Electron microscopy of the stria vascularis and its response to etacrynic acid. A study using electron-dense tracers and extracellular surface markers

Lanthanum nitrate (La3+), ruthenium red (RR) and tannic acid (TA) have been used to examine the cell coat and permeability pathways in thin sections of the stria vascularis of normal and etacrynic-acid (EA)-treated guinea pigs. The tight junctions around the stria, at both endolymphatic and spiral ligament sides, excluded tracers even when EA-associated oedema was well advanced. La3+ was also unable to enter oedematous stria from the capillaries. A dense fibrillar coat was revealed on the endolymphatic marginal cell surface. This coat was disorganised during EA intoxication. Both RR and TA crossed the apical membrane and produced enhanced contrast in some, but not all, marginal cells in EA-treated animals. The results suggest that EA may affect the structure of the apical membrane of the marginal cells and that there is heterogeneity amongst the marginal cell population. These possibilities are discussed.

The endolymphatic surface of the stria vascularis in the guinea-pig and the effects of ethacrynic acid as shown by scanning electron microscopy

Examination of the endolymphatic surface of the stria vascularis showed, in agreement with previous studies, the marginal cells to be hexagonal in shape and that most possessed microvilli. However, the cells in the apical turns were more profusely covered with microvilli than those at the base and, particularly in the basal turns, there was some variation in the surface detail of the cells. These results may indicate differences in the physiological state of the cells. In the early period of ethacrynic acid intoxication (15-30 minutes post-injection), when rapid changes in endolymphatic potential and ion-fluxes occur, the surface of the marginal cells showed only slight distortion. The most dramatic changes were noted 1 hour post-injection, and involved swelling of the cells and loss of microvilli. This distortion persisted up to 2 hours post-injection. The possible explanation for these findings is discussed.

Cell membrane alterations in the stria vascularis of the guinea pig after ethacrynic acid treatment studied by freeze-fracture

A freeze-fracture examination of the stria vascularis during the first 2 h after injection of ethacrynic acid was performed. This showed a re-distribution of the particles on the membrane fracture faces of both marginal and intermediate cells. As oedematous spaces developed, particle-poor, vesicle-like structures were found associated with both cell types. The tight junctions at the apices of the marginal cells and around basal cells were unaffected.

Morphology of egg phosphatidylcholine-cholesterole single-bilayer vesicles, studied by freeze-etch electron microscopy

Homogeneous, small, single-bilayer vesicles were prepared from egg phosphatidylcholine with various concentrations of cholesterol by ultrasonic dispersion in 0.1 M KCl, 0.01 M Tris, pH 8.0, buffer, followed by gel chromatography. The shape and size distributions of the fractionated vesicles were investigated for preparations with cholesterol compositions from 0 to 50 moles/100 moles, using freeze-etch electron microscopy. The size distribution was estimated from the shadow width of vesicles which were exposed by etching and the vesicle shape was checked by comparing the images obtained by tilting the replicas. The widths of the vesicle diameter distributions were relatively broad, corresponding to standard deviations in the range 60--90 A, but showing no systematic variation with cholesterol composition. In all cases it was found that 70% of the vesicle diameters lay within 150 A of the modal value. The apparent vesicle diameters remained constant for cholesterol compositions up to 20 moles/100 moles (modal diameter = 330 +/- 20 A, mean diameter = 350 +/- 3 A), but there was a sharp net increase in diameter at 30 moles cholesterol/100 moles reaching a model diameter of 430 +/- 20 A (mean diameter = 430 +/- 3 A) at 50 moles cholesterol/100 moles. Using the tilted microscope stage it was found that all vesicles were spherical at all cholesterol compositions studied, including those above 30 moles cholesterol/100 moles. The molecular mechanism by which cholesterol controls the vesicle size is discussed in terms of the asymmetric distribution of cholesterol across the vesicle bilayer.

Observations on the stria vascularis of the guinea pig cochlea and the changes resulting from the administration of the diuretic furosemide

An electron microscope examination of the stria vascularis of guinea pigs and the structural changes occurring after administration of furosemide has been made. The use of ruthenium-red, which stains cell coats, has shown that the atria is impermeable to the passive diffusion of material from both the endolymph and the spiral ligament. The first changes after administration of furosemide are observed 4 hours after injection; small spaces develop between the cells. After 5--6 hours the spaces have enlarged but the tight junctions between marginal cells remain intact. The intermediate cells show signs of atrophy. In severely affected animals the intermediate cells have so shrunk that the spaces are very large and the marginal cells clearly resolved. These latter cells show no structural damage. In this severely affected tissue it is seen that the blood vessels are entirely surrounded by marginal cell extensions. It is suggested that the intermediate cells may have a regulatory function. The damage appears to be recoverable, since 6 hours after injection the spaces have decreased in size although some signs of intermediate cell atrophy are still apparent.

The effects of phospholipid depletion on the cleavage pattern of the cell walls of frozen Gram-negative bacteria

Extraction of whole cells of the marine pseudomonad (B-16) with chloroform–methanol causes the disappearance of the cleavage planes, and the cross-sectioned profile of both the cytoplasmic membrane and the double-track layer of the cell wall.

Biophysical examination of the cell wall of a gram-negative marine pseudomanad. The effects of various treatments on the isolated double-track layer

The isolated double-track layer of a gram-negative marine pseudomonad was subjected to a number of treatments designed to extract or to digest either phospholipid or protein. Extraction with chloroform–methanol caused aggregation and disruption of the double-track vesicles, a loss of the cleavage plane in frozen preparations, and a disappearance of the ring-like X-ray diffraction pattern which indicates side-by-side packing of the hydrocarbon tails of the phospholipids. Digestion with phospholipase-C, following ethylenediaminetetraacetate (EDTA) treatment, had the same effects except that the diglycerides freed by the action of the enzyme formed paracrystalline lamellar aggregates. This suggests that phospholipids are very important to the structural integrity of the double-track layer, that the layer of phospholipids is centrally located, and that this component of this membrane-like structure is responsible for the X-ray diffraction pattern produced by this material.Protein depletion by urea treatment or by pronase digestion causes increased aggregation of vesicles but these structures are not significantly disrupted, the cleavage plane is present in a stronger form in frozen material, and the X-ray diffraction pattern is unchanged. This suggests that proteins are superficially located and not as important as phospholipids in the structural integrity of this layer.The effects of protein depletion and of phospholipid depletion on the double-track layer are very similar to the effects of the same treatments on membranes, which supports the suggestion that the double-track layer has a membrane-like molecular architecture. The suggestion that this layer constitutes the barrier layer in the gram-negative cell wall is supported by our observations that sodium lauryl sulfate (SLS), EDTA, and polymyxin, which affect the penetrability of the cell wall of whole cells, have been shown to affect the molecular architecture of the isolated double-track layer used in these studies.

Freeze-etching and x-ray diffraction of the isolated double-track layer from the cell wall of a gram-negative marine pseudomonad

The isolated double-track layer of the cell wall of the gram-negative marine pseudomonad studied here contains a cleavage plane. This finding localizes the single cleavage plane of the cell wall and shows that the molecular architecture of this layer provides the lipid-enriched layer which cleaves preferentially in the frozen cell. The observation that the isolated double-track layer of the cell wall is sufficiently ordered at the molecular level to yield a well-defined X-ray diffraction pattern with a d-spacing of 0.44 nm shows that its molecular architecture is very similar to that of true membranes. This specific d-spacing is produced by the highly ordered packing of the hydrophobic portions of phospholipid molecules. Therefore, the double-track layer of the cell wall has been shown, by these two biophysical means, to have a molecular architecture which would allow it to function as the membrane-like "molecular sieve" layer, whose presence has been deduced from physiological data. This layer is important in the retention of cell wall-associated enzymes and in the control of the movement of large molecules through the cell wall.