Absence of dilated lateral intercellular spaces in fluid-transporting frog gallbladder epithelium. Direct microscopy observations

Esophageal dilated intercellular spaces (DIS) and nonerosive reflux disease

Esophageal mucosal dilated intercellular spaces (DIS) are frequently observed in patients with nonerosive reflux disease (NERD) and patients with esophagitis. The specificity of DIS is questionable, as it is present in up to 30% of asymptomatic healthy subjects and in patients with other esophageal disorders. DIS occurs in parallel with a drop in potential difference, diminished transepithelial resistance, and increased esophageal mucosal permeability. These alterations arise with exposure to acid and pepsin during gastroesophageal reflux, but the exact pathway of damage to the intercellular junctions remains unclear and seems to be multifactorial. Other noxious contents of the refluxate, such as bile acids, are harmful and DIS can also be induced by acute psychological stress. DIS can be assessed quantitatively with electron microscopy (EM), but it is also recognizable with light microscopy (LM). DIS can disappear after treatment with proton pump inhibitors (PPI); however, this is not the case in all NERD patients. A recent study showed that patients with NERD who are refractory to PPI might still have DIS; and animal experiments showed that persistence of DIS might be due to esophageal mucosal exposure to bile acids and/or psychological stress. In conclusion, DIS is a frequent but nonspecific histological feature of NERD. It can be caused by acid reflux, but bile acids in the refluxate and/or psychological stress can modulate the development or persistence of DIS. Although a causal relationship between DIS and heartburn has been proposed, it still needs to be proven and the underlying mechanisms investigated before considering DIS as a target for treatment of NERD.

Dimensional changes in cells and tissues during specimen preparation for the electron microscope

Studies on dimensional changes incurred during preparation of tissue specimens for the transmission and scanning electron microscopes are reviewed, with emphasis on quantitative measurements pertinent to morphometry and three-dimensional reconstruction. The scope of the review includes fixation, dehydration, plastic embedment, critical-point drying, and freeze-drying. Recommendations are presented for monitoring dimensional changes; a strategy for the choice of method of specimen preparation is outlined.

Structural changes associated with fluid absorption by dog tracheal epithelium

During fluid absorption induced by amphotericin B, the lateral intercellular spaces (LIS) of the dog tracheal epithelium were widely dilated as compared to untreated time controls. When fluid absorption was inhibited by ouabain, or by replacement of luminal Na by choline, amphotericin B failed to cause dilation of the LIS. These data suggest that, as in other epithelia, a significant amount of transepithelial fluid flow passes down the LIS, and that these spaces may provide the local osmotic compartment which is responsible for linking transepithelial fluid movement to active ion transport.

Morphological artifacts induced in intracellularly stained neurons by dehydration: circumvention using rapid dimethyl sulfoxide clearing

In order to observe the fine details of intracellularly stained neurons in brain slices the slices must be cleared of opaque matter. This clearing process involves dehydration of the slice, which typically results in significant shrinkage of the cleared tissue. However, how this shrinkage affects neuronal morphology has not been demonstrated to date. In this paper we detail the artifacts induced in the morphology of stained neurons by this clearing process. During dehydration-induced shrinkage of the brain slices, neurons stained with the water-soluble dye, Lucifer yellow, demonstrated a dramatic decrease in size to less than two-thirds of their original dimensions. In contrast neurons stained with the horseradish peroxidase/diaminobenzidine reaction-product did not shrink with the slice; instead the dendrites bent and curled during dehydration with no loss in cell-soma size. We have managed to circumvent these artifacts by using as a clearing agent the solvent dimethyl sulfoxide, which is miscible in both aqueous and organic phases. This solvent will clear tissue slices without inducing the concomitant artifacts caused by tissue shrinkage occurring with the alcohol-dehydration process.

The tracheobronchial epithelium of the bonnet monkey (Macaca radiata): a quantitative ultrastructural study

Since there are major differences between the airway epithelium of man and that of common laboratory species, the tracheobronchial epithelium of the bonnet macaque was characterized to evaluate its usefulness as a model for study of human conducting airways. This study compared the light microscopic, scanning electron microscopic, and ultrastructural appearance of epithelium from the posterior membranous and anterior cartilaginous trachea and mainstem bronchus. Population densities, epithelial volumetric densities, and frequency distributions of cross-sectional areas of nuclei were determined for cell types present on electron micrographs. Four epithelial cell types were distinguished by ultrastructural criteria. Basal cells were 31% of the population and were similar to those described in other species. Ciliated cells were also similar to those of other species and composed 41% of the population; their nuclei were larger than those of other cell types. Mucous goblet cells had large numbers of secretory granules with electron-dense cores and a lucent periphery. They were only 8% of the population by nuclear count but composed 20% of the epithelial volume. The fourth cell type had multiple small vesicles containing small amounts of granular material and was termed a "small mucous granule cell." Small mucous granule cells (16% of the population) were present in greater numbers than mucous goblet cells but were a smaller proportion of the epithelial volume (8%). While population densities of cell types determined from transmission electron micrographs did not vary between sample sites, scanning electron microscopy demonstrated longitudinal streaks of secretory cells in the posterior trachea suggesting that regional differences in epithelial organization exist. We conclude that the macaque extrapulmonary airway epithelium differs from published descriptions of laboratory rodents in both cell types present and relative abundance of those cell types. Although detailed quantitative studies of human extrapulmonary airways are not available, the primate airways resemble those of man in both the types of cells present and the complexity of pseudostratification.

Effects of cyclic AMP on fluid absorption and ion transport across frog retinal pigment epithelium. Measurements in the open-circuit state

A modified version of a capacitance probe technique has been used to measure fluid transport across the isolated retinal pigment epithelium (RPE)-choroid of the bullfrog. The accuracy of this measurement is 0.5-1.0 nl/min. Experiments carried out in the absence of external osmotic or hydrostatic gradients show that the RPE-choroid transports fluid from the retinal to the choroid side of the tissue at a rate of approximately 10 nl/min (4-6 microliters/cm2 X h). Net fluid absorption (Jv) was abolished within 10 min by the mitochondrial uncoupler 2,4-dinitrophenol. It was also inhibited (70%) by the removal of bicarbonate from the bulk solutions bathing the tissue. Ouabain caused a slow decrease in Jv (no effect at 10 min, 70% at 3 h), which indicates that RPE fluid transport is not directly coupled to the activity of the Na-K pump located at the apical membrane of this epithelium. In contrast to ouabain, cyclic AMP (cAMP) produced a quick decrease in Jv (84% within 5 min). Radioisotope experiments in the open circuit show that cAMP stimulated secretory fluxes of Na and Cl, which accounted for the observed cAMP-induced decrease in Jv. The direction of net fluid absorption, the magnitudes of the net ionic fluxes in the open circuit, and the dependence of Jv on external bicarbonate concentration strongly suggest that fluid absorption is generated primarily by the active absorption of bicarbonate.

Increases in guinea pig small intestinal transepithelial resistance induced by osmotic loads are accompanied by rapid alterations in absorptive-cell tight-junction structure

In some epithelia, mucosal exposure to osmotic loads produces an increase in transepithelial resistance that is presumed to relate to the collapse of the paracellular spaces. Since proximal small intestinal epithelium may transiently encounter osmotic loads during normal digestion, we examined the short-term effect of osmotic loads on resistance and on epithelial structure of mucosal sheets prepared from guinea pig jejunum using Ussing-chamber, thin-section electron-microscopic, and freeze-fracture techniques. After equilibration of mucosal sheets in chambers, mucosal buffer tonicity was increased to 600 mosM with mannitol. This resulted in a 64% increase in resistance within 20 min. Concomitantly, 600 mosM produced a decrease in tight-junction cation selectivity as judged from dilution potentials, collapse of paracellular spaces, decreased cytoplasmic electron density in 10-40% of absorptive cells, and focal absorptive-cell subjunctional lateral-membrane evaginations often associated with microfilament arrays. Freeze-fracture replicas of absorptive-cell tight junctions revealed significant increases in both strand count and depth. Preincubation with 5 micrograms/ml cytochalasin D reduced the 600 mosM resistance increase caused by 600 mosM exposure by 48% but did not prevent the collapse of paracellular spaces. Lowered temperatures that produced morphologic evidence consistent with a gel-phase transition of absorptive-cell lateral membranes prevented both the resistance response and the alterations in tight-junction structure. In conclusion, transient osmotic loads produce an increase in resistance in jejunal epithelium and alter both absorptive-cell tight-junction charge selectivity and structure. These responses, which may have physiologic implications, can be reduced by cytoskeletal inhibitors and ablated by conditions that restrict mobility of absorptive-cell lateral-membrane molecules.

Effects of a small serosal hydrostatic pressure on sodium and water transport and morphology in rabbit gall-bladder

1. In order to investigate the mechanism of serosal pressure-induced inhibition of isosmotic fluid transport, the effect of 4.5 cm water serosal pressure on spontaneous water transfer (J(v)) in rabbit gall-bladders was measured (in the presence of a supporting soft nylon net on the mucosal side) in a modified Ussing chamber. This allowed unidirectional Na(+) fluxes ([Formula: see text] and [Formula: see text]), transepithelial potential difference and resistance (R(t)) to be measured simultaneously. The effects of the serosal pressure were also investigated by light and electron microscopy.2. During pressure application, R(t) increased due to a covering effect of the mucosal support. The serosal pressure caused a parallel decrease in J(v) and net Na(+) transport ([Formula: see text]) across the free epithelial surface of 80-85%. About 85% of the decrease in [Formula: see text] was due to a decrease in [Formula: see text].3. After inhibition of 93% of fluid absorption by serosal 10(-3)M-ouabain, pressure-induced change in J(v) was only 8% of the spontaneous fluid transport rate.4. Control Na(+) flux ratio ([Formula: see text]) was 3.5. The pressure-induced increase in steady-state [Formula: see text] of 30-35% therefore contributed little to the decrease in [Formula: see text]. Further, this increase in [Formula: see text] was completely prevented by mucosal 10(-3) M-amiloride.5. All pressure-induced effects on transport and electrical parameters were reversible.6. The light microscopical and scanning electron microscopical results showed that half of the epithelial surface was covered by the nylon net following serosal pressure application. Ruptures in the epithelium were not seen. Thin section and freeze fracture electron microscopy demonstrated continuous, well developed tight junctions both in control and experimental condition.7. It is concluded that a serosal pressure of only 4.5 cm water causes inhibition of a cellular active Na(+) and water transport with only minimal, if any, contribution from paracellular filtration. This would seem incompatible with the concept that an active ion transport mechanism localized in the basolateral cell membrane is responsible for transepithelial fluid transport. The possibility of a mechanical fluid transport mechanism via elements of a tubulo-cisternal endoplasmic reticulum is raised.

Surface changes in the anaesthetic conjunctiva in man, with special reference to the production of mucus from a non-goblet-cell source

This paper describes changes in the ultrastructure of the human conjunctiva which follows its denervation. The changes are mainly seen in the mucus-producing subsurface vesicles, the surface microvilli, and the intercellular spaces. Some tentative suggestions are made as to how these changes may influence the stability of the tear film in the anaesthetic state.

Fluid transport and dimensions of epithelial cells and intercellular spaces in frog gallbladder. Studies in the living state, and during processing for electron microscopy

Morphologic findings of widely dilated intercellular spaces in fluid transporting epithelia have been claimed as evidence for the existence of an epithelial compartment in which the coupling between solute and water fluxes takes place. The validity of using epithelial geometry in sectioned material as an argument can be questioned. The present report describes the morphological appearance of frog gallbladder epithelium--normal and ouabain-treated--in the living state in vitro and after fixation, dehydration and embedding. Gallbladder segments were photographed in the living state and at the end of each step of the preparative procedure. Direct observations of whole-mounted gallbladder segments were carried out, taking advantage of the possibility of optical sectioning and high resolution by Nomarski-microscopy. The same specimens were then sectioned and examined by conventional light and electron microscopy. The observations were quantitated and showed that the epithelial cells of normal and ouabain-treated gallbladders experienced an average linear shrinkage down to 70% of their length in Ringer's solution, which corresponds to a volume shrinkage down to 35%. Moreover, dilated lateral intercellular spaces appeared during the dehydration and embedding procedure in normal but only very moderately or not at all in ouabain-treated gallbladder specimens.

Hydrostatic pressure changes related to paracellular shunt ultrastructure in proximal tubule

We examined the effets of changes in hydrostatic pressures on the ultrastructural geometry of the lateral intercellular space and tight junctions in proximal tubules of contrtol (C) and volume-expanded (VE) Necturus kidney. The following groups of tubules were studied: (1) C, free-flow pressure, (2) C, stopped-flow, high-luminal pressure, (3) C, stopped-flow, low-luminal pressure, (4) VE, free-flow pressure, and (5) VE, stopped-flow, high-luminal pressure. Intratubular and peritubular capillary pressures were monitored before and during standardized perfusion-fixation for electron microscopy, and complete cross-sections of all sampled tubules were subjected to morphometric analysis. Average lateral intercellular space widths decreased significantly in C and VE stopped-flow tubules with high-luminal pressures but widened greatly in C stopped-flow tubules with low-luminal pressures. The length or width of the tight junctions did not change between the five experimental conditions. The ultrastructural changes correlate with the applied transepithelial pressure gradients rather than with transepithelial volume fluxes. The narrowing of lateral intercellular spaces in high pressure tubules correlate with the previously described increase in electrical resistance expressed per unit length tubule indicating that in these conditions part of the paracellular resistance is located in the free interspaces. The geometry of the lateral intercellular space in the proximal tubule of Necturus favors models of near-isotonic transport that do not depend on long and narrow interspaces.

Fluid transport and the dimensions of cells and interspaces of living Necturus gallbladder

The volume of the cells and lateral intercellular spaces were measured in living Necturus gallbladder epithelium. Under control conditions, the volume of the lateral spaces was 9% of the cell volume. Replacement of mucosal NaCl by sucrose or tetramethylammonium chloride (TMACl) caused intercellular spaces to collapse. During mucosal NaCl replacement, cell volume decreased to 79% of its control value. When NaCl was reintroduced into the mucosal bath, the intercellular spaces reopened and the cells returned to control volume. The NaCl active transport rate, calculated from the rate of cell volume decrease, was 266 pM/cm2.s, close to the observed rate of transepithelial salt transport. It was calculated from the decrease in cell volume that all of the intracellular NaCl was transported out of the cell during removal of mucosal NaCl. The flux of salt across the apical membrane, calculated from the rate of cell volume increase upon reintroducing mucosal NaCl, was 209 pM/cm2.s, in good agreement with estimates by other methods. The electrical resistance of the tight junctions was estimated to be 83.9% of the total tissue resistance in control conditions, suggesting that the lateral intercellular spaces normally offer only a small resistance to electrolyte movement.

The ultrastructure of the neonatal pig colon

The neonatal pig colon has several unique structural and developmental features. At birth it has a variable population of epithelial cells which in their arrangement on villus-like protrusions and in their capability for protein uptake into large preformed supranuclear vacuoles closely resemble neonatal ileal cells. Such villus-like protrusions and vacuolated cells are not present in the 2-day-old piglet. On the first day after birth absorptive epithelial cells which lack supranuclear vacuoles transiently accumulate a large number of lipid droplets, each separated from the cytoplasm only by a proteolipid interface. None of the much smaller lipid droplets bounded by a unit membrane of the smooth endoplasmic reticulum and characteristic of normal small intestinal fat uptake were ever seen in these cells. Very few of the large lipid drops remain on the second day after birth. This initial capacity of the colon for protein and lipid uptake never reappears. The pattern of colonic amino acid transport also changes markedly in the first four days of independent life and this may be correlated with the observation that the absorptive cells at birth have microvilli which are twice the length of those on similar cells at and after two days old. These morphological results are discussed in terms of implied functional changes in the neonatal period.

Size and shape of the lateral intercellular spaces in a living epithelium

The lateral intercellular spaces of Necturus gallbladder epithelium were seen and measured while the living tissue was perfused in a new chamber. The compliance of the lateral cell membranes was calculated from the measured pressure-volume characteristics of the lateral intercellular spaces.

Direct visualization of epithelial morphology in the living amphibian urinary bladder

Differential interference-contrast microscopy has been applied to the study of amphibian urinary bladders, in vitro. It is demonstrated that well-resolved images can be obtained with little loss of tissue viability. Direct observations have been made on the structure of microvilli, the distribution of mitochondria in the mitochondria-rich cells, and the patency of lateral intercellular spaces. It is noted that the effective viscosity of cytoplasm is very high--that it is apparently a gel in which there is no Brownian movement of organelles. The frequency, shape, and pattern of distribution of granular and mitochondria-rich cells is determined for the commonly studied varieties of Bufo marinus. Bladders from Colombian toads contain more and larger mitochondria-rich cells than do those of the Dominican variety. There is no specific arrangement of cell-cell contacts to suggest a structural basis for cooperativeness of action. Finally, a longitudinal study of osmotically-induced changes in the structure of the "tight" or "limiting" junctions establishes the validity of previous findings by electron microscopy.

Size changes in single muscle fibers during fixation and embedding

During fixation of single muscles fibers with glutaraldehyde, the volume of the fiber shrinks 20%, recovers in rinse and osmium tetroxide to near normal volume and shrinks 20% again when staining with uranyl acetate. This suggest that osmotic properties of membranes may not have been completely lost during fixation, post-fixation and en bloc staining. Dehydration in ethanol and propylene oxide produces a further 10% shrinkage in volume. Infiltration and embedding with Epon causes an additional 15% change in volume. This gives a total shrinkage in volume of 45% which is nearly twice that of the apparent shrinkage in the volume of the myosin lattice as determined by electron microscopy.