Histological and histochemical studies of the kidney of stickleback Gasterosteus aculeatus L. I. Morphological changes of the kidney epithelial cells during the spawning period]

1; The tubule of kidney of Gasterosteus aculeatus L. consists of four histologically different regions: Proximal tubule I and III, connection segment and collecting tubule. 2. All of tubule segments inclusively the urinary duct out of the proximal tubule I are showing synthesis of secretion. 3. There are producing two various secretion in two distinct species of cells. From the cells of proximal tubule II are secreted and extruded a granular secretion and from the cells of abducted urinary ducts (connection segment, collecting duct and urinary duct) a hyalo-mucous secretion. 4. During the breeding season the morphological variationes were divided into three stadiums, the stadium of differentiation, of producing of secretion and of reproduction. In second stadium were differenced three phases, in particular characterizing by rhythmical variationes of nucleus structure and synthesis of secretion as extrustion. 5. There are discussing the parallels to synthesis of secretion in glandular cells.     

Histological and histochemical studies of the stickleback Gasterosteus aculeatus L. kidney. III. Activity of various phosphatases depending on secretion]

1. The activity of alkaline phosphatase is intensely positive in proximal tubule I and II during the breeding season. In the kidney of secretion producing the enzyme is detectable as against to kidney of winter on the whole proximal tubule II. 2. In the kidney what is able to build a nest, concentration and size of acid phosphatase granules are very increasing in proximal tubule II. 3. The detection of unspecific esterase was negatively already. 4, The reaction of glucose-6-phosphatase is slightly demonstrable in cells of proximal segment of secretion producing what are enlarged fourfold. 5; From the varied reaction of acid and alkaline phosphatase we conclude that both are to set in relation to excretory activity, but not to process of synthesis in kidney of late-spring fish.     

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 pelvic kidney of male Ambystoma maculatum (Amphibia,urodela, ambystomatidae) with special reference to the sexual collecting ducts

This study details the gross and microscopic anatomy of the pelvic kidney in male Ambystoma maculatum. The nephron of male Ambystoma maculatum is divided into six distinct regions leading sequentially away from a renal corpuscle: (1) neck segment, which communicates with the coelomic cavity via a ventrally positioned pleuroperitoneal funnel, (2) proximal tubule, (3) intermediate segment, (4) distal tubule, (5) collecting tubule, and (6) collecting duct. The proximal tubule is divided into a vacuolated proximal region and a distal lysosomic region. The basal plasma membrane is modified into intertwining microvillus lamellae. The epithelium of the distal tubule varies little along its length and is demarcated by columns of mitochondria with their long axes oriented perpendicular to the basal lamina. The distal tubule possesses highly interdigitating microvillus lamellae from the lateral membranes and pronounced foot processes of the basal membrane that are not intertwined, but perpendicular to the basal lamina. The collecting tubule is lined by an epithelium with dark and light cells. Light cells are similar to those observed in the distal tuble except with less mitochondria and microvillus lamellae of the lateral and basal plasma membrane. Dark cells possess dark euchromatic nuclei and are filled with numerous small mitochondria. The epithelium of the neck segment, pleuroperitoneal funnel, and intermediate segment is composed entirely of ciliated cells with cilia protruding from only the central portion of the apical plasma membrane. The collecting duct is lined by a highly secretory epithelium that produces numerous membrane bound granules that stain positively for neutral carbohydrates and proteins. Apically positioned ciliated cells are intercalated between secretory cells. The collecting ducts anastomose caudally and unite with the Wolffian duct via a common collecting duct. The Wolffian duct is secretory, but not to the extent of the collecting duct, synthesizes neutral carbohydrates and proteins, and is also lined by apical ciliated cells intercalated between secretory cells. Although functional aspects associated with the morphological variation along the length of the proximal portions of the nephron have been investigated, the role of a highly secretory collecting duct has not. Historical data that implicated secretory activity concordant with mating activity, and similarity of structure and chemistry to sexual segments of the kidneys in other vertebrates, lead us to believe that the collecting duct functions as a secondary sexual organ in Ambystoma maculatum. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

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.     

Roles of ZIP8, ZIP14, and DMT1 in transport of cadmium and manganese in mouse kidney proximal tubule cells

Chronic exposure to cadmium causes preferential accumulation of cadmium in the kidney, leading to nephrotoxicity. In the process of renal cadmium accumulation, the cadmium bound to a low-molecular-weight metal-binding protein, metallothionein, has been considered to play an important role in reabsorption by epithelial cells of proximal tubules in the kidney. However, the role and mechanism of the transport of Cd(2+) ions in proximal tubule cells remain unclear. Zinc transporters such as Zrt, Irt-related protein 8 (ZIP8) and ZIP14, and divalent metal transporter 1 (DMT1) have been reported to have affinities for Cd(2+) and Mn(2+). To examine the roles of these metal transporters in the absorption of luminal Cd(2+) and Mn(2+) into proximal tubule cells, we utilized a cell culture system, in which apical and basolateral transport of metals can be separately examined. The uptake of Cd(2+) and Mn(2+) from the apical side of proximal tubule cells was inhibited by simultaneous addition of Mn(2+) and Cd(2+), respectively. The knockdown of ZIP8, ZIP14 or DMT1 by siRNA transfection significantly reduced the uptake of Cd(2+) and Mn(2+) from the apical membrane. The excretion of Cd(2+) and Mn(2+) was detected predominantly in the apical side of the proximal tubule cells. In situ hybridization of these transporters revealed that ZIP8 and ZIP14 are highly expressed in the proximal tubules of the outer stripe of the outer medulla. These results suggest that ZIP8 and ZIP14 expressed in the S3 segment of proximal tubules play significant roles in the absorption of Cd(2+) and Mn(2+) in the kidney.     

Morphology of the kidney in larvae of Bufo viridis (Amphibia, Anura, Bufonidae)

This study deals primarily with the morphology and ultrastructure of the pronephros in the green toad Bufo viridis during prometamorphosis when the pronephros and the developing mesonephros function simultaneously. Furthermore, the mesonephros was studied during pro- and postmetamorphosis with emphasis on the distal segments of the nephron. The paired kidneys consist of two cranial pronephroi immediately behind the gill region and two more caudal elongated mesonephroi. Each pronephros consists of a single convoluted tubule which opens into the coelom via three nephrostomes. This tubule is divided into three ciliated tubules, three proximal tubule branches, a common proximal tubule and a distal tubule, which in turn continues into the nephric duct. No intermediate segment is present. The length of the pronephric tubule is 12 mm, including the three branches of the ciliated tubules and proximal tubules. Primary urine is formed upon filtration from an external glomerulus, which is a convoluted capillary lined by podocytes, a specialization of the coelomic epithelium. From the coelom the filtrate is swept into the ciliated tubules. In the collecting duct system of the developing mesonephric nephron epithelial cells with conspicuous, apical osmiophilic granules appear in larvae of 9-10 mm. Heterocellularity of mixed intercalated (mitochondria rich) cells and principal cells is observed in the collecting duct system and nephric duct from a larval body length of 14 mm. As the proliferation of mitochondria-rich cells proceeds, the osmiophilic granules disappear and are completely absent from the adult amphibian mesonephros.     

Localization of carbonic anhydrase activity in the vertebrate nephron

Synopsis The localization of carbonic anhydrase activity in the vertebrate nephron has been examined with particular reference to the proximal tubule and collecting duct. In all species studied, activity was present in the proximal tubular epithelium. In the pigeon and turtle, distinctive and similar patterns of staining were observed in the glomerulus and first portion of the proximal tubule. In the rat and rhesus monkey, the entire proximal tubule exhibited activity; in these species it has been shown previously with micropuncture techniques that there is a high absorptive capacity of this nephron segment for bicarbonate. In contrast, large portions of the dog proximal tubule were inactive; similar studies in this animal have shown tubular concentrations of bicarbonate only slightly lower than plasma levels. In the rat and dog, the entire length of the collecting duct was diffusely and intensely active; in contrast, pigeon collecting duct showed no activity. An alternating pattern of inactive and intensely active cells was observed in the collecting ducts of the toad, turtle, rabbit and monkey. A similar pattern has been described in the turtle and toad bladder, tissues utilized forin vitro studies of ion transport and H⁺ secretion.     

Comparative ultrastructure of coxal glands in unfed larvae of Leptotrombidium orientale (Schluger, 1948) (Trombiculidae) and Hydryphantes ruber (de Geer, 1778) (Hydryphantidae)

Coxal glands of unfed larvae Leptotrombidium orientale (Schluger, 1948) (Trombiculidae), a terrestrial mite parasitizing vertebrates, and Hydryphantes ruber (de Geer, 1778) (Hydryphantidae), a water mite parasitizing insects were studied using transmission electron microscopy. In both species, the coxal glands are represented by a paired tubular organ extending on the sides of the brain from the mouthparts to the frontal midgut wall and are formed of the cells arranged around the central lumen. As in other Parasitengona, the coxal glands are devoid of a proximal sacculus. The excretory duct, joining with ducts of the prosomal salivary glands constitutes the common podocephalic duct, opening into the subcheliceral space. The coxal glands of L. orientale are composed of a distal tubule with a basal labyrinth, an intermediate segment without labyrinth, and a proximal tubule bearing tight microvilli on the apical cell surface and coiled around the intermediate segment. The coxal glands of H. ruber mainly consist of the uniformly organized proximal tubule with apical microvilli of the cells lacking the basal labyrinth. This tubule shows several loops running backward and forward in a vertical plane on the side of the brain. In contrast to L. orientale , larvae of H. ruber reveal a terminal cuticular sac/bladder for accumulation of secreted fluids. Organization of the coxal glands depends on the ecological conditions of mites. Larvae of terrestrial L. orientale possess distal tubule functioning in re‐absorption of ions and water. Conversely, water mite larvae H. ruber need to evacuate of the water excess, so the filtrating proximal tubule is prominent.     

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.     

Regulation of renal proximal and distal tubule transport: sodium, chloride and organic anions

Renal tubular transport and its regulation are reviewed for Na(+) (and Cl(-)), and for fluid and organic anions (including urate). Filtered Na(+) (and Cl(-)) is reabsorbed along the tubules but only in mammals and birds does most reabsorption occur in the proximal tubules. Reabsorption involves active transport of Na(+) and passive reabsorption of Cl(-). The active Na(+) step always involves Na-K-ATPase at the basolateral membrane, but the entry step at luminal membrane varies among tubule segments and among vertebrate classes (except for Na(+)-2Cl(-)-K(+) cotransporter in diluting segment). Regulation can involve intrinsic, neural and endocrine factors. Proximal tubule fluid reabsorption is dependent on Na(+) reabsorption in all vertebrates studied, except ophidian reptiles. Fluid secretion occurs in glomerular and aglomerular fishes, reptiles and even mammals, but its significance is not always clear. A non-specific transport system for net secretion of organic anions (OAs) exists in the proximal renal tubules of almost all vertebrates. Net transepithelial secretion involves: (1) transport into the cells at the basolateral side against an electrochemical gradient by a tertiary active transport process, in which the final step involves OA/alpha-ketoglutarate exchange and (2) movement out of the cells across the luminal membrane down an electrochemical gradient by unknown carrier-mediated process(es). Regulation may involve protein kinase C and mitogen-activated protein kinase. Urate is net secreted in the proximal tubules of birds and reptiles. This process is urate-specific in reptiles but in birds, it may involve both a urate-specific system and the general OA system.     

Protein localization of SLC26A2 (DTDST) in rat kidney

The SLC26 family represents a group of integral membrane anion transport proteins. Mutations in one member of this protein family, SLC26A2 (DTDST or diastrophic dysplasia sulfate transporter), result in various chondrodysplasias due to undersulfation of proteoglycans in chondrocytes, a major site of DTDST protein expression. DTDST mRNA has been detected in the kidney, but protein expression has not been characterized. Our objective for this study was to determine the protein localization of this sulfate transporter in the kidney. We used immunofluorescence (IMF) techniques with an anti-DTDST monoclonal antibody to examine kidneys harvested from adult rats. Double labeling was performed with antibodies directed against megalin, which is found in the microvillus membrane and coated pits of the proximal tubule. IMF analysis indicated that DTDST protein expression was limited to the microvillus membrane of proximal tubule cells in the renal cortex but absent in glomeruli and other nephron segments. DTDST was also detected in isolated rat kidney proximal tubule microvillus membranes by Western blot analysis, confirming the immunofluorescent localization of the DTDST transporter to this nephron segment. The functional role of the DTDST protein in the kidney is unknown, but it may play a role in proximal tubule sulfate transport.     

Primary rabbit kidney proximal tubule cell cultures maintain differentiated functions when cultured in a hormonally defined serum-free medium

Summary A primary rabbit kidney epithelial cell culture system has been developed which retains differentiated functions of the renal proximal tubule. In addition, the cells have a distinctive metabolism and spectrum of hormone responses. The primary cell were observed to retain in vitro a Na⁺-dependent sugar transport system (distinctive of the proximal segment of the nephron) and a Na⁺-dependent phosphate transport system. Both of these transport processes are localized on the apical membrane of proximal tubule cells in vivo. In addition, probenicid-sensitivep-aminohippurate (PAH) uptake was observed in basolateral membranes of the primary tubule cells, and the PAH uptake by these vesicles occurred at a rate that was very similar to that observed with membranes derived from the original tissue. Several other characteristics of the primary cells were examined, including hormone-sensitive cyclic AMP production and phosphoenolpyruvate carboxykinase (PEPCK) activity. Like the cells in vivo, the primary proximal tubule cells were observed to produce significant cyclic AMP in response to parathyroid hormone, but not in response to arginine vasopressin or salmon calcitonin. Significant PEPCK acivity was observed in the particulate fraction derived from a homogenate of primary rabbit kidney proximal tubule cells. This paper was presented at a Symposium on the Physiology and Toxicology of the Kidney In Vitro co-sponsored by The Society of Toxicology (SOT) and the Tissue Culture Association held at the 27th annual meeting of the SOT in Dallas, Texas in 1988. This work was supported by Grant 9 RO1 DK40286-07 from the National Institutes of Health, Bethesda, MD, and NIH Research Career Development Award 1 K04 CA 0088-01 to M.T.     

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.     

The kidney,adrenocortical homolog,and corpuscles of Stannius in the cockscomb pricklebackAnoplarchus purpurescens

The morphology of the kidney, adrenocortical homolog, and the corpuscles of Stannius was examined in the cockscomb prickleback,Anoplarchus purpurescens, a marine teleost which inhabits the intertidal zone. The paired kidneys of this fish are fused throughout most of their length, there is essentially a single posterior cardinal vein on the right side, they possess renal corpuscles, and there is no distal segment of the tubule. The tubule is specialized, in descending order, into ciliated neck and two proximal segments before entering the system of collecting tubules and ducts. The cells of the latter system are specialized for mucous secretion, as are cells of the main excretory ducts, the paired archinephric ducts. Tubulogenesis occurs in the kidneys in close apposition to the archinephric ducts. The presumptive adrenocortical homolog is located around the posterior cardinal veins in the head kidney while paired corpuscles of Stannius are confined to the posterior end of the kidney. All of the above features are consistent with those found in the kidneys of many other marine teleosts.     

Sodium-sensitive, probenecid-insensitive p-aminohippuric acid uptake in cultured renal proximal tubule cells of the rabbit.

In the intact kidney, renal proximal tubule cells accumulate p-aminohippurate (PAH) via a basolateral, probenecid- and sodium-sensitive transport system. Primary cultures of rabbit proximal tubule cells retain sodium-glucose co-transport in culture, but little is known about PAH transport in this system. Purified proximal tubule cells from a rabbit were grown in culture and assessed for PAH and alpha-methyl-D-glucoside uptake capacities as well as proximal tubule marker enzyme activities. Control PAH uptake on collagen-coated filters (20 +/- 3 pmol/mg protein.min; n = 8) was not significantly different from uptake in the presence of 1 mM probenecid (19 +/- 4 pmol/mg protein.min; n = 8). Uptake from the basal side of the cell was 3.9 +/- 0.7 times greater than that from the apical side. In multi-well plate studies, the uptake was significantly reduced by removing sodium from the medium and stimulated by coating the wells with collagen. Glutarate (10 mM) had no effect on the uptake of PAH. Other differentiated proximal tubule characteristics were retained in culture, including the ability to form domes and to transport glucose by a phlorizin-sensitive system. Phlorizin-sensitive 1 mM alpha-methyl-D-glucoside uptake was 134 +/- 42 pmol/mg protein.min (n = 7; P less than 0.02). The proximal tubule marker enzymes alkaline phosphatase and gamma-glutamyltranspeptidase, increased in activity in the cultures after confluence. It was concluded that whereas some differentiated properties were retained during primary culture of rabbit proximal tubule cells, the PAH transport system was selectively lost or modified from that present in the intact kidney.     

The fine structure of the elasmobranch renal tubule: intermediate, distal, and collecting duct segments of the little skate.

Sharks, skates, and rays (Elasmobranchii) have evolved unique osmoregulatory strategies to survive in marine habitats. These adaptations include a complex renal countercurrent system for urea retention. The fine structure of the complete renal tubular epithelium has yet to be elucidated in any species of cartilagenous fish. The present study, which is a companion to our recent paper describing the ultrastructure of the neck and proximal segments of the elasmobranch nephron, uses thin sections and freeze-fracture replicas to elucidate the fine structural organization of the intermediate, distal, and collecting duct segments of the little skate, Raja erinacea, renal tubule. The epithelium of the intermediate, distal, and collecting duct segments consists of two major cell types: nonflagellar cells, the major epithelial cell type; and flagellar cells, described elsewhere. The intermediate segment consists of six subdivisions lined by cuboidal-columnar cells with variously elaborated microvilli and interdigitations of lateral and basal cell plasma membranes, as well as some subdivisions with distinctive vesicles and granules. The distal segment consists of two subdivisions, both of which are lined by a simple epithelium, and are distinguished from each other by their distinctive contents; dense bodies and granules. The collecting duct segment also has two subdividions, the first lined by a simple columnar epithelium and the second by a stratified columnar epithelium. Both subdivisions have apical secretory granules. The present findings show a more highly specialized and diverse epithelium lining the renal tubule of these cartilagenous fish than is found in either of the "adjacent" phylogenetic taxa, Agnatha or Ostheichthyes, suggesting significant differences among these groups in transepithelial transport mechanisms and renal function.     

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.