Investigation of glial cells in semithin sections. II. Variation with age in the numbers of the various glial cell types in rat cortex and corpus callosum

Abstract

Semithin Epon sections stained with toluidine blue were used to enumerate astrocytes, microglia, the three subtypes of oligodendrocytes, and cells referred to as free subependymal cells, in the corpus callosum and cerebral cortex of male Sherman rats of various ages. The period covered extended from a few days before weaning (3/4 month of age) until the time when growth became negligible (5 months of age). The total number of glial cells increases with age in both cortex and corpus callosum. However, the investigation of individual cell types reveals that the number of microglia remains fairly constant throughout the period under study. The number of astrocytes in corpus callosum increases up to the age of one month, but remains constant thereafter, while their number in the cortex is the same at all investigated times. In the case of oligodendrocytes, the three subtypes behave differently. About two‐thirds of the oligodendrocytes in rats aged three‐quarters of a month are of the light or medium shade types, but the number of these gradually decreases with age and becomes very low in five‐month‐old rats. In contrast, the dark cells which constitute about one‐third of the oligodendrocytes in young rats make up nearly the whole of this group in adults. Finally, free subependymal cells are absent in cortex throughout the period under study, but are present in corpus callosum, where their number steadily declines with age. In conclusion, the numbers of astrocytes and microglia seem to remain constant in growing rats after the age of one month. Dark oligodendrocytes markedly increase in number with age, while the other types of oligodendrocytes and the free subependymal cells are reduced to negligible numbers by the age of five months.