Amyloid-β deposits in the cerebral cortex of the aged common marmoset (Callithrix jacchus): incidence and chemical composition

The incidence, distribution and chemical composition of amyloid-β (Aβ) peptide-positive deposits were investigated in the lower primate species common marmoset (Callithrix jacchus). No Aβ deposits were observed in the brains of 7 marmosets below 7 years of age. In 15 marmosets above 7 years, 60% displayed cortical Aβ-immunoreactive plaques, 80% had Aβ deposited in intracortical vessels and 87% displayed Aβ deposits in meningeal vessels. The cerebral cortex of the oldest animal (15 years) contained a substantial density of deposits. Aβ-immunoreactive plaques were found predominantly in association cortical zones followed by a lower density in paralimbic cortical areas. Deposits within vessels were most frequent in occipital cortex. Aβ40 was found primarily in vascular deposits, while Aβ42 was present in plaques. Approximately 20% of plaques and most vascular deposits displayed thioflavin S staining, indicative of the presence of fibrillar Aβ. Varying proportions of Aβ deposits contained acetylcholinesterase or butyrylcholinesterase activities and apolipoprotein E and α1-antichymotrypsin immunoreactivity. A few plaques contained immunoreactivity for amyloid precursor protein in swollen neurites. However, no abnormally phosphorylated tau immunoreactivity was present in these neurites. Survival analysis in a colony of marmosets indicated that only 6% of animals can be expected to survive beyond 7 years of age. These results indicate that the aged marmoset brain displays Aβ deposits with a distribution and chemical composition similar to those found in the human. These similarities suggest that the aged marmoset may be a useful lower primate model for the study of the pathological effects of Aβ. However, the relatively small number of animals which can be expected to reach old age severely limits the utility of this species as a model of Aβ deposition.