Studies on vertebrate kidney. V. PAS-positive lipids in kidneys of fishes from different habitats.

Five fish species living in different habitats, i.e. fresh water, estuarine and marine, were studied for the distribution of PAS-positive materia in various regions of the kidney, 10 minutes' oxidation with 0-5 per cent. HIO4 being employed prior to treatment with Schiff's reagent. PAS-positive material was detected in different sites of the kidney, i.e. brush border of proximal tubules, proximal tubule cells' cytoplasm, distal tubule cells' cytoplasm, glomerulus, basal cell border of proximal tubules and the interstitial cells. Of these sites, the brush border of the proximal tubule of Scoliodon sorrakowah showed the presence of PAS-positive lipids. Elsewhere the PAS-positive reaction was due to carbohydrates. Free aldehyde groups were absent. In Tilapia mossambica and Labeo rohita, PAS staining was enhanced after chloroform-methanol extraction, particularly in the brush border of the proximal tubule. The significance of these findings is discussed.     

Carbonic anhydrase in the monkey kidney

The distribution of carbonic anhydrase in the kidney of the cynomolgus monkey was studied by the histochemical method of Hansson. Glomeruli and Bowman's capsule were inactive. Convoluted proximal tubules showed high enzyme activity at the brush border and the basolateral membranes and the cytoplasm. Straight proximal tubules were less intensely stained. In nephrons with long loops of Henle, the descending thin limb contained weak enzyme activity, whereas the ascending thin limb was inactive. The thick limb of Henle's loop displayed most enzyme activity at the luminal cell border. In distal convoluted tubules enzyme activity was restricted to the basal part of the cells. In the late distal tubule, intercalated cells appeared among the "ordinary" distal cells and contained abundant cytoplasmic enzyme. Many intensely stained intercalated cells were also found in the cortical and outer medullary segments of the collecting duct, intermingled with more weakly stained chief cells. In the inner medullary segment of the collecting duct, enzyme activity gradually disappeared. Many capillaries were clearly stained for enzyme activity. The capillary staining apparently varied with that of the kidney tubules; virtually all capillaries in the cortex, but very few in the inner medulla, were stained. The distribution of carbonic anhydrase in the kidney tubules of the monkey is very similar to that in man and in the rat, but the primate kidney differs from the rat kidney by the presence of capillary enzyme activity. The functional importance of this difference is not clear at present.     

Histochemical evaluation of mouse and rat kidneys with lectin-horseradish peroxidase conjugates

Paraffin sections of mouse and rat kidney were stained with a battery of ten lectin-horseradish peroxidase conjugates and lectin binding was correlated with the ultrastructural distribution of periodate-reactive sugar residues as determined by the periodic acid-thiocarbohydrazide-silver proteinate technique. Various segments of the uriniferous tubule in both species showed differential affinity for labelled lectins. Significant differences were also evident between comparable tubular segments in mouse and rat kidneys. Neutral glycoconjugates containing terminal beta-galactose and terminal alpha-N-acetylgalactosamine were prevalent on the luminal surface of the proximal convoluted tubule in the rat, but alpha-N-acetylgalactosamine was absent in this site in the mouse. In both species, terminal N-acetylglucosamine was abundant in the brush border of proximal straight tubules but absent in proximal convolutions. Fucose was demonstrated in both proximal and distal segments of mouse kidney tubules but only in the distal nephron and collecting ducts in the rat. Lectin staining revealed striking heterogeneity in the structure and distribution of cellular glycoconjugates. Such cellular heterogeneity was previously unrecognizable with earlier histochemical methods. The marked cellular heterogeneity observed with several lectin-conjugates in distal convoluted tubules and collecting ducts of both species raises a prospect that lectins can provide specific markers for intercalated and principal cells in the mammalian kidney. Glycoconjugates containing terminal sialic acid and penultimate beta-galactose were present on vascular endothelium in both rodent kidneys, as were terminal alpha-galactose residues; but both species lacked reactivity for Ulex europeus I lectin in contrast to human vascular endothelial cells. The constant binding pattern of lectin conjugates allows convenient and precise differentiation of renal tubular segments and should prove valuable in the study of changes in kidney morphology promoted by experimental manipulation or pathologic changes.     

Presence of an extensive clathrin coat on the apical plasmalemma of the rat kidney proximal tubule cell

The nature of the cytoplasmic coat present on the apical invaginations of the kidney proximal tubule cell was investigated by immuneoverlay and immunocytochemistry of renal brush borders with anticlathrin antibodies. When kidney cortex was prepared for electron microscopy using methods that enhance visualization of clathrin coats, the apical invaginations at the base of the brush border microvilli were seen to be backed by a nearly continuous coating which resembles but is more extensive than the lattice-like clathrin coats found around brain coated vesicles. When isolated brush border fractions were prepared under conditions that preserve the coats, separated by SDS PAGE, and transferred to nitrocellulose, the presence of clathrin heavy and light chains was detected by immuneoverlay using two different affinity-purified anticlathrin IgGs--one that we prepared, which detects only the clathrin light chains, and the other, prepared by Louvard et al. ( Louvard , D., C. Morris, G. Warren, K. Stanley, F. Winkler , and H. Reggio , 1983, EMBO [Eur. Mol. Biol. Organ.] J., 2:1655-1664), which detects both the heavy and light chains. As viewed by light microscopy (immunofluorescence or immunoperoxidase), staining with both anticlathrins was concentrated at the base of the proximal tubule microvilli. Immunoelectron microscopic localizations carried out on brush border fractions (using peroxidase and gold conjugates) demonstrated specific binding of anticlathrin IgGs to the lattice-like cytoplasmic coat. When brush border fractions were reacted with monoclonal antibodies prepared against gp330 and maltase, proteins that serve as markers for the membrane of the apical invaginations and microvilli, respectively ( Kerjaschki , D., L. Noronha - Blob , B. Sacktor , and M. G. Farquhar , 1984, J. Cell Biol., 98:1505-1513), the two proteins retained their restrictive distribution in the brush border. The findings demonstrate (a) that the cytoplasmic coat of the proximal tubule intermicrovillar apical invaginations is composed of clathrin heavy and light chains, and (b) that the differential distribution of proteins in these two brush border microdomains is maintained in appropriately prepared brush border fractions.     

Histological and enzyme histochemical changes in the kidney of male bullhead (Cottus gobio) during the spawning period

Histological and enzyme histochemical studies were carried out on the excretory kidney of the male bullhead ( Cottus gobio ). During the spawning season striking morphofunctional changes were observed in the second proximal segment of the kidney tubule. The tubular epithelium was greatly hypertrophied, strongly basophilic and produced a PAS-positive secretion. The enzyme histochemical pattern also changed conspicuously during this time: the alkaline phosphatase activity in the brush border was greatly reduced; the acid phosphatase and non-specific esterase activity in the cytoplasm was distinctly elevated.     

Histochemical demonstration of peptidases in the human kidney

The localization of several peptidases in the human kidney was investigated histochemically. The membrane-bound peptidases, aminopeptidase A (APA), aminopeptidase M, gamma-glutamyltransferase (gamma-GT) and dipeptidylpeptidase IV, were mainly demonstrable in the brush border of the proximal tubule. In addition, APA was found in the glomeruli, while gamma-GT was found in the basal labyrinth of the proximal tubule. The lysosomal peptidases, dipeptidylpeptidase I and cathepsin B, were most strongly concentrated in the different-sized lysosomes of the proximal tubule, but they were also found in the small lysosomes of the distal tubule. Dipeptidylpeptidase II showed only a weak reaction in lysosomes of the proximal tubule. It is concluded that, in comparison with other previously studied species, the human kidney has a well-developed equipment with membrane-bound and lysosomal peptidases.     

Metabolic heterogeneity of the proximal and distal kidney tubules

Proximal and distal tubule suspensions were prepared from kidneys of Sprague-Dawley rats by an isolation procedure on a Percoll^R gradient. The marker enzymes alkaline phosphatase (brush border) and hexokinase (cytoplasmic) as well as p-aminohippurate transport capacity, gluconeogenic activity and electron microscopy were used to characterize the two kidney tubule suspensions. The results of this study indicate that cytochrome P-450 is localized to the proximal tubular cells and that the O-deethylation of 7- ethoxycoumarin was higher in the proximal than distal fraction. Both proximal and distal tubules showed glucuronidation and deacetylation capacities and a relatively equal distribution of non-protein sulfhydryls. These studies demonstrate metabolic heterogeneity of the nephron, the proximal tubule being the main site of renal xenobiotic metabolism. Understanding of metabolic heterogeneity of proximal and distal kidney tubules should provide important information regarding cell specific mechanisms of nephrotoxicity.     

Carbonic anhydrase activity in kidney and lower intestine of the European starling

A histochemical investigation of kidney and lower intestine of the European starling (Sturnus vulgaris) shows no carbonic anhydrase activity in proximal convoluted tubules, although activity is seen in similarly prepared sections of rat proximal tubules. Early distal tubule cells in the starling are stained throughout the cytoplasm and at the apical and highly infolded basolateral membranes. Late distal tubules lose apical activity and have reduced basolateral infolding, resulting in less intense staining. Darkly stained intercalated cells appear in the connecting tubules and cortical collecting ducts. Both of these segments also show intense basolateral staining. Medullary cones of the starling are highly organized, with central zones containing unstained thin descending limbs of loops of Henle, surrounded by both medullary collecting ducts with only scattered cells staining for enzyme, and by thick ascending limb segments. The latter contain many uniformly stained cells intermingled with occasional unstained cells. Scattered cells of the starling colonic villi demonstrate intense apical brush border membrane staining as well as cytoplasmic staining. Cells lining the cloaca stain less intensely. A biochemical assay for carbonic anhydrase was used to quantify enzyme activity in these tissues. Starling kidney contained 1.96 ± 0.33 (mean ± SEM) enzyme units/mg protein, less than half the activity seen in rat kidney. Stripped colonic epithelium contained 0.66 ± 0.15 enzyme units/mg protein. These quantitative results correlate well with the interpretations derived from the histochemical observations. The lack of proximal tubule carbonic anhydrase activity suggests that the avian kidney relies more on distal nephron segments to achieve net acidification of the urine.     

Renal carbonic anhydrase in the quail Coturnix coturnix japonica: I. Activity and distribution in male and female metanephros

Summary Carbonic anhydrase activity was studied in the quail metanephros by means of histochemical, histophotometrical and biochemical methods. Male and female samples were examined separately in order to show sex-related differences in enzyme activity and localization. The staining patterns revealed differential distribution of reaction product in the different, tubular segments. The initial portion of proximal tubules showed positivity on the brush border in female kidneys only.Extra situ investigations provided further evidence of sexual dimorphism resulting in higher values of enzyme activity for female than for male kidneys.In both sexes, marked staining was detected at the distal tubule level where histophotometric analysis confirmed the highest amount of reaction product. Moreover, the intracellular staining distribution at this site proved to be similar to that observed for mammalian proximal convoluted tubules.In the collecting ducts, a mosaic-like pattern was found with respect to both carbonic anhydrase staining and metachromatic properties.The functional significance of the presence of enzyme in the different renal tubules is discussed by comparison with the mammalian kidney. A model is proposed whereby the distal tubules represent the main sites of urinary acidification and bicarbonate reabsorption.     

The histochemical demonstration of brush border endopeptidase.

A histochemical method for the demonstration of a brush border endopeptidase is described based on results of biochemical and histochemical experiments. The substrate of choice is Glut-Ala-Ala-Ala-MNA which displays a very good localization ability and suitable kinetic properties. Km estimated in rat kidney homogenate amounts to 2.35 X 10(-4) M. pH optimum of this endopeptidase associated with the brush border membrane is in the alkaline range. The activity is dependent on the buffer used. In phosphate and cacodylate buffers of pH 7.2 about 30% lower activity in rat kidney and about 25% lower activity in rat small intestine than in Tris-HCl buffer of the same pH was found. The most suitable diazonium salt for the detection "in situ" is Fast Blue B. It inhibits the endopeptidase activity of rat kidney by about 85% at pH 7.2 AND BY ABOUT 55% AT PH 6.0. The best results are obtained in cryostat sections adherent to semipermeable membranes treated with chloroform-acetone before the incubation. A microdensitometric evaluation of the reaction product is possible and results are in good agreement with those of the biochemical determination. When Suc-Ala-Ala-Ala-INA is used as substrate hexazonium-p-rosaniline is the most suitable coupling agent although it inhibits more than Fast Blue B. The reaction using acylated trialanyl naphthylamides as substrates runs in two steps. Endopeptidase sets free Ala-NA which is attacked by aminopeptidase M. Aminopeptidase M is not reaction rate or localization limiting factor because its activity in the brush border is very high and the enzyme is anchored to the cell membrane very closely to endopeptidase. In homogenates of rat kidney and jejunal mucosa the endopeptidase activity was inhibted by EDTA (2X10(-3) M) by 75% in the kidney and by 68% in the jejunum, by DFP (10(-3) M) by 41% in the kidney and by 35% in the intestine, by Mn2+ (5X10(-3) M) by 25% in the kidney and by 30% in the intestine. No inhibition was exerted by E 600. In sections the results were similar. 1,10-phenanthroline (10(-2) M) caused a substantial inhibition. Endopeptidase activity was detected in the brush border of cells of proximal convuluted tubules of the kidney and in the brush border of differentiated enterocytes of the small intestine. In the same species enterocytes display a lower activity than kidney tubular cells. There are species differences in the distribution pattern of endopeptidase in the kidney. In the rabbit and man the positive reaction occurs in the whole cortex. It is distributed unevenly, however. In the rat the tubules of the inner cortex display a very high activity. In the outer cortex straight portions react strongly. In the rabbit kidney cells of the parietal layer of Bowman's capsule display a weak reaction as well. No sex differences were found in the distribution pattern of endopeptidase in the rat kidney. In the intestine of all species examined a proximo-distal gradient was found...     

Monoclonal antibodies against rat kidney antigens.

The proximal tubule of the nephron is subdivided into three structurally and functionally distinct segments, which can be differentiated with the help of special methods. With the aim of producing selective markers for these three portions of the proximal tubule, we raised monoclonal antibodies against the brush border membranes of the rat kidney. Immunohistochemistry was carried out with eleven different monoclonal antibodies to sections of rat kidney and other tissues at the light- and electron-microscopical level. These monoclonal antibodies mainly detect antigens located on the brush border of the proximal tubule, and they allow a distinction between its three segments. However, some antibodies also recognize other portions of the nephron, or even the glomerulus or stromal elements. Sites recognized by the antibodies are not limited to the kidney, but staining is observed on the intestinal brush border, the intralobular ducts of the pancreas, the bile canaliculi of the liver and on the macrophage clusters of the spleen. These antibodies are interesting reagents which can be applied to study biochemical differences between brush border membranes. In addition, they recognize antigenically related sites in other organs with reabsorptive or secretory tasks.     

Structure of the kidney in the crab-eating frog, Rana cancrivora

The structure of the nephron in the ranid frog, Rana cancrivora, was studied by light and electron microscopy. This frog is the only amphibian species to live in mangrove swamps of very high salinity. The nephron consists of the following parts: renal corpuscle, ciliated neck segment, proximal tubule, ciliated intermediate segment, distal tubule, connecting tubule, and collecting duct. The distal tubule is located in the ventromedial region of the kidney, and the other tubules are situated in the dorsolateral region. Renal corpuscles are found between the two regions. Some renal corpuscles have a wide Bowman's space because of the small glomerulus within them. The proximal tubules are composed of columnar cells with a dense luminal brush border of long microvilli and numerous apical vesicles and vacuoles. The initial part of the distal tubule consists of heavily interdigitated cells, characterized by a very regular palisade arrangement of mitochondria. In the terminal part of the distal tubule, shorter mitochondria of the infolding cells are situated irregularly around the nucleus. The connecting tubule consists of principal cells and canaliculus cells. The collecting duct consists of columnar or cuboidal cells; cytoplasmic organelles are relatively sparse. The canaliculus cells are intercalated between principal cells from the terminal distal tubule to the proximal part of the collecting duct. Our findings indicate that the kidney of R. cancrivora is structurally similar to kidneys of other amphibians. These findings are discussed with regard to probable correlations between ultrastructure and function in R. cancrivora.     

Ultrastructural evaluation of the effect of endosulfan on mice kidney

In this study, the effect of the endosulfan on mice kidney was investigated at ultrastructural level. Moreover, biochemical analyses (G6PD, CAT, SOD, GSH and MDA) were determined in supernatant of kidney tissue. Endosulfan (13mg/kg/day body weight) was administered orally to mices via intragastric-during 10 days. The presence of mitochondrial degeneration in cytoplasm of proximal convoluted tubule cells were a striking feature. Furthermore, there was lipofuscin granules and membranous structures in some of proximal convoluted tubule cells. In some glomeruli, ultrastructural changes such as fusion in pedicels and focal thickening at glomerular basal membrane were seen. There were cytoplasmic bulges in some distal convoluted tubule cells. The biochemical results of the experimental group were significant when compared to the control. The effect of the endosulfan was mainly on the proximal convoluted tubule cells. Morever, the other parts of the nephron were effected. Thus, this degeneration in kidney may be thought that oxidative stress may play a role to the mediator in changing configuration of cell membrane and seem to account for the morphologic alteration of kidney.     

The fine structure of the nephronic tubule of the mudskipper Periophthalmus koelreuteri (Pallas)

Ultrastructural examination of the head kidney of Periophthalmus koelreuteri (Pallas) (Teleostei, Gobiidae) revealed that the nephronic tubule cells are bound by tight junctions and desmosomes with little intercellular space. The first proximal segment (PI) consists of low columnar cells with well developed brush borders, indented nuclei, and numerous apical endocytic vesicles and lysosomes. A second cell type possessing clusters of apical cilia and lacking brush border and lysosomes is occasionally found between PI cells. The second proximal segment (PII) is formed of high columnar cells with brush border, regular spherical nuclei and numerous mitochondria located between well developed infoldings of the basal membrane. Single ciliary structures protrude into the lumen from PI and PII cells. The distal segment is lined by low columnar epithelium with few microvilli, regular spherical nuclei, numerous scattered mitochondria, and microbodies. The collecting tubule cells are cuboidal with few euchromatic nuclei, some mitochondria, and secondary lysosomes.     

Ultrastructure of the crayfish kidney--coelomosac, labyrinth, nephridial canal

Regions of the crayfish kidney were examined by electron microscopy. Coelsmosac cells are loosely bound together by desmosome-like spot junctions, and connected to the basal lamina via characteristic pedicels. The cytoplasm contains numerous vesicles and vacuoles of various sizes and is often crowded with large, lysosome-like granules or dense bodies. The morphology suggests a filtration mechanism with reabsorption of materials such as protein from the filtrate and secretion of other substances into the lumen. The labyrinth is composed of cuboidal to columnar cells which possess a brush border, long and narrow intercellular spaces, basal plasmalemmal invaginations and typical cytoplasmic components. Two sub-regions are distinguishable. The morphology of labyrinth I suggests that these cells move fluid isotonically across the epithelium. Labyrinth II, in addition to isotonic transport, appears to be more active in the endocytic uptake and intracellular digestion of large molecules such as protein. The nephridial canal consists of cells which lack a brush border, but display extensive basal invaginations associated with elongated mitochondria. A proximal and distal region are cytologically distinguishable. Proximally, the cells are small and filled with mitochondria throughout. Scattered within the cytoplasm are vesicles, vacuoles, diffuse glycogen, free ribosomes, dense bodies and some rough endoplasmic reticulum. Distally, the cells are less compact, larger, and cuboidal to columnar in shape. The cytoplasm is similar to that of the proximal cells, but the basal invaginations are even larger and more extensive. The morphology of cells in both regions of the nephridial canal is highly suggestive of active solute reabsorption, probably occurring against an osmotic gradient.     

Cellular differentiation in the kidneys of newborn mice studies with the electron microscope

The structure of the kidney of the Swiss albino mouse changes progressively during the first 2 weeks after birth. Cells proliferate to form new nephrons, cells differentiate by acquiring specialized membranous components, and certain cytological features which are present at birth diminish in abundance or disappear. The differentiation of the cells of the cortical tubules has been studied using the light and electron microscopes. The tubules are partially and variably differentiated at birth. During the first 2 weeks after birth the brush border develops in the proximal tubules by the accumulation of numerous microvilli on the apical cell margins. Basal striations develop in proximal and distal tubules as an alignment of mitochondria, the result of what appears to be progressive interlocking of adjacent fluted cells. The mitochondria increase in number and size, accumulate homogeneous matrix, and acquire small, very dense granules. The collecting ducts develop tight pleating of the basal cell membranes, and dark cells containing numerous small cytoplasmic vesicles and microvilli appear. At birth there are dense irregular cytoplasmic inclusions presumed to be lipide in renal cells, the cytoplasmic granules of Palade are abundant, and there are large round bodies in the cells of the proximal tubules. The lipide inclusions disappear a few days after birth, and the cytoplasmic granules of Palade diminish in abundance as the cells differentiate. The large round bodies in the proximal tubules consist of an amorphous material and contain concentrically lamellar structures and mitochondria. They resemble the cytoplasmic droplets produced in the proximal tubules of adult rats and mice by the administration of proteins. The large round bodies disappear from the proximal tubules of infant mice during the first week after birth, but the concentric lamellar structures may be found in adult mice.     

Cytoskeletal proteins of the rat kidney proximal tubule brush border

Cytoskeletal components backing the brush border of the rat kidney proximal tubule cell were identified and compared with those of the well characterized intestinal brush border by immuneoverlay and immunocytochemistry. Antibodies reactive against the intestinal microvillus core components, villin and fimbrin, as well as against the terminal web components, spectrin (fodrin) and myosin, were used. Proteins of similar molecular weight to these intestinal brush border cytoskeletal components were identified in isolated kidney brush borders by immuneoverlay. Spectrin, a major component of the terminal web region of both cell types, was more concentrated in the kidney brush border relative to both actin and myosin. By immunofluorescence, villin and fimbrin were localized in the microvilli, and spectrin and myosin were localized to the terminal web region of the brush border. In addition, spectrin was found along the basolateral membranes of the proximal tubule cell, and myosin was detected in a punctate staining pattern throughout its cytoplasm. By immunoelectron microscopy using immunogold labeling procedures, fimbrin and villin were localized in the terminal web as well as in microvilli, and spectrin and myosin were localized to fibrils in the terminal web. A key difference between the epithelia of the two organs is the extensive network of clathrin coated pits found in the terminal web region of the kidney but not the intestinal brush border. The clathrin-rich terminal web region of the kidney, like the intestinal brush border, proved to be quite stable and resistant to disruption by non-ionic detergents and harsh mechanical treatment.     

CELLULAR DIFFERENTIATION IN THE KIDNEYS OF NEWBORN MICE STUDIED WITH THE ELECTRON MICROSCOPE

The structure of the kidney of the Swiss albino mouse changes progressively during the first 2 weeks after birth. Cells proliferate to form new nephrons, cells differentiate by acquiring specialized membranous components, and certain cytological features which are present at birth diminish in abundance or disappear. The differentiation of the cells of the cortical tubules has been studied using the light and electron microscopes. The tubules are partially and variably differentiated at birth. During the first 2 weeks after birth the brush border develops in the proximal tubules by the accumulation of numerous microvilli on the apical cell margins. Basal striations develop in proximal and distal tubules as an alignment of mitochondria, the result of what appears to be progressive interlocking of adjacent fluted cells. The mitochondria increase in number and size, accumulate homogeneous matrix, and acquire small, very dense granules. The collecting ducts develop tight pleating of the basal cell membranes, and dark cells containing numerous small cytoplasmic vesicles and microvilli appear. At birth there are dense irregular cytoplasmic inclusions presumed to be lipide in renal cells, the cytoplasmic granules of Palade are abundant, and there are large round bodies in the cells of the proximal tubules. The lipide inclusions disappear a few days after birth, and the cytoplasmic granules of Palade diminish in abundance as the cells differentiate. The large round bodies in the proximal tubules consist of an amorphous material and contain concentrically lamellar structures and mitochondria. They resemble the cytoplasmic droplets produced in the proximal tubules of adult rats and mice by the administration of proteins. The large round bodies disappear from the proximal tubules of infant mice during the first week after birth, but the concentric lamellar structures may be found in adult mice.     

The pronephros of the early ammocoete larva of lampreys (Cyclostomata,Petromyzontes): Fine structure of the renal tubules

Summary The renal tubules of the paired pronephros in early larvae (ammocoetes) of two lamprey species, Lampetra fluviatilis and Petromyzon marinus, were studied by use of light-, scanning- and transmission electron microscopy. They consist of (1) a variable number of pronephric tubules (3 to 6), and (2) an excretory duct. By fine-structural criteria, the renal tubules can be divided into 6 segments. Each pronephric tubule is divided into (1) the nephrostome and (2) the proximal tubule, the excretory duct consisting of (3) a common proximal tubule followed by (4) a short intermediate segment, and then by a pronephric duct composed of (5) a cranial and (6) a caudal section. The epithelium of the nephrostome displays bundles of cilia. The cells of the proximal tubule possess a brush border, many endocytotic organelles and a system of canaliculi (tubular invaginations of the basolateral plasmalemma). The same characteristics are encountered in the epithelium of the common proximal tubule; however, the number of these specific organelles decreases along the course of this segment in a posterior direction. In the intermediate segment, the epithelium appears structurally nonspecialized. The cells of the cranial pronephric duct lack a brush border; they have an extensive system of canaliculi and numerous mitochondria. The caudal pronephric duct is lined by an epithelium composed of light and dark cells; the latter are filled with mitochondria and the former contain mucus granules beneath the luminal plasmalemma. The tubular segments found in the pronephros are the same in structure and sequence as in the lamprey opisthonephroi. However, only the nephrostomes and proximal tubules occur serially in the pronephros, while the common proximal tubule, the intermediate segment and the cranial pronephric duct form portions of a single excretory duct.This paper is dedicated to the memory of Professor W. Bargmann, long-time editor of Cell and Tissue Research, the author of a splendid review on the structure of the vertebrate kidney and a master of German scientific writing.     

The fine structure of the mesonephros of the lamprey,Entosphenus japonicus Martens

Summary The fine structure of the mesonephric kidney of the lamprey, Entosphenus japonicus Martens, has been investigated with the electron microscope and discussed from the viewpoint of comparative morphology of the mesonephros.The structure of the capillary wall of the glomerulus essentially coincides with that of higher vertebrates, though its basement membrane is remarkably thick (300–400 m) because of a dense accumulation of fibrillar material between the endothelium and the basal lamina of epithelial cell. No obvious fenestration of the endothelial cell has been observed in the glomerulus or capillaries in any part of this organ.The kidney tubule is divided into three segments: 1. neck segment composed of ciliated cells with numerous mitochondria and glycogen particles, 2. proximal tubule composed of brush bordered cells provided with extensive pinocytotic vesicles and lysosomal granules in the apical cytoplasm and with lamellar membranes in the basal, and 3. distal tubule characterized by cells which, with their abundant mitochondria and branched tubular endoplasmic reticulum (about 500 Å diameter) with a central core, closely resemble the chloride cells in the gill filament of some teleosts. The possibility that the lamellar membranes in the proximal tubule cells correspond to basal infoldings is discussed.The extensive development of the tubular reticulum and of the mitochondria in the distal tubule cells is believed to reflect the active absorption of urine chloride in the urinary tubule of lamprey mesonephric kidney evidenced by physiologists. The proximal tubule is suggested to take a part also in the urinary transport of water and ions, as the lamellar membranes found in the cells of this portion likely correspond to the basal infoldings in more advanced forms of the kidney.The epithelial cells of the ureteric duct are characterized by granules suggesting a mucous secretion. No fine structure implying an absorptive activity in this duct has been observed.