Preservation of hippocampal neuron numbers in aged rhesus monkeys.

To investigate whether or not aging of nonhuman primates is accompanied by a region-specific neuron loss in the hippocampal formation, we used the optical fractionator technique to obtain stereological estimates of unilateral neuron numbers of the hippocampi of eight young (0-4 years) and five aged (18-31 years) male rhesus monkeys (Macaca mulatta). Our results show a preservation of neurons (mean x 10 (3)+/-S.D.x10(3)) in the subiculum (young=588+/-124, aged=612+/-207), CA1 (young=1051+/-249, aged=1318+/-311), CA2 (young=100+/-18, aged=113+/-12), CA3 (young=478+/-125, aged=509+/-139), hilus (young=337+/-115, aged=394+/-90), and dentate gyrus (young=5550+/-1725, aged=7799+/-2087) of the hippocampal formation. These results confirm a previous stereological study in rhesus monkeys, but are in conflict with data for humans, showing age-dependent region-specific alterations in the hippocampal formation.