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×103±S.D.×103) 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.