Structure of avian loop of Henle as related to countercurrent multiplier system

The ultrastructural detail of the avian loop of Henle was examined, and comparisons were made to the loop of Henle of mammalian kidneys. Birds are the only group of vertebrates other than mammals that have the capability of elaborating a urine more concentrated than the plasma. Therefore, a comparison of the principal tubular element responsible for this phenomenon was made. Gambel's quail (Callipepla gambelii) were used as experimental animals. The animals were anesthetized with Inactin, and the kidneys were perfused-fixed in situ. The results show that the transition from the pars recta of the proximal tubule to the thin descending limb of Henle's loop (DLLH) is very abrupt. The upper part of the DLLH appears to be composed of type 2 and the lower part of type 3 epithelia, a condition somewhat similar to mammals. The junctions of the upper portion of DLLH are of the zonula adherens type and those of the lower portion of DLLH are of the macula adherens type. The epithelium of the loop always thickens before the formation of the hairpin turn with the thick descending limb being approximately 15% of the total DLLH. The structure of the avian renal medulla, in particular that of the loop of Henle, appears to parallel quite closely that postulated by the original formulation of the countercurrent multiplier system for the mammalian kidney.