Amyloid in the brains of aged squirrel monkeys.

In this immunocytochemical study, the brains of nine squirrel monkeys (Saimiri sciureus), ranging from 8 to 27 years of age, were examined for the presence and distribution of beta/A4 amyloid, a 4-kilodalton peptide. In aged squirrel monkeys, amyloid is associated primarily with intracerebral and meningeal capillaries and arterioles and occurs to a lesser degree as small and/or diffuse deposits in the neural parenchyma and in the dense cores of senile plaques. Cerebrovascular amyloid is found primarily in neocortex, amygdala, and septum verum and is rare or nonexistent in other subcortical gray structures, white matter, cerebellum, and spinal cord; this pattern of localization is comparable to that in humans with cerebral amyloid angiopathy. There is a significant correlation between cerebrovascular and parenchymal deposits of amyloid. However, cerebrovascular amyloid is always the most abundant form in squirrel monkeys, even in cases of severe cerebral amyloidosis. In contrast to squirrel monkeys, aged rhesus monkeys (Macaca mulatta) develop mostly parenchymal deposits of amyloid and have relatively less vascular amyloid. This species difference in the histological distribution of amyloid suggests that separate mechanisms may influence the accumulation of amyloid in cerebral blood vessels and in the neural parenchyma. These data also indicate that the squirrel monkey can serve as a model for investigations of cerebrovascular amyloidosis.