Ultrastructure of Amphiuma distal nephron: evidence for cellular heterogeneity.

To obtain more information on the ultrastructure of the distal nephron of the salamander, Amphiuma, we conducted freeze-fracture electron microscopy and morphometric experiments. In the early distal tubule, the organization of the tight junction is variable, containing from one to two strands in the proximal region and four strands in distal regions. The length density of the tight junction in this segment varies from greater than 60 m/cm2 of apical membrane surface to less than 10/cm2 of apical membrane surface. These observations agree with a previous study demonstrating that the junction of this segment exhibits considerable axial heterogeneity. The junctions of the late distal tubule and collecting tubule are more complex. In the late distal tubule, the tight junction is composed of 6-8 strands, whereas the tight junction of the collecting tubule is composed of 8-12 strands. The collecting tubule contains principal cells and two types of intercalated cells: alpha and beta. The alpha-cells contain a high density of rod-shaped particles in the apical plasma membrane and in membranes of apical cytoplasmic vesicles. The beta-cells contain rod-shaped particles only in the basolateral membrane. In principal cells, we observed a novel organization of intramembranous particles within the apical plasma membrane. A model describing the relationship of the two types of intramembranous particles within the membrane is presented. This study demonstrates that the amphibian and mammalian distal nephron share many morphological characteristics including cellular and axial heterogeneity.