Electronmicroscopic observation of amyloid deposits in the vascular walls of the choroid plexus in systemic amyloidosis

This report concerns an ultrastructural study of the vessels of the choroid plexus in 3 cases of systemic amyloidosis, sections of formalin-fixed brains being used. Small-sized arteries and arterioles were mainly affected by deposition of amyloid fibrils. In the arterioles, the amyloid fibrils were often so densely accumulated that they separated individual smooth muscle cells of the media and reached the subendothelial region. The basal surface of the endothelium of such vessels showed frequent invaginations filled with amyloid tufts; some of the tufts displaced the thin cytoplasm of the endothelial cells toward the lumen. Occasionally, tufts of amyloid fibrils were found to be exposed to and to float in the vascular lumen. These observations were made only in severely amyloid-laden vessels. Among the several possibilities regarding the tuft-endothelial relationship, the transendothelial transport of amyloid tufts is most likely, but the possibility of intraendothelial processing of amyloid precursor proteins and their secretion to form fibrils extracellularly cannot be excluded.