CELLULAR MECHANISMS OF PROTEIN METABOLISM IN THE NEPHRON : I. THE STRUCTURAL ASPECTS OF PROTEINURIA; TUBULAR ABSORPTION, DROPLET FORMATION, AND THE DISPOSAL OF PROTEINS

When proteins pass the glomerular filter they are in part directly absorbed by the epithelial cells of the proximal convolution of the nephron with no apparent alteration of the cytological pattern. If the capacity of the tubule cells to thus absorb protein from the tubule fluid is exceeded either by the amount or the nature of the protein the accessory mechanism of droplet formation occurs. This accessory mechanism is an intracellular process in which cytoplasmic elements, the mitochondria with their enzymes, and the absorbed protein combine to form droplets. As the droplets form and then disappear from the renal cells their evolution presents a constantly changing picture depending on the varying nature of their protein and cytoplasmic content. The droplet is therefore not a cytological structure of fixed characteristics (hyaline droplet) but a locus of metabolic activity and varied structural aspect.