The ultrastructure of the nephrons of the desert rodent (Psammomys obesus) kidney. I. Thin limb of Henle of short-looped nephrons

Renal calcium and magnesium handling during pregnancy: modeling and analysis

Pregnancy is associated with elevated demand of most nutrients, with many trace elements and minerals critical for the development of fetus. In particular, calcium (Ca2+) and magnesium (Mg2+) are essential for cellular function, and their deficiency can lead to impaired fetal growth. A key contributor to the homeostasis of these ions is the kidney, which in a pregnant rat undergoes major changes in morphology, hemodynamics, and molecular structure. The goal of this study is to unravel the functional implications of these pregnancy-induced changes in renal handling of Ca2+ and Mg2+, two cations that are essential in a healthy pregnancy. To achieve that goal, we developed computational models of electrolyte and water transport along the nephrons of a rat in mid and late pregnancy. Model simulations reveal a substantial increase in the reabsorption of Mg2+ along the proximal tubules and thick ascending limbs. In contrast, the reabsorption of Ca2+ is increased in the proximal tubules but decreased in the thick ascending limbs, due to the lower transepithelial concentration gradient of Ca2+ along the latter. Despite the enhanced transport capacity, the marked increase in glomerular filtration rate results in elevated urinary excretions of Ca2+ and Mg2+ in pregnancy. Furthermore, we conducted simulations of hypocalcemia and hypomagnesemia. We found that hypocalcemia lowers Ca2+ excretion substantially more than Mg2+ excretion, with this effect being more pronounced in virgin rats than in pregnant ones. Conversely, hypomagnesemia reduces the excretion of Mg2+ and Ca2+ to more similar degrees. These differences can be explained by the greater sensitivity of the calcium-sensing receptor (CaSR) to Ca2+ compared with Mg2+.NEW & NOTEWORTHY A growing fetus' demands of minerals, notably calcium and magnesium, necessitate adaptations in pregnancy. In particular, the kidney undergoes major changes in morphology, hemodynamics, and molecular structure. This computational modeling study provides insights into how these pregnancy-induced renal adaptation impact calcium and magnesium transport along different nephron segments. Model simulations indicate that, despite the enhanced transport capacity, the marked increase in glomerular filtration rate results in elevated urinary excretions of calcium and magnesium in pregnancy.

Kidney Single-cell Transcriptomes Predict Spatial Corticomedullary Gene Expression and Tissue Osmolality Gradients

BACKGROUND

Single-cell transcriptomes from dissociated tissues provide insights into cell types and their gene expression and may harbor additional information on spatial position and the local microenvironment. The kidney's cells are embedded into a gradient of increasing tissue osmolality from the cortex to the medulla, which may alter their transcriptomes and provide cues for spatial reconstruction.

METHODS

Single-cell or single-nuclei mRNA sequencing of dissociated mouse kidneys and of dissected cortex, outer, and inner medulla, to represent the corticomedullary axis, was performed. Computational approaches predicted the spatial ordering of cells along the corticomedullary axis and quantitated expression levels of osmo-responsive genes. In situ hybridization validated computational predictions of spatial gene-expression patterns. The strategy was used to compare single-cell transcriptomes from wild-type mice to those of mice with a collecting duct-specific knockout of the transcription factor grainyhead-like 2 (Grhl2^CD-/-), which display reduced renal medullary osmolality.

RESULTS

Single-cell transcriptomics from dissociated kidneys provided sufficient information to approximately reconstruct the spatial position of kidney tubule cells and to predict corticomedullary gene expression. Spatial gene expression in the kidney changes gradually and osmo-responsive genes follow the physiologic corticomedullary gradient of tissue osmolality. Single-nuclei transcriptomes from Grhl2^CD-/- mice indicated a flattened expression gradient of osmo-responsive genes compared with control mice, consistent with their physiologic phenotype.

CONCLUSIONS

Single-cell transcriptomics from dissociated kidneys facilitated the prediction of spatial gene expression along the corticomedullary axis and quantitation of osmotically regulated genes, allowing the prediction of a physiologic phenotype.

Effects of variations in food intake on renal sodium pump activity and its gene expression in Psammomys kidney

Psammomys obesus lives in an arid environment and feeds on saltbush. When animals are fed a laboratory diet, urine osmolarity drops. To explore the mechanism(s) of water conservation, we measured renal function, kidney solute content, Na-K-ATPase activity, and mRNA in several groups: group I (saltbush diet, 18 g/day, 4.2 g protein); group II (laboratory diet, 10 g/day, 1.8 g protein); and group III, the same as group I, and group IV, the same as group II, both plus a 1-day fast. Urine osmolarity was 2,223 +/- 160, 941 +/- 144, 1,122 +/- 169 and 648 +/- 70.9 mosM in groups I, II, III, and IV, respectively. Tissue osmolarities in cortex, outer medulla, and inner medulla, respectively, were 349 +/- 14, 644 +/- 63, and 1,152 +/- 34 microosM/mg tissue in group I; 317 +/- 24, 493 +/- 17, and 766 +/- 60 microosM/mg tissue in group II; 335 +/- 6, 582 +/- 15, 707 +/- 35 microosM/mg tissue in group III; and 314 +/- 18, 490 +/- 22, and 597 +/- 29 microosM/mg tissue in group IV. There were no differences in Na-K-ATPase activity and mRNA in cortex and in medulla between groups I and II, whereas in group III Na-K-ATPase activity and mRNA increased in cortex and outer medulla. These results suggest a key role for urea in corticomedullary osmotic gradient of Psammomys. The absence of differences in Na-K-ATPase activity and mRNA between groups I and II despite differences in tissue sodium concentrations is consistent with Na-K-ATPase-independent Na absorption. Increased Na-K-ATPase activity and mRNA in fasting suggest transition to Na-K-ATPase- dependent Na transport.

Renal anatomy in sparrows from different environments

The renal anatomy of three species of sparrows, two from mesic areas, the House Sparrow (Passer domesticus) and Song Sparrow (Melospiza melodia), and one salt marsh species, the Savannah Sparrow (Passerculus sandwichensis) was examined. Electron microscopy was used to describe the ultrastructure of the nephron. In addition, stereology was used to quantify the volumes of cortex, medulla, and major vasculature of the kidneys, and the volumes and surface areas occupied by individual nephron components. There appeared to be no differences in the ultrastructural anatomy of the nephrons among the sparrows. Proximal tubules contained both narrow and wide intercellular spaces filled with interdigitations of the basolateral membrane. The thin limbs of Henle contained very wide intercellular spaces which were absent in the thick limbs of Henle. The distal tubule cells contained short, apical microvilli and infoldings of the basolateral membrane. In cross section, the medullary cones of all birds display an outer ring of thick limbs of Henle which surround an inner ring of collecting ducts, which in turn surround a central core of thin limbs of Henle. The Savannah Sparrow has a significantly higher volume of medulla compared to the two more mesic species. Within the cortex, the Savannah Sparrow also has a significantly higher volume of proximal tubules but a significantly lower volume of distal tubules than the other species. Within the medulla, the Savannah Sparrow has a significantly higher volume and surface area of capillaries, and a significantly higher surface area of thick limbs of Henle and collecting ducts than the mesic species. These data suggest that the salt marsh Savannah Sparrow has the renal morphology necessary to produce a more highly concentrated urine than the mesic zone species.

Kidney structure and function of obligate and facultative hibernators: the white-tailed prairie dog (Cynomys leucurus) and the black-tailed prairie dog (Cynomys ludovicianus)

The white-tailed prairie dog is an obligate hibernator that enters a heterothermic phase when maintained in the cold with low intensity light and ad libitum food and water. The black-tailed prairie dog (a facultative hibernator) will not hibernate under similar conditions. It has been suggested that the black tailed prairie dog remains active during the winter because it can conserve water more effectively due to a more efficient kidney. The present study revealed no significant differences between the species in renal morphology: relative medullary thickness, nephron heterogeneity, renal vasculature, or fornix dimensions, all of which are structures associated with the urinary concentrating mechanism. In addition, there was no difference in number of nephrons between the two species. The black-tailed prairie dog does produce a more concentrated urine when food and water deprived. However, this difference was not observed when the animals were salt loaded. The water-deprivation and salt-loading experiments suggest that the higher urine osmolality produced by the back-tailed prairie dog during fasting is a result of a higher urea load due to a greater protein catabolism and not because of a differential capacity to concentrate urine.

Mammalian renal modifications in dry environments

The literature on the role of the kidney and renal morphological modifications in places of limited water supply is reviewed. The anatomical structures for urine concentration found in animals living in desert or arid environments, although not all occurring in one particular animal, are wide medullae, long loops of Henle, long proximal tubules, long collecting tubules, small renal corpuscles, extension of the renal pelvis, well developed elongated papillae, occurrence of giant vascular bundles, specialized ultrastructure of Henle's loops, epithelial changes in the collecting tubule, zonation of the vasa recta and peculiarity of the arterial supply to the kidney. The renal renin content is moderately high in these species. The renin-angiotensin-aldosterone system is very active, retaining Na+ with water. The urine is concentrated at the expense of other electrolytes. Both the renal blood and urinary flow rates are lower than in species with access to unlimited water supply. The juxtaglomerular apparatus components are topographically intimate for effective tubuloglomerular autoregulation of renal blood flow.

Kidney morphology: update 1985

Many new observations have been made correlating changes in renal morphology with changes in renal function. These changes have led to a better definition of the various segments which make up the renal tubule. These morphological patterns vary in different species as is documented by morphologic studies and by differences in the patterns of enzymes in various nephron segments. The subdivision of cells located in the mesangial region of the glomerulus into several types may provide new areas to study with respect to disease processes.

Scanning electron microscopy of basolateral surfaces of rat renal tubules isolated by sequential digestion

Renal tubular cells and segments isolated by a trypsin, pepsin, pronase E digestion procedure were studied with scanning electron microscopy. The basal and lateral surfaces of S1, S2, S3 proximal tubular (PT) segments, descending and ascending thin limbs of Henle (TL), distal ascending thick limb of Henle, or distal straight tubule (DST) and distal convoluted tubule (DCT) segments, connecting tubules (CNT), and collecting ducts (CD) were identified and characterized. The basal processes of the S1 and S2 PT cells were fan shaped, were oriented in a circumferential direction, and terminated in microvilli at the basement membrane. S3 PT cells had microvillous basal processes mainly on the lateral edges of the cells. The basal processes of DST and DCT were similar to PT in orientation but terminated on the basement membrane with flattened, thin attachments. The long-loop descending TL and the ascending TL exhibited distinctive interdigitating cell processes. TL segments with simple contours were present in smaller numbers and were characteristic of short-loop descending limbs. CNT showed some cells with basal surfaces resembling DCT cells and others resembling CD cells. Both cortical and medullary CD segments exhibited intercalated cells with round basal contours and a sparse pattern of basal infolding clefts. The cortical CD principal cells revealed a much more elaborate mosaic of plicae, clefts, and microvilli than those of the medullary CD. These observations extend the previous knowledge gained from transmission electron microscopy and assist in the interpretation of that knowledge.

Functional heterogeneity of the descending limbs of Henle's loop. I. Internephron heterogeneity in the hamster kidney

By using the in vitro microperfusion technique, were examined functions of the descending limbs of Henle's loop obtained from either the short-loop nephron (SDL) or the upper portion of the long-loop nephron of hamsters (LDLu). Morphological distinctions between these segments were confirmed by light and electron microscopic observations. Both segments were highly permeable to water. The LDLu was highly permeable to sodium and to chloride: efflux coefficients (10(-7) cm2 x s-1) for 22Na and 36Cl were 41.0 +/- 5.4 and 3.8 +/- 0.6, respectively. The SDL were less permeable to sodium and to chloride: efflux coefficient for 22Na and 36Cl were 2.9 +/- 1.4 and 0.9 +/- 0.2, respectively. In contrast, the SDL was more permeable to urea as compared to the LDL, efflux coefficients for urea being 5.1 +/- 1.4 vs 1.4 +/- 0.3, respectively. When composition of the perfusate was identical to that of the bathing fluid, no transepithelial voltage was demonstrated. The volume flux was very small or undetectable. From these observations, we propose that the internephron heterogeneity must be taken into consideration for constructing a model of countercurrent system in the renal medulla.

Functional heterogeneity of the descending limbs of Henle's loop. II. Interspecies differences among rabbits, rats, and hamsters

Permeability properties of the descending limbs of Henle's loop were compared among rabbits, hamsters, and rats by measuring transepithelial voltage (Vt) across the isolated renal tubules perfused in vitro. From the deflection of the Vt when the composition of the bathing fluid was varied, the permeabilities of sodium and of potassium relative to chloride (Pna/PCl and PK/PCl, respectively) were determined in either the descending limbs of the short-loop nephron (SDL) or the segments of the upper protion of the long-loop nephron (LDLu). In hamsters and rats, the values of PNa/PCl of the LDLu (3.98 +/- 0.66 and 5.03 +/- 0.79) were higher than those of the SDL (0.68 +/- 0.03 and 0.61 +/- 0.00). In contrast, in rabbits the value of PNa/PCl of the LDLu (0.96 +/- 0.05) was only slightly higher than that of the SDLu (0.75 +/- 0.03). The similar tendency was also noted in the values of PK/PCl. In hamsters and rats, the PK/PCi ratios were 4.90 +/- 0.82 and 6.44 +/- 0.90, respectively, in the LDLu and 1.09 +/- 0.04 and 1.02 +/- 0.0, respectively in the SDL. When a transepithelial osmotic gradient was imposed by adding raffinose to the bath, a lumen-negative streaming voltage of about -8 mV was generated in the hamster and the rat LDLu. Taken together with the findings in the preceding paper, these observations support the view that the descending limbs of rabbits are different from those of hamsters and rats in that internephron heterogeneity is less remarkable, and that the LDLu of hamsters and rats is highly permeable to sodium and to potassium as well as to water.

Freeze-fracture studies on the thin limbs of Henle's loop in Psammomys obesus

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 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 mu 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 mu 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 mu 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 mu 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.

Ultrastructural organization of the transition from the distal nephron to the collecting duct in the desert rodent Psammomys obesus

The transition from the nephron to the collecting duct is formed by three tubular segments (convoluted part of the distal tubule, connecting tubule, cortical collecting duct), which in the desert rodent, Psammomys obesus, transform gradually from one segment to the next, due to intermingling of their different cell types. The convoluted part of the distal tubule (DTC) starts abruptly, shortly beyond the macula densa and initially is homogeneously composed of characteristic DTC-cells. Subsequently, the DTC-cells intermingle with intercalated cells. The first appearance of the connecting-tubule cell, which gradually replaces the DTC-cell, is regarded as the beginning of the connecting tubule. The major portion of the connecting tubule is lined by connecting-tubule cells and intercalated cells. The first appearance of the principal cell between them defines the beginning of the cortical collecting duct, which in the medullary ray is lined by principal and intercalated cells only. Each cell type is described in detail and discussed in relation to the assumed function of the tubular segments. Interspecies differences in the cellular composition of the transitional zone from the nephron to the collecting duct are discussed in relation to the different organization of the collecting duct system.

Regional membrane specialization in the thin limbs of Henle's loops as seen by freeze-fracture electron microscopy

Rat thin limbs of Henle were studied by freeze-fracture electron microscopy. Thin limb segments in both short- and long-looped nephrons were identified by previously developed ultrastructural criteria, continuity with known thick segments, and architectural relationships in the outer medulla. Intramembrane particle (IMP) density and the number of intramembrane fibrils comprising the zonula occludens were determined for each morphologically identifiable thin limb segment. The IMP density on the protoplasmic faces of both the luminal and abluminal membranes of the upper portion of the descending thin limb (DTL) of the long-looped nephron is quantitatively greater than in the short-looped thin limb, lower portion of the long-looped DTL, and in the ascending thin limb. The zonulae occludens in the long-looped upper DTL consists of a single fibril; the long-looped lower DTL contains 3.13 +/- 0.14 fibrils; the ascending thin limb contains 1.31 +/- 0.09 fibrils; and the short-looped DTL contains 3.75 +/- 0.19 fibrils. These studies further support the contention that there is anatomic heterogeneity among the thin limb segments. Because direct physiologic studies in the thin limbs are incomplete and conflicting, the need for correlative physiologic studies on anatomically characterized structures is indicated.

The ultrastructural localization of membrane ATPase in rat thin limbs of the loop of Henle

The cytochemical distribution of nonspecific membrane ATPase activity in the epithelial membranes of the thin limbs of the loops of Henle of rat nephrons was studied at the ultrastructural level. Membrane ATPase activity was localized in the luminal, lateral, and (to a lesser extent) basal membranes of only the outer medullary segment of the thin descending limbs of long nephrons (Type II epithelium). The reaction product was lacking in the thin limb of short nephrons (Type I epithelium) as well as in the inner medullary descending (Type III epithelium) and ascending (Type IV epithelium) segments of the thin limbs of long nephrons. These data reinforce the concept of thin limb heterogeneity and may indicate a specialized role for the outer medullary segment of thin descending limbs of long nephrons in the concentrating mechanism.