Size frequency distributions of abnormal protein deposits in Alzheimer's disease and variant Creutzfeldt-Jakob disease.

The size frequency distributions of beta-amyloid (A beta) and prion protein (PrPsc) deposits were studied in Alzheimer's disease (AD) and the variant form of Creutzfeldt-Jakob disease (vCJD) respectively. All size distributions were unimodal and positively skewed. A beta deposits reached a greater maximum size and their distributions were significantly less skewed than the PrPsc deposits. All distributions were approximately log-normal in shape but only the diffuse PrPsc deposits did not deviate significantly from a log-normal model. There were fewer larger classic A beta deposits than predicted and the florid PrPsc deposits occupied a more restricted size range than predicted by a log-normal model. Hence, A beta deposits exhibit greater growth than the corresponding PrPsc deposits. Surface diffusion may be particularly important in determining the growth of the diffuse PrPsc deposits. In addition, there are factors limiting the maximum size of the A beta and florid PrPsc deposits.