Morphology of renal tubular hyaline casts

The structure of renal tubular hyaline casts in kidney biopsies from patients with the nephrotic syndrome was studied and defined by light, immunofluorescence, and electron microscopies. Two distinct types of casts were identified. One, intensely stained with periodic acid-Schiff's reagent, was formed almost exclusively of myriads of fine, hollow tubular fibrils of indefinite length and approximately 40 Angstroms in width. The other, of slight periodic acid-Schiff positivity, was made up chiefly of a finely granular, medium dense matrix, often, however, with similar filaments as above but in considerably lesser quantities, as well as numerous remnants of degenerating cytoplasm. The fibrillar matrix of both of these casts is clearly Tamm-Horsfall protein, previously demonstrated to have identical fine structural features when studied as filaments isolated from urine. Furthermore, with the use of specific fluoresceinated antiserum, most casts were shown to consist of Tamm-Horsfall protein. The filaments comprising the casts may be confused with amyloid because of ultrastructural similarities, especially when observed in urinary sediments.