method of Ramón y Cajal. In the present work, the cells which did not belong in one of these two groups were examined in the light microscope using semithin sections stained with toluidine blue and in the electron microscope using thin sections stained with uranyl‐lead. These cells make up a large, somewhat heterogeneous group, but they have a few common features: regular nucleus, cytoplasm rich in ribosomes and microtubules, and a variable number of narrow, non‐branching fine processes of uniform diameter. Because of these features, the cells are all taken to be oligodendrocytes. Three classes may be described within this group of cells: (a) light oligodendrocytes, which are large cells with pale nucleus, a large nucleolus, a cytoplasm containing rather small organelles as well as many free ribosomes, and giving off numerous fine processes; they make up about 6% of the glial cells in the corpus callosum; and they undergo mitosis at a rapid rate; (b) medium‐shade oligodendrocytes, which are somewhat smaller cells with moderately dense nucleus and cytoplasm, containing well developed organelles, and giving off a fair number of fine processes; they make up about a quarter of the glial cells; and they undergo mitosis at a moderate rate; (c) dark oligodendrocytes, which are even smaller cells with very dense nucleus and cytoplasm, containing a prominent Golgi and structures referred to as lamellar bodies, and giving off very few fine processes; these cells make up about 40% of the glial cells; they do not undergo mitosis and probably arise from divisions of medium oligodendrocytes.