The ultrastructure of the nephrons of the desert rodent <i>(Psammomys obesus)</i> kidney I. Thin limb of henle of short‐looped nephrons

Abstract

Abstract Previous studies by Kaissling et al. ('75) showed that superficial and midcortical glomeruli of the Psammomys kidney give rise to short nephrons which do not enter the inner medulla. The thin descending limbs (TDLs) of such short nephrons descend into the inner stripe of the outer medulla only within vascular bundles. The TDLs of long nephrons are excluded from these bundles. Transmission EM was used in the present study to examine the ultrastructure of short TDLs in relation to the arterial and venous vasa recta. In the interbundle region of the uppermost outer medulla, proximal tubules of short nephrons give rise to TDLs classified as having Type II (interbundle type) epithelium. Type II is simplified to Type I (bundle type) epithelium where the short TDL enters the vascular bundle. It persists throughout the vascular bundle and continues briefly into the interbundle region until its transition to the distal tubule. Type I epithelium is extremely thin and simple. These cells are poorly inter‐digitated and have simple junctional complexes. An intimate association with the venous vasa recta is seen. Comparison of these data with other micropuncture studies shows that the 4‐fold concentration of urea in the distal nephron is probably correlated with the ultrastructural characteristics of short TDLs and their isolation within vascular bundles. This is augmented by the uptake of urinary urea from the pelvic extensions (Kaissling et al., '75). Thus, the loop of Henle of short nephrons must be regarded as an intrinsic part of the complex multiplier system of the Psammomys kidney.