No abstract

This study gives a three-dimensional (3D) structural analysis of rat nephrons and their connections to collecting ducts. Approximately 4,500 2.5-m-thick serial sections from the renal surface to the papillary tip were obtained from each of 3 kidneys of Wistar rats. Digital images were recorded and aligned into three image stacks and traced from image to image. Short-loop nephrons (SLNs), long-loop nephrons (LLNs), and collecting ducts (CDs) were reconstructed in 3D. We identified a well-defined boundary between the outer stripe and the inner stripe of the outer medulla corresponding to the transition of descending thick limbs to descending thin limbs and between the inner stripe and the inner medulla, i.e., the transition of ascending thin limbs into ascending thick limbs of LLNs. In all nephrons, a mosaic pattern of proximal tubule (PT) cells and descending thin limb (DTL) cells was observed at the transition between the PT and the DTL. The course of the LLNs revealed tortuous proximal "straight" tubules and winding of the DTLs within the outer half of the inner stripe. The localization of loop bends of SLNs in the inner stripe of the outer medulla and the bends of LLNs in the inner medulla reflected the localization of their glomeruli; i.e., the deeper the glomerulus, the deeper the bend. Each CD drained approximately three to six nephrons with a different pattern than previously established in mice. This information will provide a basis for evaluation of structural changes within nephrons as a result of physiological or pharmaceutical intervention.rat kidney morphology; three-dimensional structural analysis; digital tracing THE RAT KIDNEY HAS FOR MANY years been the target of a large number of functional and morphological investigations (10 -13, 17) and was recently reviewed (5). In recent years, especially the rat renal medulla has been the subject of intensive comparative structural-functional studies and computer modeling [see e.g., Refs. 15 and 20 and very recently a very comprehensive study on modeling calcium transport in the rat nephron was published (28)]. A prerequisite for conducting these kinds of studies is that the detailed morphology of the kidney is known. However, such a detailed three-dimensional (3D) analysis enabling identification of structural changes at a precisely defined distance from the glomerulus of the different segments along the nephron and collecting duct (CD) in the kidney is currently not available. We have previously described the 3D organization and ultrastructural segmental variation of the mouse kidney nephrons and CDs (29, 32). To describe the variation in kidney morphology in different species, we decided to study the morphology of the rat kidney in detail. We have developed a combined histological and computerized technique, which enables the reconstruction of a large number of rat nephrons in 3D space. Consequently, the present study gives a detailed 3D analysis of rat nephrons, both short-loop nephrons (SLNs) and long-loop nephrons (LLNs), including a description of t...

The aim was to quantify the glomerular capillary surface area, the segmental tubular radius, length, and area of single nephrons in mouse and rat kidneys. Multiple 2.5-µm-thick serial Epon sections were obtained from three mouse and three rat kidneys for three-dimensional reconstruction of the nephron tubules. Micrographs were aligned for each kidney, and 359 nephrons were traced and their segments localized. Thirty mouse and thirty rat nephrons were selected for further investigation. The luminal radius of each segment was determined by two methods. The luminal surface area was estimated from the radius and length of each segment. High-resolution micrographs were recorded for five rat glomeruli, and the capillary surface area determined. The capillary volume and surface area were corrected for glomerular shrinkage. A positive correlation was found between glomerular capillary area and proximal tubule area. The thickest part of the nephron, i.e., the proximal tubule, was followed by the thinnest part of the nephron, i.e., the descending thin limb, and the diameters of the seven identified nephron segments share the same rank in the two species. The radius and length measurements from mouse and rat nephrons generally share the same pattern; rat tubular radius-to-mouse tubular radius ratio ≈ 1.47, and rat tubular length-to-mouse tubular length ratio ≈ 2.29, suggesting relatively longer tubules in the rat. The detailed tables of mouse and rat glomerular capillary area and segmental radius, length, and area values may be used to enhance understanding of the associated physiology, including existing steady-state models of the urine-concentrating mechanism.