Biomedical subjects
Publications and source records attributed to W Kriz.
The microscopic structure of the various nephron segments comprising the medullary counterflow system is described. The architectural organization of the tubules and blood vessels in the inner and outer medulla is also described. Species differences are noted. A hypothesis that the thicker parts of the descending limbs of Henle actively secrete salt is advanced.
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The blood vessels of the renal medulla have several particular features. The supplying vessels of the renal medulla are the efferent arterioles of the juxtamedullary glomeruli. Thus, the blood supply is entirely postglomerular. The distribution of the blood within the renal medulla is effected by the descending vasa recta. Descending and ascending vasa recta form the vascular bundles. Descending vasa recta leave the bundles at any level of the medulla to feed the adjacent capillaries; the longest descending vasa recta reach the papillary tip. The capillary plexus are differently developed. A dense capillary plexus is present only in the inner stripe of the outer medulla. In the inner medulla the capillary plexuses are poorly developed. At these sites ascending recta contribute to the capillarisation. The ascending vasa recta originate from capillaries at any level of the medulla and ascend without joining together towards the intrarenal veins at the cortico-medullary border. They are capillary vessels with wide lumina. Within the outer stripe of the outer medulla these vessels are very narrowly associated with the tubules; interstitial spaces are very poorly developed at this site. These relationships are interpreted as a possible weak point of the medullary circulation in pathological situations.
The vascular poles of 30 renal corpuscles were studied in critical-point dried renal cortex of the Munich-Wistar rat by scanning electron microscope (SEM). In 28 cases one afferent and one efferent arteriole were observed; one was done in the Sprague-Dawley rat, where 34 renal corpuscles were examined. One afferent and a single efferent arteriole were found in 32 cases; in one case only one afferent and two efferent arterioles were observed. In another case it was questioned whether a single or a double efferent arteriole is present. In addition, the specimens allowed an evaluation of the area where the extraglomerular mesangium passes into the mesangial cells of the glomerular tuft. This area was found to be rather small to represent a "punctum fixum" for a contraction of the entire mesangium.
In the kidney of the Syrian hamster the descending thin limbs of both the short and long loops of Henle are not spatially separated from each other and descend between the vascular bundles. Ultrastructurally, five different epithelial types are distinguished in the thin limbs of the short and long loops of Henle. Short loops possess only a descending thin limb with a simply organized epithelium (type 1). Long loops comprise an upper and a lower part of the descending thin limb and the ascending thin limb. The upper part of the long descending thin limb is equipped with a complex and highly interdigitating epithelium with shallow junctions (type 2), which gradually transforms into the simple noninterdigitating type-3 epithelium of the lower part. In a minor portion of long descending thin limbs, however, the upper part begins with an even more complexly organized epithelium (type 2a) than type 2. Type-2a epithelium is conspicuously thicker and possesses a more elaborate mode of cellular interdigitation. Along the descent of this tubular part through the inner stripe of the outer medulla, type-2a epithelium transforms into type-2 epithelium. It is suggested that the long descending thin limbs, which start with type-2a epithelium, belong to the longest loops. The type-4 epithelium of the ascending thin limbs is characterized by flat and extensively interdigitating cells with shallow junctions. The unique pattern of the type-2a epithelium favors the assumption that solute secretion essentially contributes to the increase in concentration of tubular fluid in long descending thin limbs.
Thin kidney cortex explants from newborn rabbits, consisting of the capsula fibrosa with an adherent layer of nephrogenic material (s-shaped bodies and collecting duct anlagen) were isolated and cultured for various periods (up to 15 days) by conventional procedures. Within the first 24 h of culturing, it was observed that the explants form globular bodies covered by a monolayer epithelium. The cells are cuboidal or cylindrical, and may, in later stages, consist of two or even more cell layers. The epithelium shows a polar differentiation, with deep tight junctions at the apical poles and lateral intercellular spaces opening to the basal surface. A basal lamina is not consistently developed. The support of the epithelium consists of fibroblasts and degenerating tissue elements of the explants. It can be shown that the epithelium is derived from the anlagen of the collecting ducts. Structurally, the epithelium resembles a collecting duct epithelium. The experimental system is considered as a useful model to study metabolism and function of the renal collecting duct epithelium, as well as developmental problems, such as cell migration, outgrowth, and differentiation.
Goormaghtigh cells of the JGA are characterized by an extensive cellular ramification. In order to elucidate the shape and arrangement of the cell processes a three-dimensional model of a Goormaghtigh cell and of an adjacent granular cell has been constructed based on electron micrographs of a series of ultrathin sections. The model shows that a Goormaghtigh cell has the shape of a flatly pressed cylinder with both ends splitting up into a bunch of parallel processes. The processes maintain a close neighboring position and do not intermingle with processes of other Goormaghtigh cells. This feature is most puzzling when considering that Goormaghtigh cells and their processes are extensively connected by gap junctions. Even processes belonging to the same cell are electrically coupled with each other through gap junctions. The granular cells are clearly different in shape from Goormaghtigh cells. In granular cells bunches of processes are lacking. Granular cells obviously ramify into a few, large processes. The present findings are consistent with the assumption of a functionally central position of Goormaghtigh cells within the feedback mechanism of the JGA.
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The macula densa in rabbits and rats was studied by transmission electron microscopy (TEM). After different experimental procedures, the kidneys were fixed by direct perfusion with the fixative without prior flushing of the blood. With this technique, the renal cortex is consistently well preserved. The lateral intercellular spaces of all proximal and distal nephron segments were constantly found to be closed, whereas those of the macula densa varied: depending on the functional situation of the kidney, they were found to be either dilated or closed. Dilated intercellular spaces in the macula densa were encountered in control, sodium-rich, and sodium-deficient rabbits and rats and, in addition, in rats with hypotonic and isotonic hypervolemia. In contrast, in rats with hypertonic hypovolemia and in rats undergoing furosemide or mannitol diuresis, the lateral intercellular spaces of the macula densa were closed. Whether these findings reflect the in vivo state of the macula densa interspaces remains uncertain. The association with specific functional stages demonstrates, at least, a specific behavior of the macula densa cells that is different from that of all other proximal and distal nephron segments and appears to be similar to that of the collecting duct epithelium. The findings suggest that the macula densa is a water-permeable cell plaque within the otherwise water-impermeable thick ascending limb.
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The thin limbs of short and long loops of Henle of the desert rodent Psammomys obesus were studied by freeze-fracture techniques. Intercellular junctions and internal membrane characteristics of thin-limb epithelia are of interest with regard to the high urine-concentrating capacity of this animal. The epithelium of the descending thin limbs of short loops is remarkably undifferentiated and equipped with multistrand tight junctions. In the descending thin limb of long loops, two segments are to be distinguished. The upper parts are characterized by an extensive cellular interdigitation and single-strand tight junctions. Thus, the paracellular pathways are prominent from two aspects: the junctional belt is elongated by interdigitation, and its apico-basal depth is shallow. The transition from the upper to the lower part appears to be abrupt, as indicated by the change in intramembrane particle density. The lower parts are characterized by a noninterdigitating epithelium with junctions consisting of few, but always more than two, strands. In addition, this thin-limb segment is characterized by regularly distributed infoldings of the basal cell membrane. The ascending thin limbs are established by an interdigitation epithelium, with junctions generally consisting of one strand. Once again, the elongated junctional belt is shallow. This study presents further evidence that remarkable species difference occur among thin-limb epithelia, especially concerning the descending thin limbs of long loops. Those differences may well explain discrepant functional findings concerning the transport properties of this segment in various species.
The renal medulla develops very differently among species, being more prominent in those with a high urinary concentrating capacity. Attempts to correlate structure and function must consider loops of Henle, collecting ducts, vessels, interstitium, and pelvis. Two types of loops of Henle, long and short, are distinguished. Numerical relationships between both differ among species. Based on the epithelial lining a short loop consists of a thick descending limb (pars recta of proximal tubule), a thin descending limb, and a thick ascending limb. Long loops, in addition, have a thin ascending limb; their descending thin limbs are different from those of short loops and are site of considerable interspecies differences. Collecting ducts form in the cortex by joining several nephrons. Patterns with and without arcade formation are distinguished. On entering inner medulla, collecting ducts fuse successively. Collecting duct epithelium consists of principal and intercalated cells whose individual functions are subject to debate. Blood vessels are arranged in a very strict pattern reflecting that, in addition to nourishment, unique requirements in maintaining the corticomedullary osmotic gradient are to be met. Ultrastructural organization of medullary vessels is less specific compared to cortical vessels. Two types of renal medulla are distinguished. The simple type has vascular bundles consisting only of de- and ascending vasa recta; in the complex type, descending thin limbs of short loops are also integrated into vascular bundles. Functional implications of this difference are considerable. Striking interspecies differences also occur in the renal pelvis.
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Isolated glomerular preparations are known for their capability to synthesize proteins. A thorough biochemical separation and characterization of newly synthesized proteins, however, is lacking. In this study isolated glomeruli from rat kidney (enriched by sieving technique alone or by a combination of sieving and subsequent further enrichment by either sucrose or Ficoll discontinuous centrifugation) were incubated for various periods with protein and glycoprotein precursors. The synthesis of soluble and membrane-bound proteins was studied. Separation of glomerular proteins was carried out by means of SDS-electrophoresis in 6 M urea. The radioactivity of newly synthesized proteins was determined by the scintillation technique. In the leucine-incubated glomeruli, a broad spectrum of newly synthesized monomer proteins ranging from 90,000 to 25,000 daltons was detected, whereas in the glucosamine and galactose incubated glomeruli synthesis of glycoproteins of molecular weights ranging mainly between 90,000 and 60,000 daltons could be observed. Most of the radioactivity incorporated into isolated glomeruli was regained in 600 g (containing basement membrane with residual mesangial/endothelial nuclei and cellular debris) and 12,000 g (membrane structures and mitochondria) pellets, indicating that most of the newly synthesized proteins are membrane bound. A component of newly synthesized glycoprotein (72,000 daltons) could be extracted by mild NaCl treatment and is partly soluble in desoxycholate. Further experiments utilizing isoelectrofocusing and two-dimensional electrophoresis are in progress.
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