Deterioration threshold of synaptic morphology in aging and senile dementia of Alzheimer's type.

OBJECTIVE

To search for a deterioration threshold of synaptic ultrastructure in physiologic aging and senile dementia of Alzheimer's type (SDAT).

STUDY DESIGN

The numerical density (Nu), average area (S) and surface density (Su) of the synaptic contact zones were investigated in the hippocampus and cerebellum from adults, elderly and SDAT patients by means of computer-assisted morphometry.

RESULTS

The Nu and surface Su densities of the synapses decreased while the size of S enlarged in elderly and SDAT patients vs. the adults. By plotting Nu vs. S, the adult group showed a very high percentage of small junctions. In contrast, elderly and dementia patients demonstrated two overlapping populations of enlarged contacts. In SDAT the synapse-to-neuron ratio was decreased by 48% in the hippocampus and by 56% in the cerebellum.

CONCLUSION

Our findings support the concept that the degeneration of synaptic contacts per se should be considered a crucial step in the progression of senile dementia, but the identification of a discrete deterioration threshold of synaptic morphology between aging and SDAT is not feasible at present.