Electron microscopic features and proliferation of astrocytes in the corpus callosum of the rat

Abstract The gold chloride sublimate method of Ramón y Cajal was used to stain astrocytes in the corpus callosum of 60–80 g male rats. The opacity of the metal‐stained astrocytes to the electron beam allowed their study in the electron microscope. It was thus found that metal deposits are absent from the nucleus but are present over two cytoplasmic structures: bundles of filaments and dense bodies. Once the features of astrocytes had been observed in metal stained preparations, it became possible to identify them in sections stained for electron microscopy by the routine uranyl‐lead sequence. The presence of filament bundles in the cytoplasm was the most useful diagnostic feature. Dense bodies also helped. The nucleus was relatively large and light with a very distinct, often irregular nuclear envelope. The rather light cytoplasm contained glycogen granules, with few ribosomes and cisterns of endoplasmic reticulusm. However, a few otherwise typical astrocytes had fairly dark nucleus and cytoplasm. After gaining experience with the electron microscope, it was possible to identify most astrocytes in the light microscope by their nuclear features. Counts of the frequency of astrocytes using either the light or electron microscope revealed that they make up about one quarter of the glial population of the corpus callosum. The main properties of astrocytes seemed to be: (1) plasticity of the cell surface, which infiltrates all available spaces around it; (2) apparent rigidity of the bundles of filaments present in the processes, where they may play a supporting role; and (3) ability of astrocytes to undergo division, as demonstrated both by the uptake of 3 H‐thymidine into 1.7% of these cells and by the presence of mitotic figures.