Three-dimensional functional reconstruction of inner medullary thin limbs of Henle's loop.

Digital three-dimensional (3-D) functional reconstructions of inner medullary nephrons were performed. Antibodies against aquaporins (AQP)-1 and -2 and the chloride channel ClC-K1 identified descending thin limbs (DTLs), collecting ducts (CDs), and ascending thin limbs (ATLs), respectively, through indirect immunofluorescence. Tubules were labeled in transverse sections and assembled into 3-D arrays, permitting individual tubule or combined surface representations to depths of 3.3 mm to be viewed in an interactive digital model. Surface representations of 75 tubules positioned near the central region of the inner medulla were reconstructed. In most DTL segments that form loops below 1 mm from the inner medullary base, AQP1 expression begins at the base, becomes intermittent for variable lengths, and continues nearly midway to the loop. The terminal DTL segment exhibiting undetectable AQP1 represents nearly 60% of the distance from the medullary base to the tip of the loop. AQP1 expression was entirely undetectable in shorter long-looped DTLs. ClC-K1 is expressed continuously along the terminal portion of all DTLs reconstructed here, beginning with a prebend region approximately 164 microm before the bend in all tubules and continuing through the entire ascent of the ATLs to the base of the inner medulla. CDs express AQP2 continuously and extensive branching patterns are illustrated. 3-D functional reconstruction of inner medullary nephrons is capable of showing axial distribution of membrane proteins in tubules of the inner medulla and can contribute to further development and refinement of models that attempt to elucidate the concentrating mechanism.