The Surface Ultrastructure of Urinary Casts

Abstract

A method for the scanning electron microscopic (SEM) study of elements contained in the urinary sediment is described. Abnormal urinary sediments were obtained from patients having a wide variety of intrinsic renal diseases as well as from “normals.” The sediments were initially observed using ordinary light, phase and interference microscopy. For study using the scanning electron microscope they were then filtered through a membrane filter and fixed with cold phosphate-buffered 3.5 glutaraldehyde. The filters containing urinary sediment were washed, postfixed in 1% osmium tetroxide, dehydrated and critical-point dried using carbon dioxide, then mounted and sputter-coated with gold–palladium for study using scanning electron microscopy. Preliminary data have indicated numerous, previously undescribed, ultramicroscopic peculiarities of cast structure. A basic and fundamental structure was found to be common to all casts studied, and consisted of a fibrillar protein meshwork. Hyaline casts were composed entirely of this complex proteinaceous material. The same structural material was present in cellular and waxy casts. Blood and epithelial cells were found to be closely adherent to the fibrillar cast matrix in cellular casts. In many instances they were attached by means of encompassing fibrils, in contrast to the time-honored theory of their incorporation within the matrix during cast formation.