A study of the fine structure of the kidney in casein-induced amyloidosis in rabbits.

Renal amyloidosis, produced in rabbits by subcutaneous casein injections, was studied by light and electron microscopy. The earliest change before amyloid was detected was the appearance of endothelial cell blebs and mild endothelial cell proliferation. Amyloid was deposited between the basement membrane and the endothelial cell. The cytoplasm of the latter, when over amyloid, was devoid of fenestrations but otherwise had normal inclusions. Basement membrane excrescences were observed both early and late in the development of the disease, while basement membrane thickening was common with advanced disease. The integrity of the lamina densa of the basement membrane appeared to be preserved through the amyloid deposit. Epithelial cell foot process broadened and fused with advanced disease. The absence of the foot processes was particularly noticeable in areas adjacent to amyloid deposits. Epithelial cell cytoplasm contained multiple vacuoles. The cytoplasmic strands connecting the foot processes to the body of the cell were significantly attenuated. The substance of the amyloid consisted of fibrils that were oriented in parallel bundles in areas adjacent to the endothelial cell. The fibrils were <300 A wide and ranged in length up to 5000 A.