Decrease in the number of synapses in the senescent brain: a quantitative electron microscopic analysis of the dentate gyrus molecular layer in the rat.

Axo-dendritic synapses were counted in electron micrographs taken from the middle third of the dentate gyrus molecular layer of young adult and senescent Fischer 344 rats. A significant decrease in the number of synapses was found in senescent animals relative to young ones. This loss of synapses, which involved all the morphological varieties of axo-dendritic synaptic contacts in the dentate gyrus molecular layer, appeared to be unrelated to changes in dimensions of synapses, tissue volume or number of postsynaptic granule cells. It is proposed that the age-related loss of synaptic contacts might be attributed to a reduced capacity of senescent brains for synaptic regeneration and remodelling.