The histotopography and ultrastructure of the thin limb of the Henle's loop: A scanning electron microscopic study of the rat kidney.

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Abstract
The three-dimensional construction of the thin limb epithelium was demonstrated in all four segments previously determined by transmission electron microscopy. Special reference was given to the pattern and extent of both intercellular and intracellular interdigitation. In the thin limb of short-looped nephrons, epithelial cells were simple polygonal in shape. The cells showed fine intercellular and intracellular interdigitations of basolateral microprojections. The upper descending thin limb of long-looped nephrons was characterized by well-developed intercellular and intracellular interdigitations. Both types of interdigitations showed a double structure, as they were composed of primary lateral processes or deep basal grooves, with numerous secondary microprojections on their walls. This segment additionally revealed apical interdigitations of small laminar processes, which caused the fine undulation of luminal cell boundaries. In the lower descending thin limb of long-looped nephrons, the intercellular and intracellular interdigitations also showed the double structure, though diminished in degree. This segment lacked the fine apical interdigitations. The ascending thin limb was characterized by an elaborate pectineal pattern of intercellular interdigitations, and by the absence of intracellular interdigitations. The intercellular interdigitations in this segment were formed by thin primary processes oriented parallel to each other. No secondary microprojections were found in their lateral surfaces.