Identification of microglia in light and electron microscopy

Abstract

Abstract The corpus callosum of 60–80 gm male rats was processed by the weak silver carbonate method of del Rio‐Hortega for the detection of microglia in the light microscope. Because of the opacity of the metal stain taken up by these cells, they could be recognized in the electron microscope. The characteristic features of microglia were thus found to be a small nucleus in which dark chromatin contrasts with light nucleoplasm, and a cytoplasm containing dense bodies (presumed to be lysosomes) and poor in endoplasmic reticulum. After gaining experience with microglia in the electron microscope, it became possible to recognize their features in 0.5–1 μ thick toluidine blue stained Epon sections examined in the light microscope. In the electron microscope, some microglia are found within expansions of the basement membrane of capillaries ( pericytal microglia ); they usually have little cytoplasm, few dense bodies and few processes. There is evidence that these cells can break out of their basement membrane enclosure and enter the brain tissue proper. There, they scatter singly between other cells and fibers. These cells ( interstitial microglia ) send of long processes. They may contain numerous dense bodies and be surrounded by irregular spaces (suggesting the existence of surface activity). These observations support the classical view of microglia as brain macrophages. Microglia constitute 5.5% of the glial population of the corpus callosum. After injection of 3 H‐thymidine and radioautography in the light or electron microscope, none showed labeling. Hence, it is presumed that microglia do not proliferate under normal conditions.