Synaptic density in the inner molecular layer of the hippocampal dentate gyrus in Alzheimer disease.

We examined the inner molecular layer (IML) of the hippocampal dentate gyrus for possible changes in synaptic density. Material was obtained from 9 individuals with Alzheimer disease (AD) and compared to samples obtained from 10 age-matched, postmortem-matched neurologically normal controls, employing standard ultrastructural techniques. Statistical analyses demonstrated a significant decline in synaptic numbers between controls and AD subjects. This decline was accompanied by a significant increase in apposition length and resulted in a significant correlation with the synaptic density. As the number of synapses declined, the apposition length increased. Assessment was also made of the granule cells density and the analyses showed a significant decline in the synapse to granule cell ratio in the AD group. This decline in the density of synaptic contacts in the IML reflects a more widespread decline in plasticity in AD and may be related to the memory problems associated with the disease.