Similarities of membrane structure in freeze-fractured Xenopus laevis kidney collecting tubule and urinary bladder.

The collecting tubule of Xenopus laevis kidney is formed of two main types of cell: the socalled flask cells (or mitochondria-rich cells) and the remaining, more cuboidal epithelial cells. It has previously been shown that the flask-cell plasma membrane contains a population of elongated intramembrane particles similar to those found in mitochondria-rich cells of the toad bladder. It is now clear that the structure of the apical membrane of the remaining epithelial cells of the collecting duct is similar to the apical membrane of the amphibian urinary bladder granular cells. The P-face of the apical membrane has relatively few particles, whereas the E-face has many more. The E-face particles are of large diameter (16 nm), and many of them have an apical dense spot, which may represent a pit or depression in the particle. Such particles are not found on the lateral E-face below the level of the tight junctions. At the present time, the functional significance of these particles is unknown, but since vasopressin fails to elicit a hydrosmotic response in Xenopus laevis they are probably not involved in transepithelial water permeability. The fact that the different membrane specializations which characterize these mitochondria-rich and non-mitochondria-rich cells are found both in the bladder and the collecting tubule suggests that, at least in Xenopus, the 2 epithelia may share some common functions at the level of the apical membranes of their constituent cells.