A further evaluation of the effect of age on striate cortex of the rhesus monkey.

The brains of 14 rhesus monkeys (Macaca mulatta) between 4 and 35 years old were examined to determine the effects of aging on the thickness, neuronal frequency, fine structure, surface area, and volume of striate cortex. The effects of aging were ascertained by comparing the striate cortex in the six monkeys between 4 and 12 years of age with that of the eight monkeys over 25 years of age. The brains of the monkeys were all fixed by vascular perfusion and except for one of the old monkeys, whose age was estimated, the exact ages of all of the monkeys are known. One micron thick sections of plastic embedded cortex from one hemisphere of each monkey were examined by light microscopy to determine the thickness of the striate cortex, and neuronal frequency was determined by counting the numbers of neurons displaying nuclei in 250 microns-wide strips passing through the thickness of the cortex. When young monkeys were compared with the old ones, no differences were found in either the thickness of the cortex or in the numbers of neuronal profiles beneath unit areas of cortical surface. This suggests that neurons are not lost with age, and when the cortices were examined by electron microscopy there was no indication that the cell bodies of neurons are degenerating, except possibly in layer 1. Serial, 30 microns-thick, Nissl stained frozen sections from the other hemisphere of each monkey were used to determine both the surface area and the volume of the striate cortex. Overall, the surface area varied between 702 and 1480 mm2, with a mean value of 956 mm2, but there was no indication that the surface area decreased with age, and the same is true for the volume of striate cortex. The conclusion is that while there is a large variation in the amount of cortex occupied by area 17, there is no indication that its thickness, volume, or number of neurons is altered by age.