PRESERVATION INTO OLD-AGE OF SYNAPTIC NUMBER AND SIZE IN THE SUPRAGRANULAR LAYER OF THE DENTATE GYRUS IN RHESUS-MONKEYS

In order to determine whether there are age-related changes in the sup ragranular layer of the dentate gyrus of the rhesus monkey, we examine d this layer in monkeys 4-35 years of age, spanning the entire range o f adulthood of this species. Electron microscopic analyses were conduc ted to determine whether there is an age-related change in the number of synapsing axon terminals, in the cross-sectional area of these term inals, or in the length of the postsynaptic density at the synaptic ju nction. Only asymmetrical synapses in the anterior dentate gyrus were evaluated. Tn a subset of our monkeys (n = 6, ages 4-31 years), we com pared three different approaches to the estimation of synaptic density : (1) the conventional profile method, in which synapse numbers are ex pressed per unit area of the examined tissue section; (2) the empirica l formula of Colonnier and Beaulieu for converting areal densities int o number of synapses per unit volume, and (3) the 'disector' method, a stereological approach to the estimation of the number of synapses pe r unit volume that makes no assumption about the shape of the objects. Data are presented validating the small-fold method of estimating sec tion thickness for the disector. The three methods were highly interco rrelated (r's approximate to 0.89), and none of the methods revealed a n age-related loss of synapses. Analysis of the thickness of the denta te gyrus molecular layer suggests that the reference volume in which t hese synapses were counted does not change with age. In addition, the conventional profile method showed no age-related change in the number of axodendritic or axospinous synapses, the cross-sectional area of t he synapsing terminals, or the length of the postsynaptic densities of synapses. Together, these data suggest a remarkable age-related prese rvation of synapses in the normal aging monkey.