Differentiation of cell function should be studied in homogeneous cell populations. The differentiating mammalian kidney is a thoroughly heterogeneous organ. Consequently, the single epithelial segment was studied in situ and, particularly, by in vitro analysis of physical, chemical, and ultrastructural phenomena in perfused and nonperfused dissected nephrons.
Results: The changes of hydraulic permeability in the proximal nephron appear to be important in utilizing extracellular forces for absorption, since cellular transport is restricted by low basolateral membrane area and by ion pump sites (Na+-K+-ATPase). Salt transport in the diluting segment, by contrast, changes at constant osmotic permeability and at increasing ion concentration gradients across the epithelium. Extracellular volume and osmotic homeostasis in the neonate testify to the competence of organ function, although cell structure and function differentiate. In the future, interactions between nephron cell functions will be studied by altering the natural sequence of phenomena during differentiation.