Rat thin limbs of Henle (TL) were studied by electron microscopy. Descending and ascending thin limbs (DTL, ATL) were identified by tracing their continuity with known structures and by histotopography. Two types of TL were identified. Type I showed cellular interdigitation and very shallow intercellular occluding junctions. Junctions between noninterdigitating cells in type II had longer apical basal depths. In short loops, DTL were type II. In long loops, DTL were type I in the outer medulla and possessed 'complex' features such as microvilli and basal cytoplasmic invaginations. 'Complex' type I DTL extended to varying distances in the inner medulla, where they underwent transition to type II limbs. ATL were type I, but appeared 'simple' compared to the upper portions of DTL (no microvilli or basal invaginations). Transition from type II to 'simple' type I epithelium occurred in the terminal portions of DTL at short distances (56 to 133μ) before their junctions with ATL at the bends. Counts showed that profiles of 'complex' type I DTL were outnumbered by type II DTL in the upper one half, and virtually absent in the lower one half of the inner medulla. Ultrastructural studies suggest that the degree of passive ion permeation across epithelia may be determined by the depth of their occluding junctions. Thus, type I cell junctions may be more permeable than type II. Cellular interdigitation provides for a larger paracellular pathway in type I limbs, and thus may enhance their permeability. These considerations suggest a role for TL structural differences in medullary countercurrent mechanisms.
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