Segmental Differentiation in the Proximal Convoluted Tubule of the Mammalian Nephron

Edwards, in discussing the proximal convoluted tubule of the mammalian nephron, refers to “its functional differentiation as manifest segmentally unaccompanied by obvious cytological change.” Edwards and Schnitter state that the proximal convoluted tubule of the mammal “is cytologically uniform throughout its length.” In his most recent publication, Edwards mentions the “unisegmental appearance” of the mammalian proximal convolution, and states that “it is probable that the obvious presence of definite segments in the proximal convolution of the renal tubule in fish has been cytologically concealed but functionally retained in this convolution in the kidney of other vertebrates.” Contrary to the statements of Edwards, convincing evidence of at least a bisegmental differentiation of the mammalian proximal convoluted tubule on cytological grounds was presented 25 years ago by Zimmermann. In the dog and cat he found, in the pars recta, an abrupt transition from an epithelium with markedly irregular cell outlines (to be designated here as first segment) to one with sharp, rectilinear cell outlines, in general hexagonal in shape (to be designated here as second segment). In the cat the cells of the first segment showed abundant vacuoles (fat), while the second segment was entirely free of them. In the dog, on the other hand, just the opposite was true, the vacuoles being confined to the second segment. The striking difference, with respect to fat storage, in the terminal portion of the proximal convoluted tubule in these two animals had been described previously by Peter. In his study of the cat Model16 missed the fat-free terminal portion entirely, and has been rightly criticized by Nakamra.7