Morphologic Analysis of Tubular Transport

Abstract

Transporting epithelia are layers of cells which separate compartments with different composition. Any epithelium therefore represents a barrier, which on the one hand prevents equilibration and on the other hand provides the transportation of substances from one side to the other and thus permanently contributes to the maintenance in composition differences between the compartments being separated. The transporting epithelium of the kidneys’ functional units, the nephrons, is single-layered and composed of polarized cells (Fig. 1). The apical or luminal pole directly contacts the urinary space, whereas the basal pole faces the interstitial fluid and is further attached to the basement membrane which stabilizes this tubular type of epithelium. If one traces a nephron from its beginning at the renal corpuscle down to the renal papilla, it must be recognized that the epithelium changes its assembly quite drastically. Therefore, we may divide the nephron in a sequence of segments each assembled by different cell types. From studies performed over the last decade we know that these various types of epithelium are confined to sometimes very differentiated transport mechanisms. By this reason we must pose the question: What does cellular or epithelial morphology tell us about transport performance? In fact we must confess that there are only a very few clearcut morphologic correlates to transport and only our experience, also obtained from studies on other transporting epithelia, leads to this correlation. The most prominent correlate is an enlarged cell surface at the basal part produced by folds or interdigitations of cells and a surface enriched in glycoproteins — a glycocalyx — at the apical side.