Synapse loss and gliosis in the molecular layer of the cerebral cortex in Alzheimer's disease and in frontal lobe degeneration.

Changes in density of synapses and astrocytes in the molecular layer of the frontal and parietal cortex were compared in Alzheimer's disease (AD) and frontal lobe degeneration of non-Alzheimer type (FLD). The investigation was limited to the molecular layer because it is possible in this part of the cortex to measure changes in synapses and astrocytes without contamination by nerve cell body changes. In the frontal pole synapse density declined by 40% in both FLD and AD whereas in the parietal area there was a 50% decrease in synapse density in AD but no significant change in FLD. Number of astrocytes showed an inverse relationship to synapse density. There was a significant increase in astrocytes in the frontal cortex in both FLD and AD but in the parietal cortex such an increase was seen only in AD. These results confirm previous reports of synapse loss in AD and demonstrate a similar loss in FLD in the frontal, but not parietal cortex. The findings underscore the regional pattern changes of FLD, previously shown for other parameters, and its difference from that of AD. We propose that these changes in molecular layer may be representative of the pathology (and the functional deficit) within the underlying cortical layers.