| Abstract: | Long nephrons are derived from juxtamedullary glomeruli. In their descent through the outer medulla toward the inner medulla, the thin descending limbs (TDLs) of long loops of Henle are consistently excluded from the vascular bundles and occupy the interbundle region. The outer medullary segment of long TDLs (Type II epithelium) is elaborately developed with numerous cellular interdigitations. microvilli, and a cytoplasm well equipped with numerous organelles. The inner medullary segment of these long TDLs is characterized by yet another epithelium that is markedly reduced from its Type II predecessor and is designated as Type III epithelium. It is a very low, flattened epithelium with few cytoplasmic details. In a cross section of the inner medullary TDL, the Type III epithelium appears poorly interdigitated, with only two to four cell processes and their junctional complexes. Shortly before the bend of a long loop of Henle, the epithelium is again altered to one that is well-interdigitated and better equipped with cytoplasmic organelles (Type IV epithelium). It persists through the actual bend of the loop and throughout the entire thin ascending limb (TAL), until the latter's transition to the distal tubule at the level of the inner/outer medullary border. We conclude that the Type II epithelium of the outer medullary segments of long TDLs in Psammomys is suitably constructed for involvement in energized transport of solutes. The possibility for a similar role in the Type IV epithelium of the bends and TALs is not excluded by our data. However, the Type III epithelium of the inner medullary segments of long TDLs is most suitably constructed for the concentration of its luminal fluids via water extraction. A comparison between these epithelial types and their permeabilities in various species is presented. |
