The effects of age on the cells in layer 1 of primate cerebral cortex.

Although there is significant thinning of layer 1 with age in both occipital area 17 and prefrontal area 46 of the rhesus monkey, there are no significant age-related changes in the numbers of neurons, astrocytes, or microglia and oligodendrocytes in this layer. A few profiles of degenerating neurons have been encountered in old monkeys, but they are uncommon. Some astrocytes undergo hypertrophy with age, as evidenced by the increased thickness of the glial limiting membrane, and throughout layer 1 the amount of filaments in the cytoplasm of both their cell bodies and processes increases. The astrocytes also come to contain phagocytic material in the old monkeys, as do the microglial cells. We have previously shown that in both areas 17 and 46 there is an age-related loss of synapses from layer 1 and a concomitant loss of dendritic branches from the apical tufts of pyramidal cells from layer 1. These may be the sources of the material phagocytosed by the astrocytes and microglial cells.