The ultrastructure of the thin limbs of henle in kidneys of the desert heteromyid Perognathus penicillatus

The thin limbs of both long‐ and short‐looped nephrons in Perognathus kidneys were studied with transmission and scanning electron microscopy. The superficial nephrons have a short thin limb located in the vascular bundles of the outer medulla and are characterized by a simple, low‐lying epithelium (0.4 ± 0.1 μ thickness). In contrast, the first descending part of the thin limb of the majority of midcortical and juxtamedullary nephrons has a relatively thick epithelium (1.7 ± 0.6 μ in thickness) with marked lateral and basal interdigitation and a dense surface covering of microvilli. The remaining part of the long descending thin limb is relatively simple with a low‐lying epithelium (0.6 ± 0.1 μ in thickness), decorated on its surface by sparse microplicae. The bend of the loop and the ascending limb are covered by a very simple low‐lying epithelium (0.6 ± 0.2 μ in thickness) with relatively little surface modification. The extreme urine‐concentrating ability of Perognathus does not appear to be due to the development of a unique thin loop epithelium but rather to the extensive length of the inner and outer medulla.