Lateral Intercellular Spaces Research Articles

Quantitative ultrastructure and functional correlates in proximal tubule of Ambystoma and Necturus.

The functional properties of the proximal tubule in the salamander Ambystoma tigrinum have been well characterized but its ultrastructure has not been examined. We therefore analyzed the qualitative and quantitative ultrastructure of the proximal tubule in this species as a basis for structure-function correlations. In addition, a comparative study between species was performed between Ambystoma and Necturus. In Ambystoma the basal cell membrane but not the lateral cell membrane has a highly elaborate organization and is greatly amplified at the basal cell surface. Therefore, the bulk of the basolateral membrane does not face the lateral intercellular space but faces a basal extracellular labyrinth immediately adjacent to the peritubular space. We suggest that this intraepithelial compartment may serve as a site for solute-solvent interactions. The morphometric comparative analysis provides quantitative estimates of tubule dimensions, volume of cells and extracellular channels, areas of luminal, lateral, and basal cell membranes as well as averaged dimensions of the lateral intercellular spaces. Structure-function correlations show that when certain functional parameters are normalized on the basis of ultrastructural rather than epithelial dimensions the interspecies variability decreases.

The uptake of cationized ferritin by guinea-pig gall bladder in vitro.

Tissue pieces of guinea-pig gall bladder were grown in vitro for up to ten days. Over this period at different intervals, specimens were exposed to cationized ferritin in culture medium for 1 h and then grown in ferritin free medium for up to 24 h. Other specimens were grown in culture medium containing cationized ferritin for up to 24 h. Both treatments produced a similar morphological sequence. Electron microscopy at all intervals studied showed the cationized ferritin was first bound by the apical cell membrane, clumped and internalized in large 400 nm vesicles. It was then carried to lysosomes in the region of the Golgi apparatus. Within 1 h, the marker was exocytosed in clumps into the lateral intercellular space, accumulating against the basement membrane in a roughly regular approximately 60 nm array. This pathway of cationized ferritin through the gall bladder epithelium is the same as that followed in vivo although the time taken was shorter in vitro.

Morphologic response of the rabbit cortical collecting tubule to peritubular hypotonicity: quantitative examination with differential interference contrast microscopy.

The isolated and perfused cortical collecting tubule of the rabbit was examined by differential interference contrast microscopy in order to characterize the morphologic response of this nephron segment to peritubular hypotonicity. Computer-assisted, morphometric procedures were developed to obtain measurements of cell volume and lateral intercellular space geometry from interference contrast images of perfused nephron segments. Following dilution of the bath from 290 to 190 mOsm in the absence of antidiuretic hormone (T = 25 degrees C), the cells swelled rapidly to a new steady-state volume which was maintained for at least 20 to 30 min and which was about 90% of that predicted for ideal osmometric behavior. The increase in cell volume was accomplished entirely by bulging of the cells into the lumen; lateral space width and outside tubule diameter were unaffected by peritubular hypotonicity. In addition, the swelling of the cells was associated with an apparent swelling of intracellular organelles, e.g., nuclei and mitochondria. Our results indicate that cells of the mammalian collecting tubule swell without the capacity for significant volume regulation at 25 degrees C and without the cytoplasmic vacuolation and dilation of the lateral intercellular spaces observed following the onset of antidiuretic hormone-dependent volume reabsorption (E. Ganote , J. Grantham , H. Moses, M. Burg and J. Orloff , J. Cell Biol. 36:355, 1968).

Quantitative analysis of the structural events associated with antidiuretic hormone-induced volume reabsorption in the rabbit cortical collecting tubule.

We quantitatively examined the influence of antidiuretic hormone (ADH)-dependent volume reabsorption on the morphology of the rabbit cortical collecting tubule. Estimates of cell volume and the geometry of the lateral intercellular spaces were extracted from differential interference contrast images of perfused nephron segments using the morphometric procedures described in the preceding paper (K.L. Kirk , D.R. DiBona and J.A. Schafer, J. Membrane Biol. 79:53-64, 1984). The results indicate that ADH addition in the presence, but not absence, of a lumen-to-bath osmotic gradient (130 to 290 mOsm) stimulated transepithelial volume flow and simultaneously increased the volumes of both the cells (+28%) and the lateral intercellular spaces (+78%). In addition, the formation of cytoplasmic vacuoles could be observed during the latter stages of the swelling response, and vacuole formation continued well after new steady-state values for transepithelial water flow and cell volume had been reached. Two main conclusions can be drawn from these results. First, the cytoplasmic vacuoles comprise a slowly filling compartment that lies in parallel to the transepithelial pathway for ADH-stimulated volume reabsorption. Second, from the magnitude of the cell volume increase, we estimate that the hydraulic conductivities of the opposing cell membranes are nearly equal during maximal ADH stimulation.

<b>IMMUNOCYTOCHEMICAL EVIDENCE FOR A VESICULAR TRANSPORT OF ALBUMIN ACROSS THE EPITHELIUM IN THE RAT INTERLOBULAR BILE </b><b>DUCT </b>

Mechanism of transport of serum albumin from intercellular fluid into the bile duct was investigated by a pre-embedding immunocytochemical technique. Rat livers were fixed by perfusion, and vibratome sections were incubated with anti-rat serum albumin (Fab fragments) conjugated with peroxidase. After washing in phosphate-buffered saline (PBS), sections were fixed in glutaraldehyde, followed by incubation in diaminobenzidine (DAB)-H2O2 medium for peroxidase, The sections were then embedded in Epon. On light microscopy, the DAB reaction product (antigenic sites for albumin) was seen in the interlobular connective tissue in which bile ducts were embedded. On electron microscopy, electron dense reaction products were located in interdigitating lateral intercellular space between biliary epithelial cells. The staining was confined to the space at the level of desmosome; zonula adherens and zonula occludens were not stained. On the basal and lateral surface of the epithelial cells, numerous albumin-positive endocytotic vesicles were seen. Multivesicular bodies and peripheral dilatations of Golgi lamellae were also frequently stained. The results suggest that serum albumin is transported across the biliary epithelium by vesicular, intracellular traflic and that diffusion of albumin through the intercellular space between the biliary epithelial cells does not occur under normal conditions.

Role of tight-junctional pathways in bile salt-induced increases in colonic permeability.

The effects of a dihydroxy bile salt, taurochenodeoxycholate (TCDC), on the permeability and conductance of isolated, short-circuited segments of the rabbit descending colon were examined using conventional Ussing chamber techniques. Increasing concentrations of TCDC (1-4 mM) produced dose-dependent increases in sodium backflux (JNas leads to m) and tissue conductance (Gt) when applied to either the mucosal or serosal salines. However, mucosal addition was twice as potent in increasing JNas leads to m and Gt at 4 mM. Tracer experiments indicated that the transepithelial serosal-to-mucosal fluxes of sodium and mannitol are via an aqueous, unrestricted, free-solution pathway, while albumin movements are restricted through this pathway both in the absence and presence of mucosal TCDC. The changes in JNas leads to m, JMans leads to m, and Gt caused by 4 mM mucosal TCDC were largely reversed by rinsing the mucosal chamber with fresh buffer. It was also observed that osmotically induced volume flows in the serosal-to-mucosal direction could offset or reverse the changes in Gt produced by 2 mM mucosal TCDC, suggesting that the enhanced conductance pathway is in series with the lateral intercellular spaces. Taken together, these results suggest that low concentrations of TCDC alter the integrity of tight-junctional complexes between the epithelial cells of the rabbit colon.

Early functional and morphological changes in renal tubular necrosis due top-aminophenol

AbstractEarly functional and morphological changes in renal tubular necrosis due to p-aminophenol. Functional and morphological changes developed rapidly in rats after the intravenous administration of the organic nephrotoxin p-aminophenol. Proximal intratubular pressure remained close to its mean control value of 14.9 ± 0.9 mm Hg up to 40 min after injection of the nephrotoxin then rose rapidly over the following 50 min to a maximum of 38.7 ± 7.4 mm Hg. Distal tubular pressure also rose in the same manner. Renal blood flow remained constant, but GFR fell to 11% of control values while fractional excretion of sodium and water rose 12 and five times, respectively. Morphological changes developed in parallel with the functional changes. They were widespread, varied in intensity from cell to cell, were more severe in the distal third of the proximal convoluted tubule and consisted of cytoplasmic swelling, reduced organelle concentration, reduction or loss of basal infoldings, widening of lateral intercellular spaces, extrusion of bubbles of cell sap into the tubular lumen; brush borders were preserved. No casts were present up to 90 min. Similar results were seen when p-aminophenol was added to the perfusate of the isolated perfused kidney. It is proposed that metabolic and morphological damage leads rapidly to both impairment of proximal tubular sodium reabsorption and increased flow resistance in the cortical collecting system. Both effects contribute to a rise in tubular pressures which oppose glomerular filtration.Modifications précoces fonctionnelles et morphologiques au cours de la nécrose tubulaire rénale par le p-aminophénol. Des modifications fonctionnelles et morphologiques se sont développées rapidement chez des rats après administration intraveineux de p-aminophénol, une néphrotoxine organique. La pression intra-tubulaire proximale est restée proche de la valeur contrôle moyenne de 14,9 ± 0,9 mm Hg jusqu'à 40 min après injection de la néphrotoxine, puis s'est élevée rapidement pendant les 50 min suivantes jusqu'à un maximum de 38,7 ± 7,4 mm Hg. La pression tubulaire distale s'est également élevée de la même façon. Le flux sanguin rénal est resté constant, mais GFR a chuté à 11% des valeurs contrôles tandis que les excrétions fractionnelles de sodium et d'eau se sont élevées 12 et cinq fois, respectivement. Des modifications morphologiques sont apparues parallèlement aux modifications fonctionnelles. Elles étaient diffuses, variaient d'intensité d'une cellule à l'autre, étaient plus sévères dans le tiers distal du tubule contourné proximal et consistaient en un gonflement cytoplasmique, une réduction de la concentration des organelles, une réduction ou une perte des invaginations basales, un élargissement des espaces intercellulaires latéraux, une extrusion de bulles de liquide cellulaire dans la lumière tubulaire; les bordures en brosse étaient préservées. Aucun cylindre n'était présent jusqu'à 90 min. Des résultats identiques ont été observés lorsque le p-aminophénol était ajouté au perfusat d'un rein isolé perfusé. Il est proposé que la lésion métabolique et morphologique conduit rapidement à une altération de la réabsorption tubulaire proximale du sodium et à une augmentation de la résistance au flux dans le système collecteur cortical. Ces deux effets contribuent à une augmentation des pressions tubulaires qui s'opposent à la filtration glomérulaire.

Paracellular shunt ultrastructure and changes in fluid transport in Necturus proximal tubule

AbstractParacellular shunt ultrastructure and changes in fluid transport in Necturus proximal tubule. To define the structure-function relationships of the lateral intercellular space (LIS), we examined the ultrastructure of Necturus proximal tubule following experimental variation of fluid absorption with or without alteration of active sodium transport. Comparisons were made between (1) control (blood-perfused) kidneys and kidneys doubly perfused using either (2) Ringer solution with organic substrate, (3) Ringer solution with substrate in capillaries but steady-state solution in tubules, (4) Ringer without substrate, or (5) Ringer with low sodium. Intratubular and peritubular capillary pressures were monitored before and during standardized perfusion-fixation for electron microscopy and complete cross-sections of sampled proximal tubules were analyzed by morphometry. All morphometric parameters were the same for proximal tubules in kidneys perfused with blood and kidneys perfused with substrate Ringer. Morphometric parameters of cells and lateral intercellular spaces were the same for tubules with normal volume reabsorption rate (Jv) and with Jv = 0 in substrateperfused kidneys, and for tubules in kidneys perfused without substrate. However, in kidneys perfused with low sodium, cell height, cell volume, LIS-volume, and average maximum width of LIS were significantly decreased while tight junction length and peritubular length of LIS were unchanged. The results suggest that the measured dimensions of the lateral intercellular space in Necturus proximal tubule are independent of the magnitude of transepithelial salt and fluid movement per se but influenced by hydrostatic pressure gradients and active sodium transport.Ultrastructure du shunt paracellulaire et modifications du transport de liquides dans le tubule proximal de Necturus. Afin de déterminer la relation structure-fonction de l'espace latéral intercellulaire (LIS), nous avons examiné l'ultrastructure du tubule proximal de Necturus après variation expérimentale de l'absorption liquidienne, avec ou sans altération du transport actif de sodium. Les comparaisons ont été faites entre (1) des reins contrôles (perfusés avec du sang) et des reins doublement perfusés avec soit (2) une solution de Ringer avec des substrats organiques, (3) une solution de Ringer avec des substrats dans les capillaires mais une solution contrôle dans les tubules, (4) du Ringer sans substrat, ou (5) du Ringer avec une faible concentration de Na. Les pressions capillaires intratubulaires et péritubulaires ont été mesurées avant et pendant une perfusion-fixation standardisée pour microscopie électronique, et des sections croisées complètes d'échantillons de tubules proximaux ont été analysées par morphométrie. Tous les paramètres morphométriques étaient les mêmes pour les tubules proximaux de reins perfusés avec du sang ou de reins perfusés avec du Ringer avec substrats. Les paramètres morphométriques des cellules et des espaces intercellulaire latéraux étaient les mêmes dans les tubes ayant un débit de réabsorption volumique (Jv) normal, avec Jv = 0 dans les reins perfusés avec du substrat, et dans les tubules de reins perfusés sans substrat. Cependant, dans les reins perfusés avec un faible Na, la hauteur cellulaire, le volume cellulaire, le volume des LIS, et la largeur maximale moyenne des LIS étaient significativement diminués tandis que la longeur des jonctions serrées, et la longeur péritubulaire des LIS étaient inchangées. Ces résultats suggèrent que les dimensions mesurées de l'espace intercellulaire latéral du tubule proximal de Necturus sont indépendantes de l'importance des mouvements transépithéliaux de sel et d'eau per se mais sont influencées par les gradients de pressions hydrostatiques et le transport actif de sodium.

Nonequilibrium thermodynamic model of the rat proximal tubule epithelium

The rat proximal tubule epithelium is represented as well-stirred, compliant cellular and paracellular compartments bounded by mucosal and serosal bathing solutions. With a uniform pCO2 throughout the epithelium, the model variables include the concentrations of Na, K, Cl, HCO3, H2PO4, HPO4, and H, as well as hydrostatic pressure and electrical potential. Except for a metabolically driven Na-K exchanger at the basolateral cell membrane, all membrane transport within the epithelium is passive and is represented by the linear equations of nonequilibrium thermodynamics. In particular, this includes the cotransport of Na-Cl and Na-H2PO4 and countertransport of Na-H at the apical cell membrane. Experimental constraints on the choice of ionic conductivities are satisfied by allowing K-Cl cotransport at the basolateral membrane. The model equations include those for mass balance of the nonreacting species, as well as chemical equilibrium for the acidification reactions. Time-dependent terms are retained to permit the study of transient phenomena. In the steady state the energy dissipation is computed and verified equal to the sum of input from the Na-K exchanger plus the Gibbs free energy of mass addition to the system. The parameter dependence of coupled water transport is studied and shown to be consistent with the predictions of previous analytical models of the lateral intercellular space. Water transport in the presence of an end-proximal (HCO3-depleted) luminal solution is investigated. Here the lower permeability and higher reflection coefficient of HCO3 enhance net sodium and water transport. Due to enhanced flux across the tight junction, this process may permit proximal tubule Na transport to proceed with diminished energy dissipation.

The intercellular junctions of guinea-pig placental capillaries: a possible structural basis for endothelial solute permeability

The endothelial cell junction in guinea-pig placental capillaries consists of a continuous ribbon desmosome (zonula adherens) within which lies a particulate tight junction consisting of between one and five anastomosing strands. The intercellular space at these tight junctions is narrowed and is subdivided by junctional bars which are probably continuous with the intramembrane particle rows seen in freeze-fracture replicas of the junctions. Perfusion with lanthanum salts shows the gaps between the junctional bars to be lanthanum-filled and the entire junction to be lanthanum permeable. The estimated size of the spaces between the junctional bars is consistent with the junctional pore size indicated by previous ultrastructural tracer studies. The wider lateral intercellular space of the ribbon desmosome is spanned by more widely spaced "linkers" which may act as a coarser three-dimensional filter in series with size-limiting pores between the tight junctional bars.

Iron loading in the liver of parasitic adult lampreys, Petromyzon marinus L.

Electron microscopic and histochemical procedures were used to describe iron loading in the normal liver of parasitic adult sea lampreys. Petromyzon marinus L. Ferric iron is present in large quantities as ferritin in the cytoplasmic matrix, in hemosiderin granules, and in large vacuoles of hepatocytes. Ferritin is also found in the lateral intercellular space, in the perivascular space, and in the sinusoidal lumen. Large crystals in the cytoplasmic and nuclear matrices of some hepatocytes likely represent excessive aggregations of ferritin molecules. Ferrous iron is localized at the lateral cell membrane and vacuolar membranes of hepatocytes. Iron accumulation is less significant in Kupffer cells which are present in small numbers. The relationship of iron loading in the adult lamprey liver to the sanguivorous diet, the rate of erythrophagocytosis, the absence of bile ducts and canaliculi, and the nature of the iron-binding proteins in the plasma are considered. Emphasis is placed on the similarity of the above morphological findings to those resulting from experimentally induced and pathological states of iron loading in other vertebrates, and on the potential use of the lamprey as a model for studying iron loading in the vertebrate liver.

Functional properties of the paracellular pathway in some leaky epithelia.

We here review the functional properties of the paracellular pathway of leaky epithelia such as gallbladder and renal proximal tubule. These epithelia are characterized by leaky terminal bars between adjacent cells which allow small ions, non-electrolytes and water to leak from lumen to interstitial fluid or back. In the past 10 years a great deal of information has been obtained about the properties of the misnamed 'tight' junctions in the terminal bars, by assuming that the overall permeation pattern reflected predominantly the junctional permeation properties. Although recent trans- and intraepithelial impedance analyses indicate that this assumption is not always justified (the contribution of the lateral intercellular space to the paracellular shunt resistance is not negligible, when the spaces are collapsed) it seems that the major conclusions are correct. The properties of the terminal junctions may thus be summarized as follows. (1) Large molecules such as horseradish peroxidase are not able to pass. (2) Passage of lipophilic substances is insignificant, as these substances permeate by the cellular route. (3) Depending on the tissue, ion permeation is either governed by channels with negative fixed charges, or positive fixed charges, or both. As inferred from ion selectivity patterns the channels of different epithelia are either wide and highly hydrated or narrow and poorly hydrated, thus allowing more or less water molecules to pass besides the ions. In narrow channels single-file diffusion may occur. (4) Besides the selective channels a free solution shunt seems to be present in some epithelia. (5) When applied in millimolar concentrations 2,4,6-triaminopyrimidinium and amiloride block negatively charged junctional channels. However these substances do not simply turn leaky epithelia into tight epithelia, because they have additional effects on the cell membranes. (6) As observed in cell cultures, formation of tight junctions requires connecting particles to be present on the cell surface--which seems to be controlled by the cytoskeleton-- and requires the presence of calcium ions as ligands. (7) Cellular control over paracellular permeability may be exerted through changes of intracellular calcium concentration.

Water permeability and pathways in the proximal tubule.

The route of water transport in the proximal tubule could be either transjunctional or transcellular. A transjunctional route is supported by data showing high osmotic-to-diffusive water permeability ratios, the possible correlation of junctional leakiness to ions and nonelectrolytes with water permeability, and solvent drag of nonelectrolytes and ions. These data, however, are not convincing. A transcellular route of water transport is supported by data showing that the osmotic water permeability (Pf) for apical and/or basolateral cell membranes is sufficiently high to account for the transepithelial Pf, making a tentative conclusion for a transcellular route of water transport possible. In addition, measurements of Pf have yielded insights into the mechanism of solute-solvent coupling. Pf has been reported to be mostly between 0.1 and 0.3 cm/s. In the rabbit proximal straight and the Necturus proximal convoluted tubule, in which water transport rates are low, this range of Pf will account for volume absorption with only small osmotic gradients (less than 6 mosmol). Higher osmotic gradients are required in the rat and possibly the rabbit proximal convoluted tubule, where water transport rates are higher. Solute-solvent coupling in all species is probably due to both luminal hypotonicity and lateral intercellular space hypertonicity. These two processes are directly linked. Mass balance requires that generation of luminal hypotonicity also generates a hypertonic absorbate and, thus, some degree of lateral intercellular space hypertonicity. It is likely that, in the rabbit at least, effective osmotic pressure gradients due to differences in solute reflection coefficients play little role in solute-solvent coupling.

Use of AC Impedance Analysis to Study Membrane Changes Related to Acid Secretion in Amphibian Gastric Mucosa

We have applied transepithelial AC impedance techniques to gastric mucosa to reconcile ultrastructural and electrophysiological findings about gastric acid secretion and the mucosal barrier. By fitting impedance data measured at different HCl secretion rates to equivalent circuit models, we extracted capacitances and resistances (as measures of membrane area and ionic conductance, respectively) for the apical and basolateral membranes. The impedance measurements were found to be incompatible with earlier equivalent circuit models that modeled membrane electrical properties as lumped circuits based on one or two cell types. A distributed circuit model was developed that assumed only one dominant electrical pathway (i.e., one cell type), but that incorporated electrical effects arising from long and narrow membrane-lined structures present in the epithelium (e.g., gastric crypts, tubulovesicles, lateral intercellular spaces). This morphologically based model was found to represent the measured data accurately, and to yield values for membrane capacitances consistent with morphometric measurements of membrane areas. The main physiological conclusions from this analysis were as follows: (a) The dominant transepithelial current pathway may reside in the oxyntic cells. (b) The transepithelial conductance increase associated with the onset of acid secretion is entirely due to increased conductance of the apical membrane. This is in turn due entirely to increased area of this membrane, resulting from incorporation of tubulovesicular membrane. (c) When membrane conductances are normalized to actual membrane area by use of membrane capacitances, it turns out that acid secretion is not associated with a change in specific ionic conductance (change in conductance per unit area) at either the apical or basolateral membrane. (d) The puzzlingly low value of transepithelial resistance (</=400 Omega-cm(2)) arises because there are hundreds or thousands of square centimeters of actual membrane area per square centimeter chamber area. Apical membrane resistance is 25 kOmega-cm(2) (actual membrane area), implying a tight barrier to back-diffusion of protons.

Does calcium mediate slowing of gastric emptying by fat in humans?

The relative potencies of 47 compounds in slowing gastric emptying in one subject are given. The majority of the most effective compounds are detergent, but all detergents do not slow gastric emptying. It is proposed that the property common to those detergents that slow gastric emptying is the binding of ionized calcium. Disodium edetate (EDTA) is not a detergent. It has a high affinity for calcium, and it slows gastric emptying strongly. Reasons are given for proposing that decreasing the volumes of the lateral intercellular spaces of the luminal half of the duodenal villi mediates the slowing of gastric emptying by carbohydrate, protein, fats, and acids. Osmotic stimuli in the duodenal lumen, or in the enterocytes, reduce the flux of water into the lateral intercellular spaces. Anions of fatty acids and hydrogen ions displace calcium from a site on the tight junctions at the luminal border of the enterocytes. As a result, the tight junctions become more permeant to water and the lateral intercellular spaces shrink as their content drains into the duodenal lumen. Shrinking of the lateral intercellular space is proposed as the final common event in the transduction of the duodenal stimuli that slow gastric emptying.

Structure of Necturus gallbladder epithelium during transport at low external osmolarities.

Gallbladders transport isotonically over a wide range of osmolarities. This ability has been assumed to depend on the geometry of the lateral intercellular spaces. We report that this geometry in the Necturus gallbladder varies extensively with the external osmolarity and depends in vitro on the integrity of the subepithelial tissues. The structure of the living epithelium was studied by Nomarski light microscopy while ultrastructural effects were revealed by electron microscopy. The short-term effects (less than 60 min) of low external osmolarities were: 1) the cells became bell-shaped with an increased cell height measured centrally, 2) lateral intercellular spaces lost their convoluted character; and 3) numerous membrane-bound cavities appeared in the cells. Furthermore, long-term exposure to the low external osmolarities caused an uneven density of epithelial cells. With subepithelial tissues intact, blistering of the epithelium cell layer was evident. Qualitative electron-microscopic data indicate that the membrane of the cavities was recruited from the basolateral cell membrane. This agrees well with light-microscopic observation that the cavities were initiated as invaginations of this cell membrane.

Steady states and the effects of ouabain in the Necturus gallbladder epithelium: a model analysis.

A simple numerical model for the Necturus gallbladder epithelium is presented. K+, Na+ and Cl- cross the mucosal and serosal membranes as well as the junctions by means of electrodiffusion; furthermore the mucosal membrane contains a neutral entry mechanism for NaCl and the serosal membrane contains an active pump for K+ and Na+. The values which have been used for the model are taken from the literature. The model can only attain steady states if the resistance of the serosal membrane is lower than 1000 omega cm2. Values reported in the literature for the resistance of this membrane vary from about 3000 to about 100 omega cm2. We shall argue, however, that the higher estimates are in error because they are derived from a model of the tissue in which each membrane and the junction are modeled by a resistor; this procedure is invalid because the resistance of the lateral intercellular space relative to the resistance of the tight junctions is neglected and consequently the resistance of the serosal membrane is overestimated by a factor of about four. Apart from predicting a realistic steady state at normal external concentrations the model can predict quantitatively several experimental results obtained from the living epithelium. We have focused on the experiments which test the permeabilities of the serosal membrane and the properties of the pump: i) Replacement of serosal Cl- by an impermeant ion. ii) Replacement of serosal K+ by Na+. iii) Inhibiting the (Na+, K+)-pump. The best correspondence between model and experiments is obtained when the pump is assumed to be electrogenic (or rheogenic) with a ratio of coupling between Na+ and K+ of 3:2. In this case both model and direct experiments (also presented in this paper) show an initial abrupt depolarization of 6 to 7 mV. The model also shows that it cannot be concluded from i and ii that the Cl- permeability of the serosal membrane is low. The model explains, even with high passive Cl- permeabilities, why the intracellular Cl- concentration is relatively unaffected by paracellular currents, a fact which in other epithelia has been taken as an implication of a low Cl- permeability of the serosal membranes.

Effect of bile salt perfusion and intraduct pressure on ionic flux and mucosal ultrastructure in the pancreatic duct of the cat.

Net ionic flux and mucosal ultrastructure were examined following perfusion of the cat pancreatic duct with bicarbonate or sodium taurocholate solutions (5-40 mM). Taurocholate perfusion increased net Cl- gain, net HCO3- loss and net K+ gain and was associated with significant widening of lateral intercellular spaces and increased complexity of intercellular labyrinths. Increased perfusion pressure (30 mm Hg) did not affect flux or ultrastructure during perfusion with bicarbonate but increased net ion flux significantly during perfusion with 40 mM sodium taurocholate. Ultrastructural changes during perfusion of 40 mM taurocholate at increased pressure were not consistent but focal epithelial disruption and cell shedding were seen occasionally. The hypothesis is advanced that taurocholate perfusion triggers physiological transport mechanisms and may make the duct mucosa more vulnerable to other potentially harmful agents. The significance of these changes in the pathogenesis of acute pancreatitis in man remains uncertain and care must be exercised before extrapolating from observed net ion flux data in this animal model.

The morphology of the kidney in young adult anadromous sea lampreys, Petromyzon marinus L., adapted to seawater. I. General morphology and fine structure of the renal corpuscle and the proximal segments

The morphology of the opisthonephric kidney of young adult anadromous sea lampreys, Petromyzon marinus L., was examined at regular intervals during the gradual acclimation of these recently metamorphosed individuals to full-strength seawater. No alteration of the renal corpuscle, the ciliated neck segment, or the proximal pars convoluta and pars recta segments is observed through 40% seawater, but dilation of ventral tubules, the accumulation of a white precipitate, and specific fine structural changes in the cells of the proximal segments are noted in 60% seawater. The ultrastructural alterations are more conspicuous in 80 and 100% seawater and may reflect the involvement of the proximal segment in secretion, likely magnesium and sulphate ions. Flattening of the epithelium in proximal segments, enlargement of lateral intercellular spaces in the recta segment, and apparent increases in numbers of intermediate segments at levels of 80% seawater is more obvious in animals adapted to 100% seawater. The smaller cross-sectional area of the renal corpuscle in animals adapted to 80 and 100% seawater compared with the area in lower salinity might explain the previously found reduced glomerular filtration rate which accompanies seawater acclimation in anadromous lampreys.

In vitro histological and tetracycline staining properties of surface layer rat incisor enamel also reflect the cyclical nature of the maturation process.

Rat incisors cleaned of overlying enamel organ cells stain unevenly with several common histological stains producing two kinds of banding pattern. In one a pattern of widely spaced stained bands run in an oblique to transverse direction from a more apical level on the medial side to a more incisal level on the lateral side of each tooth. The pattern correlated directly with that produced by both in vivo and in vitro short-term tetracycline labelling in the same teeth: it also resembled the patterns previously demonstrated in a) horseradish peroxidase penetration into lateral intercellular spaces between maturation ameloblasts, b) the distribution of enamel labelling in vivo with 45Ca, c) of etching of enamel surfaces demineralized with EDTA with the glutaraldehyde fixed enamel organ in situ and d) in the distribution of smooth-ended and ruffled border types of cell specializations. We conclude that these bands demonstrate further and previously unrecognised aspects of the cyclical phenomena in enamel maturation-mineralization and show cyclical differences in the physico-chemical status of the organic matrix in the maturation process. A second, incremental growth type of banding pattern occurred at 200-225 micron intervals in the lower incisors and had the same distribution at the beginning and end of the maturation zone.