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Specialized junction in the distal convoluted tubule of rat kidney

The ultrastructure and permeability of the apical junctions between epithelial cells of the distal nephron have been studied in rat kidney using a colloidal lanthanum tracer and uranyl acetate staining en bloc. The apical intercellular junctions of the macula densa and juxtaglomerular segment of the early distal convoluted tubule measure up to 0.5 μ in length and about 50 Å in width. Lanthanum permeates the occluding portion of these junctions in a discontinuous manner, defining a series of closely spaced and parallel lines of fusion that run in a direction perpendicular to the apical‐basal axis of the tubular cells. The apical junctions of the remainder of the distal convoluted and cortical collecting tubules are impermeable to lanthanum. This distinctive apical tight junction can account for the greater permeability to ions of the early distal convoluted vs. late distal convoluted and cortical collecting tubules.