Microscopic anatomy of the triton kidney

The structure of the nephron, length of all its segments and the renal architectonics as a whole have been studied in the newt (Triturus vulgaris L.) by means of microdissection and microinjection methods. The microinjection of latex or lissamine green is performed into the blood vessels and nephrons. In one kidney 85-95 nephrons are counted, their number is not the same along the kidney length and increases in the caudal direction. There are nephrons of the ventral, intrarenal and medial populations. The length of the former is 2.7 times as great as that of the latter. A relative length of the nephron segments changes slightly. In all the ventral nephrons a nephrostome is detected. A specific peculiarity of the newt nephron that differs it from that in other vertebrates is the presence of a long distal segment, heterogeneous by its structure. Superficial canaliculi of the kidney are strictly oriented: medially to the renal corpuscles there are loops of the first part of the distal segment, they are vascularized out of the efferent veins system; laterally to the renal corpuscles the loops of the proximal, connective and second part of the distal segments are localized; they receive their blood from some branches of the afferent vein.     

Developing nephrons in adolescent dogfish, Scyliorhinus caniculus (L.), with reference to ultrastructure of early stages, histogenesis of the renal countercurrent system, and nephron segmentation in marine elasmobranchs

Light and electron microscopy of the excretory kidney of adolescent dogfish, Scyliorhinus caniculus (L.), revealed immature and mature nephrons as well as four developmental stages of nephrons. At stage I the nephron was characterized by a condensed mass of mesenchymal cells in the center of several concentric layers of connective tissue. At stage II of the nephron, the S-shaped body was an elongate cyst with a high prismatic epithelium that was connected by a developing collecting tubule with the collecting duct system. At stage III, the developing nephrons already possess the essential features of the mature nephron but lack complete differentiation. Developing renal corpuscles had one afferent arteriole and two efferent vessels. Developing tubules ran four times between the lateral bundle zone and the mesial tissue zone before they joined the collecting duct system. A continuous sheath of flat cells, encompassing the collecting duct system, extended around the developing lateral bundle. A rudimentary central vessel ran from the developing lateral bundle to the venous sinusoid capillaries between the mesial convolutions. Developmental stage IV was similar to the mature nephron, however, renal corpuscles and tubular segments were smaller than those of mature nephrons. Conclusive evidence for morphological homology of elasmobranch nephron segments and collecting tubule-collecting duct system with those of other vertebrates is provided. The origin and nature of the central vessel and the bundle sheath is clarified. These specific structures of marine elasmobranch kidney supposedly are of great functional relevance for the renal countercurrent system that in turn is essential for ion- and osmo-regulation.     

Structural organization of nephrons in the kideny and proximal reabsorption in representatives of different vertebrate classes (according to microdissection and micropuncture studies)]

By microdissection method, single nephrons have been isolated from the river lamprey Lampetra fluviatilis, teleost fish Myoxocephalus scorpius (White sea), frog Rana temporaria, lizard Agama caucasica, hen Gallus domesticus and albino rat Rattus norvegicus (Wistar strain). Tubular reabsorptive capacity was measured in kidneys of lampreys and rats by the split oil droplet method. In the animals studied, each of the nephrons consists of the glomerulus, proximal, thin and distal segments. Relative length of the proximal segment does not depend on phylogenetic position of a species. Transtubular isotonic water reabsorption from the proximal tubule in rats is significantly higher than in lampreys. The level of proximal reabsorption is not related to the length of the proximal tubule in the Verterbrates studied but depends on the intensity of tubular transport. The structure which is similar to Henle's loop is present already in Cyclostomes [12]. Further stage of the progressive development of the uriniferous tubules is presented by uneven localization of nephron populations within the kideney in Reptiles. These data suggest that the effective system of osmotic concentration appeared in evolution not as the result of Henle's loop formation, but on the basis of the development of the medullar substance formed mainly by intercortical and juxtamedullar nephron populations.     

Development of various nephron populations in birds during ontogenesis

Microanatomy of nephrons during ontogenesis has been studied in domestic hens. In the bird kidneys, as well as in mammalian kidneys, three populations of nephrons can be detected: juxtamedullary, or deep nephrons, superficial and situating between them intracortical nephrons. In formation of the bird renal medulla only loops of the juxtamedullary nephrons participate. During ontogenesis in the kidneys of birds, like in mammalians, juxtamedullary nephrons are the first to be formed; they can be obtained from kidneys of 14-day-old chicken embryos, however, a complete formation of the nephron structure is terminated after hatching. During development the length of the juxtamedullary nephrons increases nearly by one order, while the length of the superficial and intracortical nephrons increases no more than twice. The diameter of the glomeruli in the juxtamedullary nephrons during development increases nearly twice, in the superficial and intracortical nephrons only a slight increase is noted. A relative length of the proximal canaliculus of the intracortical and superficial nephrons gradually increases during ontogenesis and practically does not change in the juxtamedullary nephrons, in them the nephron loop becomes longer. The developmental pattern in various parts of the superficial, intracortical and juxtamedullary nephrons in general features is similar in mammalians and birds.     

Structure of the kidney in the coelacanth Latimeria chalumnae with reference to osmoregulation

The morphology of the nephrons of the coelacanth Latimeria chalumnae was investigated by light microscopy. Each nephron is composed of a large renal corpuscle with well‐vascularized glomerulus, non‐ciliated neck segment, proximal convoluted tubule divided into distinct first and second segments, non‐ciliated intermediate segment, distal tubule, collecting tubule and collecting duct. The parietal layer of the Bowman's capsule of the renal corpuscle is composed of low cuboidal cells. The short non‐ciliated neck segment is lined by cuboidal epithelium. The first and second proximal segments display a prominent brush border and contain amorphous material in their lumen. The second proximal segment differs from the first segment in having taller columnar epithelium and a relatively narrow lumen. The intermediate segment is lined by non‐ciliated columnar epithelium and its lumen appears empty. The distal tubule is narrow in diameter and its cuboidal epithelium is devoid of intercalated cells. A unique feature of L. chalumnae is having binucleate cells in the tubule and collecting duct epithelium. The renal arteries have poorly developed tunica media and its cells contain granular material. The structure of L. chalumnae nephrons correlates well with their osmoregulatory function and resembles those of euryhaline teleosts.     

Recapitulation in the development of the components of the counterflow system in the vertebrate metanephros

The investigation has been performed on 107 renal preparations obtained from persons of various age (from 5-month-old fetuses up to 45 years of age), certain representatives of other classes of the Vertebrata are also included: fish, amphibia, reptile and mammalia at various stages of pre- and postnatal periods of ontogenesis by means of preparing graphic and plastic reconstructive models, histological investigation and microdissection. The complexity of the intrarenal branching of derivatives of the mesonephric duct diverticulum, development and structure of the canalicular part in nephrons directly depend on the phylogenetic position of the animal. Complexity of the nephron architectonics occurs along the progressive line of taxonomic groups of higher Vertebrata. The nephron loop becomes longer, thin segment of the nephron canalicular part increases in its length and, at last, in mammalia a cone-shaped fasciculus appears as a structural-functional unit of the osmoregulating apparatus of the constant kidney. In the comparative anatomical and comparative embryological aspects recapitulation is observed concerning certain morphological signs of derivatives of the metanephric duct and nephron.     

Morphology of the Nephron in the Mesonephros of Bufo bufo (Amphibia,Anura, Bufonidae)

The present study deals with the morphology and ultrastruclure of the nephron in the mesonephros of the toad, Bufo bufo (Linnaeus, 1758). Based on serial sections in paraffin, Araldite and Epon, the position of the different segments of the nephron within the kidney tissue was determined, and a nephron subsequently reconstructed. The nephron consists of the following parts: Malpighian corpuscle, neck segment, proximal tubule, intermediate segment, early distal tubule, late distal tubule and collecting tubule. The late distal tubule was subdivided into three morphologically different sections. The total number of nephrons in the toad mesonephros was estimated at 6000 units. The length of the segments in the reconstructed nephron was calculated. The cytology of the epithelial cells constituting the segments was described using transmission and scanning electron microscopy. Heterocellularity was found in the late distal tubule section I and III and in the collecting tubule. The proportional distribution and number of intercalated (mitochondria-rich) cells in the late distal tubule and collecting tubule was calculated. Only one morphological type of intercalated cell could be distinguished. Late distal tubules were removed from fresh Bufo kidneys for preliminary studies of the intercalated cells with Nomarski optics.     

FGF8 is required for cell survival at distinct stages of nephrogenesis and for regulation of gene expression in nascent nephrons

During kidney morphogenesis, the formation of nephrons begins when mesenchymal nephron progenitor cells aggregate and transform into epithelial vesicles that elongate and assume an S-shape. Cells in different regions of the S-shaped body subsequently differentiate into the morphologically and functionally distinct segments of the mature nephron. Here, we have used an allelic series of mutations to determine the role of the secreted signaling molecule FGF8 in nephrogenesis. In the absence of FGF8 signaling, nephron formation is initiated, but the nascent nephrons do not express Wnt4 or Lim1, and nephrogenesis does not progress to the S-shaped body stage. Furthermore, the nephron progenitor cells that reside in the peripheral zone, the outermost region of the developing kidney, are progressively lost. When FGF8 signaling is severely reduced rather than eliminated, mesenchymal cells differentiate into S-shaped bodies. However, the cells within these structures that normally differentiate into the tubular segments of the mature nephron undergo apoptosis, resulting in the formation of kidneys with severely truncated nephrons consisting of renal corpuscles connected to collecting ducts by an abnormally short tubular segment. Thus, unlike other FGF family members, which regulate growth and branching morphogenesis of the collecting duct system, Fgf8 encodes a factor essential for gene regulation and cell survival at distinct steps in nephrogenesis.     

Nephron formation adopts a novel spatial topology at cessation of nephrogenesis

Nephron number in the mammalian kidney is known to vary dramatically, with postnatal renal function directly influenced by nephron complement. What determines final nephron number is poorly understood but nephron formation in the mouse kidney ceases within the first few days after birth, presumably due to the loss of all remaining nephron progenitors via epithelial differentiation. What initiates this event is not known. Indeed, whether nephron formation occurs in the same way at this time as during embryonic development has also not been examined. In this study, we investigate the key cellular compartments involved in nephron formation; the ureteric tip, cap mesenchyme and early nephrons; from postnatal day (P) 0 to 6 in the mouse. High resolution analyses of gene and protein expression indicate that loss of nephron progenitors precedes loss of ureteric tip identity, but show spatial shifts in the expression of cap mesenchyme genes during this time. In addition, cap mesenchymal volume and rate of proliferation decline prior to birth. Section-based 3D modeling and Optical Projection Tomography revealed a burst of ectopic nephron induction, with the formation of multiple (up to 5) nephrons per ureteric tip evident from P2. While the distal–proximal patterning of these nephrons occurred normally, their spatial relationship with the ureteric compartment was altered. We propose that this phase of nephron formation represents an acceleration of differentiation within the cap mesenchyme due to a displacement of signals within the nephrogenic niche.     

Nephron structure and immunohistochemical localization of ion pumps and aquaporins in the kidney of frogs inhabiting different environments

Amphibians inhabit areas ranging from completely aqueous to terrestrial environments and move between water and land. The kidneys of all anurans are similar at the gross morphological level: the structure of their nephrons is related to habitat. According to the observation by light and electron microscopy, the cells that make up the nephron differ among species. Immunohistochemical studies using antibodies to various ATPases showed a significant species difference depending on habitat. The immunoreactivity for Na+,K(+)-ATPase was low in the proximal tubules but high in the basolateral membranes of early distal tubules to collecting ducts in all species. In the proximal tubule, apical membranes of the cells were slightly immunoreactive to H(+)-ATPase antibody in aquatic species. In the connecting tubule and the collecting duct, the apical membrane of intercalated cells was immunoreactive in all species. In aquatic species, H+,K(+)-ATPase immunoreactivity was observed in cell along the proximal, distal tubule to the collecting duct. However, H+,K(+)-ATPase was present along the intercalated cells of the distal segments from early distal to collecting tubules in terrestrial and semi-aquatic species. In the renal corpuscle, the neck segment and the intermediate segment, immunoreactivities to ion pumps were not observed in any of the species examined. Taking together our observations, we conclude that in the aquatic species, a large volume of plasma must be filtered in a large glomerulus and the ultrafiltrate components are reabsorbed along a large and long proximal segment of the nephron. Control of tubular transport may be poorly developed when a small short distal segment of the nephron is observed. On the contrary, terrestrial species have a long and well-developed distal segment and regulation mechanisms of tubular transport may have evolved in these segments. Thus, the development of the late distal segments of the nephron is one of the important factors for the terrestrial adaptation.     

Functional morphology of the avian medullary cone

The organization of the renal medulla of the Gambel's quail, Callipepla gambelii, kidney was examined to determine the number of loops of Henle and collecting ducts and the surface area occupied by the different nephron segments as a function of distance down the medullary cones. Eleven medullary cones were dissected from the kidneys of four birds, and the tissue was processed and sectioned for light microscopy. In addition, individual nephrons were isolated on which total loop thin descending segment and thick prebend segment lengths were measured. The results show no correlation between the absolute number of loops of Henle and the length of the medullary cones. The number of thick and thin limbs of Henle and collecting ducts decrease exponentially with distance toward the apex of the cones and the rate of decrease is similar for cones of different lengths. Initially there is a rapid decrease in the number of thin limbs of Henle, indicating that most nephrons do not penetrate the cones a great distance. Thick descending limbs of Henle (prebend segment) ranged in length from 50 to 770 microm, and there was little correlation with the total length of the loop of Henle. However, the length of the thin limb of Henle correlated well with total loop length. The cell surface areas of the limbs of the loop of Henle and the collecting ducts decreased toward the apex of the cones.     

Microanatomy of the renal cortex in the domestic fowl

Two aspects of the avian renal cortical microanatomy previously were unclear. The precise in situ folding patterns and orientations of the nephrons with respect to the other cortical elements had not been demonstrated. It also was not known whether certain nephron segments are supplied exclusively by either the arterial or the portal blood flow. In the present study, a new casting compound was developed to allow selective examination of the cortical components by light microscopy. Cortical nephrons at the surface of the kidney were serially sectioned and reconstructed in order to determine: (a) their relationships to the vasculature and collecting ducts; (b) the location and characteristics of the tubule segments; and (c) the primary and secondary folding patterns of the tubules. The anatomical findings were documented individually and then summarized in a comprehensive diagram of the superficial cortical microanatomy. In addition, an in vivo method was used to determine the extent of portal blood distribution to the nephron segments. It was demonstrated that renal portal blood suffuses all of the segments except for the loops of Henle.     

Maturation of glomerular size distribution profiles in domestic fowl (Gallus gallus)

Previous histological evaluations of chick kidneys indicated nephrons continue to develop from embryonic foci for up to 6 weeks after hatching. The present study was conducted using an in vivo alcian blue staining technique to quantify posthatch changes in glomerular numbers and sizes in female domestic fowl at 1, 3, 5, 9, 12, 21, and 30 weeks of age. Changes in glomerular size distributions reflect changes in the heterogeneous nephron populations of avian kidneys. Foci of embryonic tissue were observed at the periphery of renal lobules up to 12 weeks of age. Glomerular numbers increased from 69,800/kidney at 1 week to 586,000/kidney at 12 weeks, with no further significant increase up to 30 weeks (599,000/kidney). The increase in glomerular number per gram kidney weight remained constant as kidney mass increased up to 12 weeks of age, after which the number of glomeruli per gram kidney weight declined significantly as kidney size increased without further addition of new nephrons. Glomerular size distribution profiles were constructed using eleven circumference categories. The peak number of glomeruli fell within the 0.11-0.14 mm category at 1 and 3 weeks; within the 0.15-0.18 mm category at 5, 9, and 12 weeks; and within the 0.19-0.22 mm category at 21 and 30 weeks. One and 3-week-old chicks had no glomeruli within the largest (greater than or equal to 0.35 mm circumference) size categories, and 9-12-week-old birds had significantly fewer glomeruli in these categories than 21-30-week-old birds. These results demonstrate that posthatch renal maturation in domestic fowl involves the ongoing formation of new nephrons up to 12 weeks of age, with subsequent kidney growth (12-30 weeks of age) accomplished by enlargement of existing nephrons (nephron hypertrophy). The cumulative evidence indicates that nephrons destined to develop loops of Henle (mammalian-type) develop first, with shorter (reptilian-type) nephrons developing later as the kidneys enlarge.     

Collectrin/tmem27 is expressed at high levels in all segments of the developing Xenopus pronephric nephron and in the Wolffian duct

Collectrin/tmem27 encodes a transmembrane protein that plays a critical role in amino-acid transport. Originally described as being expressed only in collecting ducts, it has subsequently also been shown to also be expressed in the S1 segment of the proximal tubule of mammalian metanephric nephrons. In this report we describe the expression of collectrin in the simple embryonic kidney of amphibians, the pronephros. Each pronephros contains a single large nephron with a proximo-distal segmentation very similar to that of mammalian metanephric nephrons. Analysis of collectrin expression in pronephroi at a variety of embryonic stages indicates that this gene is expressed at very high levels throughout the pronephric system, including proximal and distal segments and the Wolffian duct. Expression in the pronephros commences at Xenopus embryonic stage 28 which corresponds to when epithelialization begins within the pronephric mesenchyme. Like the Na+K+ATPase/atp1a1, another highly expressed pronephric marker, collectrin is also expressed in the cloaca but not in the cloacal derived posterior segment of the Wolffian duct, the rectal diverticulum. Unlike the Na+K+ATPase, which is expressed at lower levels in proximal portions of the pronephric nephron, expression of collectrin is even throughout all of the pronephric epithelia. This expression domain extends far beyond that shown to express amino-acid transporters and indicates collectrin may function in facilitating additional transport processes. Its high level of expression and broad distribution make it an excellent marker with which to examine pronephric kidney development.     

Transepithelial potential in mesonephric nephrons of 7-day-old chick embryos in relation to the histochemically detected sodium pump

In order to obtain basic information on the transport properties of differentiating embryonic nephrons, we examined the 7-day-old chick mesonephros by measuring the transtubular epithelial potential difference (TPD) and by histochemical detection of Na,K-ATPase activity. TPD as an indicator of the electrogenic transport was measured in individual segments of superficial nephrons in vivo. Their electric polarity was always lumen-negative. TPD was reduced by addition of 10 mM KCN applied to the mesonephric nephrons from the outside. In the proximal tubules, TPD was significantly lower (mean+/-SD: -1.0+/-0.5 mV) than in the distal and collecting tubules (-2.2+/-1.0 mV, p< or =0.05). Activity of the sodium pump was evaluated histochemically by detection of ouabain-sensitive potassium-dependent p-nitrophenyl phosphatase in cryostat sections of the mesonephros. The enzyme activity was demonstrated only in distal tubules and in the collecting ducts, but not in the proximal tubules. These findings have revealed significant differences between embryonic nephron segments: the distal tubule, in contrast to the proximal one, is supplied by the sodium pump and is able to generate higher TPD. Therefore, we consider that it is only the distal nephron, which possesses the ability of active transport.     

LIF, the ES-cell inhibition factor, reversibly blocks nephrogenesis in cultured mouse kidney rudiments.

Mouse kidney induction proceeds in vitro much as it does in vivo: the ureteric bud bifurcates to give collecting ducts while the mesenchyme condenses into aggregates which epithelialise and then elongate into tubules with glomerular and other nephron structures. We report here that the factor known as LIF (leukaemia inhibitory factor), which regulates the differentiation and growth of embryonic-stem (ES) and other cells in culture, has little effect in vitro on growth or on ureteric-bud morphogenesis other than to stimulate the bifurcation process. It does however exert a striking effect on the mesenchyme. At about four times the concentration required to inhibit ES-cell differentiation, LIF strongly but reversibly blocks the effects of metanephric mesenchyme induction: although mesenchyme condenses around growing duct tips, the number of mature nephrons that form over 6 days is reduced by 75% or more. The few nephrons that do develop in the presence of LIF probably come from mesenchyme already induced at the time of culture and are indistinguishable from those that form in controls as assayed by morphology, by X-gal staining of endogenous galactosidase and by antibodies to brush-border and CD15 antigens. There is a further unexpected feature of rudiments cultured in LIF which is absent in controls: they contain an unexpectedly high number of stable epithelialised aggregates that express laminin around their periphery and which do not develop further. These results argue that the process of nephrogenesis involves at least two distinct stages which can be blocked by LIF: the effect of the initial induction and the future development of epithelialised aggregates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Microdissectional investigation of the nephrons in some fishes,amphibians, and reptiles inhabiting different environment

The structure of nephrons in 83 species of fishes, amphibians, and reptiles was investigated by microdissection. Glomerular diameter, nephron length, length of nephron segments, and the ratios between glomerular size and nephron length are presented. In diifferent groups of fishes (Elasmobranchii, Acipenseridae, Teleostei), the adaptation to freshwater or seawater environment may lead to diverse changes of nephron structure. The kidneys of euryhaline teleosts capable of living in fresh water may be aglomerular, as are those of some marine fishes. In contrast, the diameter of glomeruli in some marine fishes is larger than in true freshwater fishes. In amphibians, the adaptation to freshwater environment, as in teleost fishes, has led to different changes of nephron structure. The size of glomeruli in freshwater reptiles is larger in comparison to terrestrial animals, and the distal tubule in desert and freshwater reptiles is longer than in nondesert species. This probably reflects the adaptive changes of the reptilian nephron to freshwater and desert environments. The results of this study show that the nephron structure of lower vertebrates is predominantly determined by the different environments they occupy rather than by progressive changes within the vertebrate sequence. © 1994 Wiley-Liss, Inc.