During the past five years studies on proteinuria induced in rats by the intraperitoneal injection of various proteins have been carried on in the laboratory of the Renal Clinic at Stanford Hospital. The integrating ideas behind the design of the studies were that the undamaged glomerulus of the rat is normally permeable to small amounts of protein; that in the course of the formation of the large daily glomerular filtration volume a relatively large amount of protein is filtered through; that some reabsorption of the protein takes place along the course of the renal tubule; that the renal epithelium can reabsorb a certain maximum amount; and that, therefore, proteinuria will result if the renal epithelium is overloaded with protein. These studies were carried on without systematic histological investigation of the available material. It is the purpose of the present paper to report some of the preliminary findings, the primary concern here being to trace the fate of the injected hemoglobin with the aid of histological and histochemical techniques.The notion of glomerular filtration and tubular reabsorption of large molecules, including proteins, is not new. Athrocytosis has been described by GCrard (1) as a "characteristic property of the vertebrate nephron" arising after the absorption of electro-negative colloids through the apical surfaces of renal epithelium, followed by intracellular "flocculation" of the absorbed material. The term athrocytosis, according to Lison (2), was first used by Burian, later by von Mollendorf, then resurrected, after a period of disuse, by G~rard and Cordier (1); it is equivalent to the German term Speicherung. Randerath (3), in a comprehensive survey of the histogenesis of the various forms of nephritis, has reviewed the work of recent German and Belgian authors. He concludes that in "nephrosis" altered permeability of the glomerular capillaries permits the passage of large amounts of protein which produces the characteristic histological alterations; i. e., thickening of the glomerular basement membrane and hyaline droplet formation in the renal epithelium, the latter being the visible evidence of athrocytosis or Speicherung.The human glomerular membrane is usually considered to be impermeable to protein, for all practical purposes, by most authors in this country. Passage of protein, indicated by its appearance in the urine, is taken as evidence of damage to the glomerular "filter." However, Oliver (4) has recently reviewed certain considerations which 153 on
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