Studies on the cornea. II. The uptake and transport of colloidal particles by the living rabbit cornea in vitro

The Evaluation of Ophthalmic Findings in Women Patients With Iron and Vitamin B12 Deficiency Anemia

Purpose

To determine whether there are differences in the central corneal thickness (CCT), retinal nerve fiber layer (RNFL), and ganglion cell-inner plexiform layer (GC-IPL) in women with iron deficiency anemia (IDA) and vitamin B12 deficiency anemia (B12DA).

Methods

The patients were divided and evaluated in three groups as IDA (group 1), combined IDA-B12DA (group 2), and healthy females (group 3). Each group constituted 35 subjects. CCT, endothelial cell density (CD), the coefficient of variation (CV), and hexagonality (HEX) values were evaluated. Superior, nasal, inferior, and temporal retinal nerve fiber analysis were done, and mean thickness of GC-IPL complex was also evaluated.

Results

CCT was found to be less (thin CCT) in group 2 as compared to group 1 and group 3 (P = 0.016 and P = 0.013, respectively). While CD measurements did not vary significantly between the groups, CV value was found to be lower in the control group as compared to that in the other groups (P = 0.001 and P = 0.002, respectively). HEX and the thicknesses of all RNFL and GC-IPL were lower in both the anemic groups as compared to the control group (P = 0.012 and P = 0.013, respectively); however, no significant difference was determined between the anemia groups.

Conclusions

It should be noted that anemia is a common clinical picture particularly seen in women, and there may be a tendency toward corneal and retinal disorders in these patients. The present study found that presence in the patients with IDA and with combined IDA-B12DA may have influenced endothelial functions; also the patients with anemia had significantly thinner CCT than healthy controls.

Translational Relevance

Optical coherence tomography and specular microscopy measurements may provide important data for early diagnosis and treatment of both corneal and retinal diseases according to the type of anemia in anemic women.

The emergence of the concept of tight junctions and physiological regulation by ouabain

The exchange of substances between metazoan and the environment takes place across transporting epithelia that have two fundamental differentiated features: tight junctions (TJ) and apical/basolateral polarity. Usually, reviews of the structure and function of transporting epithelia follow a historical description of major biological findings, but seldom refer to the fact that it also required fundamental theoretical changes in the physics and chemistry involved. We make a brief description of the concatenation of both types of achievements, in which it becomes clear that the major source of conflicts was the enzyme Na(+),K(+)-ATPase (also referred to as "the pump"), because of its intrinsic mechanisms and its asymmetric expression on one side of epithelial cells only (polarity). This enzyme is also the receptor of the newly recognized hormone ouabain, whose chief function is to modulate cell contacts, such as TJs, several types of cell-cell contacts participating in polarization (as gauged through ciliogenesis).

Studies on the cornea. I. The fine structure of the rabbit cornea and the uptake and transport of colloidal particles by the cornea in vivo

Physiological studies have demonstrated that ions, as well as large molecules such as hemoglobin or fluorescein, can diffuse across and within the cornea. Most of the substrates for corneal metabolism are obtained from aqueous humor filling the anterior chamber. In order to receive its nutrients and in order to maintain its normal conditions of hydration, the avascular cornea must transport relatively large amounts of solute and solvent across the cellular layers which cover this structure. It has been suggested in the past that there may be a morphological basis for the transport of large amounts of solvents and solutes by cells by the mechanism of pinocytosis. The use of electron-opaque markers to study fluid movements at the electron microscope magnification level was described by Wissig (29). The present study describes the fine structure of the normal rabbit cornea and the pathways of transport of colloidal particles by the cornea in vivo. Rabbit corneas were exposed in vivo to suspensions of saccharated iron oxide, thorium dioxide, or ferritin by injection of the material into the anterior chamber. In other experiments thorium dioxide or saccharated iron oxide was injected into the corneal stroma, producing a small bleb. Particles presented at the aqueous humor surface of the rabbit corneal endothelium are first attached to the cell surface and then pinocytosed. It appears that the particles are carried around the terminal bar by an intracellular pathway involving the pinocytosis of the particles and their subsequent transport in vesicles to the lateral cell margin basal to the terminal bar. Particles introduced at the basal surface of the endothelium (via blebs in the corneal stroma) are apparently carried through the endothelial cells in membrane-bounded vesicles without appearing in the intercellular space. There appears to be free diffusion of these particles through Descemet's membrane and the corneal stroma. The stromal cells take up large quantities of the particles when blebs are injected into the stroma.

Junctional complexes in various epithelia

The epithelia of a number of glands and cavitary organs of the rat and guinea pig have been surveyed, and in all cases investigated, a characteristic tripartite junctional complex has been found between adjacent cells. Although the complex differs in precise arrangement from one organ to another, it has been regularly encountered in the mucosal epithelia of the stomach, intestine, gall bladder, uterus, and oviduct; in the glandular epithelia of the liver, pancreas, parotid, stomach, and thyroid; in the epithelia of pancreatic, hepatic, and salivary ducts; and finally, between the epithelial cells of the nephron (proximal and distal convolution, collecting ducts). The elements of the complex, identified as zonula occludens (tight junction), zonula adhaerens (intermediary junction), and macula adhaerens (desmosome), occupy a juxtaluminal position and succeed each other in the order given in an apical-basal direction. The zonula occludens (tight junction) is characterized by fusion of the adjacent cell membranes resulting in obliteration of the intercellular space over variable distances. Within the obliterated zone, the dense outer leaflets of the adjoining cell membranes converge to form a single intermediate line. A diffuse band of dense cytoplasmic material is often associated with this junction, but its development varies from one epithelium to another. The zonula adhaerens (intermediate junction) is characterized by the presence of an intercellular space ( approximately 200 A) occupied by homogeneous, apparently amorphous material of low density; by strict parallelism of the adjoining cell membranes over distances of 0.2 to 0.5 micro; and by conspicuous bands of dense material located in the subjacent cytoplasmic matrix. The desmosome or macula adhaerens is also characterized by the presence of an intercellular space ( approximately 240 A) which, in this case, contains a central disc of dense material; by discrete cytoplasmic plaques disposed parallel to the inner leaflet of each cell membrane; and by the presence of bundles of cytoplasmic fibrils converging on the plaques. The zonula occludens appears to form a continuous belt-like attachment, whereas the desmosome is a discontinuous, button-like structure. The zomula adhaerens is continuous in most epithelia but discontinuous in some. Observations made during experimental hemoglobinuria in rats showed that the hemoglobin, which undergoes enough concentration in the nephron lumina to act as an electron-opaque mass tracer, does not penetrate the intercellular spaces beyond the zonula occludens. Similar observations were made in pancreatic acini and ducts where discharged zymogen served as a mass tracer. Hence the tight junction is impervious to concentrated protein solutions and appears to function as a diffusion barrier or "seal." The desmosome and probably also the zonula adhaerens may represent intercellular attachment devices.

An electron microscopic study of the passage of colloidal particles from the blood vessels of the ciliary processes and choroid plexus of the rabbit

The thinnest areas of the capillaries of the choroid plexus and ciliary processes in the eye of the rabbit are characterized by the presence of fenestrae. When various colloidal particles opaque to the electron beam (thorotrast, gold sol, and saccharated iron oxide) were injected into the blood stream, none were found in fenestrae or in areas that might suggest their having passed through fenestrae. The passage of marker particles from the lumen to the surrounding connective tissue does take place on occasion in the areas of thicker walls in the capillaries and venules rather than in the attenuated and fenestrated endothelial walls. The pathway taken by these markers may be either through the cytoplasm of the endothelial cells via membrane-bounded vesicles and vacuoles or through the intercellular spaces of the vessels. An altered aqueous humor (cloudy and plasmoid) was produced by endotoxin injection or by making a draining fistula in rabbit cornea. Both methods gave rise to the same changes in the blood vessels of the ciliary processes. Under such conditions of inflammation the passage of colloidal particles through the thicker walls of the capillaries and venules was greatly increased and occurred primarily as an intercellular passage between the endothelial cells. The attenuated and fenestrated areas of the endothelium of the small capillaries remained unchanged with no particles passing through them. These results on the altered vessels of the ciliary processes parallel those of Majno and Palade (26) on the rat cremaster muscle.

The fine structure of the basement membrane in epidermal tumors

Epidermal tumors were induced in Swiss female mice by a topical application of 9,10-dimethyl-1,2-benzanthracene solution followed by repeated applications of croton oil solutions. Fourteen benign and malignant tumors were sampled 25 weeks after the treatment had begun, fixed in osmium tetroxide or potassium permanganate, and embedded in Epon. Sections stained with lead hydroxide were examined. In three tumors defects of the epidermal basement membrane were seen. These defects accompanied local invasion of the tumors. The possible mechanisms of the development of this unusual anatomical situation are discussed.

INTERCELLULAR ELECTRICAL COUPLING AT A FORMING MEMBRANE JUNCTION IN A DIVIDING CELL

Ion communication between the halves (blastomere) of a dividing cell (Asterias forbesi egg), as determined electrically, diminishes progressively during division as a cell membrane junction forms at the plane of cleavage. Virtually all communication is lost towards the end of division by the time the first intercellular space becomes continuous with the exterior. Resting membrane potentials in both cell halves are essentially equal and constant throughout division.

EFFECTS OF CALCIUM DEPLETION ON THE JUNCTIONAL COMPLEX BETWEEN OXYNTIC CELLS OF GASTRIC GLANDS

In order to ascertain the role of calcium in maintaining the structure of the junctional complex between oxyntic cells, control gastric mucosae of the frog Rana pipiens were compared with those exposed to 1 to 4 mM ethylenediaminetetraacetic acid (EDTA). Changes in transmucosal potential difference and mucosal conductance were monitored. In one case a piece of EDTA-treated mucosa was washed and placed in a Ca⁺⁺-containing solution. Material from all three categories was prepared for electron microscopy (glutaraldehyde and OsO₄ fixation with Epon 812 embedding). Electron micrographs showed that after Ca⁺⁺ depletion the intercellular electron-opaque material of the desmosome disappears and the walls of this component separate. Similar changes were observed in the zonula adhaerens. Consistent changes were difficult to detect in the zonula occludens although in some instances disorganization or separation within the tight junction was seen. These effects on the components of the junctional complex were reversible on readministration of Ca⁺⁺. The results indicate that Ca⁺⁺ is important in maintaining the integrity of the junctional complex. A model correlating the fine structural changes with physiological data is presented.

THE ENTRY AND DISTRIBUTION OF HERPES VIRUS AND COLLOIDAL GOLD IN HELA CELLS AFTER CONTACT IN SUSPENSION

The way in which herpes virus of a well adapted strain penetrates susceptible HeLa cells has been investigated using thin sectioning techniques for electron microscopy. Mature virus particles and cells were mixed together in suspension cultures for 15, 30, 60, or 120 minutes so that the stages in virus uptake could be followed in sequence. The ingestion of particles of colloidal gold by HeLa cells under similar conditions was studied for comparison in parallel experiments. After 15 minutes' contact, the mature virus was found adsorbed on the surface of the cells but separated from them by a narrow gap in which phosphotungstic acid staining was sometimes able to reveal an extraneous coat which appeared as an amorphous layer on the outer aspect of the plasma membrane. When mixing continued for longer the particles were present in deep invaginations or actual cytoplasmic vacuoles, with their outer layers in various stages of stripping and digestion. The stripped, naked, central portion of the virus was occasionally found in these vacuoles but was more commonly free in the cytoplasmic matrix; the mode of transition between these sites could not be determined. Where contact continued for 2 hours these phenomena were much less frequently observed. The larger particles of colloidal gold were ingested in the same way as the virus, but smaller ones were taken up in micropinocytosis vesicles. The gold passed through membrane-bounded cytoplasmic spaces to accumulate in vacuoles from which, in contrast to herpes particles, it did not escape. These findings are discussed, and considered with particular reference to their bearing on the initiation of infection, the uptake and disposal of particles by cells, and the influence on the latter of virus morphology.

ELECTRON MICROSCOPY OF ABSORPTION OF TRACER MATERIALS BY TOAD URINARY BLADDER EPITHELIUM

The absorption of Thorotrast and saccharated iron oxide by the epithelium of the toad urinary bladder was studied by electron microscopy. Whether the toads were hydrated, dehydrated, or given Pitressin, no significant differences in transport of colloidal particles by epithelial cells were observed. This implies that these physiological factors had little effect on the transport of the tracer particles. Tracer particles were encountered in three types of epithelial cells which line the bladder lumen, but most frequently in the mitochondria-rich cells. Tracer materials were incorporated into the cytoplasm of epithelial cells after being adsorbed to the coating layer covering the luminal surface of the cells. In the intermediate stage (1 to 3 hours after introducing tracer) particles were present in small vesicles, tubules, and multivesicular bodies. In the later stages (up to 65 hours), the particles were more commonly seen to be densely packed within large membrane-bounded bodies which were often found near the Golgi region. These large bodies probably were formed by the fusion of small vesicles. Irrespective of the stages of absorption, no particles were found in the intercellular spaces or in the submucosa. Particles apparently did not penetrate the intercellular spaces of the epithelium beyond the level of the tight junction.

STUDIES ON AN EPITHELIAL (GLAND) CELL JUNCTION. I. MODIFICATIONS OF SURFACE MEMBRANE PERMEABILITY

Membrane permeability of an epithelial cell junction (Drosophila salivary gland) was examined with intracellular microelectrodes and with fluorescent tracers. In contrast to the non-junctional cell membrane surface, which has a low permeability to ions (10^-4 mho/cm²), the junctional membrane surface is highly permeable. In fact, it introduces no substantial restriction to ion flow beyond that in the cytoplasm; the resistance through a chain of cells (150 Ω cm) is only slightly greater than in extruded cytoplasm (100 Ω cm). The diffusion resistance along the intercellular space to the exterior, on the other hand, is very high. Here, there exists an ion barrier of, at least, 10⁴Ω cm². As a result, small ions and fluorescein move rather freely from one cell to the next, but do not leak appreciably through the intercellular space to the exterior. The organ here, rather than the single cell, appears to be the unit of ion environment. The possible underlying structural aspects are discussed.

The assessment of endothelial integrity by scanning electron microscopy and fluorescein diacetate staining following treatment with cryoprotective additives

As part of the development of methods of corneal cryostorage for transplantation, a toxicity study was carried out on the rabbit corneal endothelium using four cryoprotective additives (CPA's) 1) dimethyl sulphoxide (Me2SO), 2) propane-1,2-diol (PG), 3) glycerol (GLY), 4) polyvinylpyrrolidone (PVP). A fifth group, based upon a CPA combination of Me2SO and PVP, was used to characterize both the assays, and the response of the endothelial layer to osmotic stress. The effect upon the cell membrane was assessed using scanning electron microscopy (SEM) and fluorescein diacetate with ethidium bromide staining (FDA/EB). Two sampling points were used, one immediately after treatment and the other following an incubation period. Calculations were performed to predict the maximum relative volume of cells during CPA exchange. Immediately following serial addition and removal of 2 or 3 mol/L (M) PG or GLY, the cells exhibited adverse morphological changes shown with SEM, and the proportion of intact cells judged by FDA/EB staining was significantly reduced when CPA equilibration was performed at 37 degrees C rather than at 20 degrees C. A 3M Me2SO concentration gave less morphological change than 3M PG or GLY, but even after treatment with 4M Me2SO more than 95% cells were judged intact by FDA/EB staining. PVP at 40% w/v showed minimal damage with both assays, and the fifth experimental group suggested that PVP may protect from injury during hypotonic stress. With all groups, the integrity of the cell layer recovered during incubation, so that for each sample the percentage of intact cells was high. However, although confluency was often restored following incubation, total cell density was usually reduced. The results indicate that serial addition and removal of 3M Me2SO is tolerated by the cornea, whereas PG or GLY cannot be used at 2 or 3M without inducing osmotic damage. There was low toxicity to PVP, and it was an effective osmotic buffer.

Corneal endothelial changes under induced intraocular pressure elevation: a scanning and transmission electron microscopic study in rabbits

Intraocular pressure was artificially raised to 60--70 mmHg in 7 albino rabbits for periods of 15 minutes to 4 hours. The corneal endothelium of these eyes was studied by transmission and scanning electron microscopy. A correlation between exposure time to elevated IOP, clinical signs observed by slit-lamp examination, and extent of morphological damage is clearly shown. In eyes exposed to high pressure for 15 and 30 minutes corneas remained transparent and only minimal changes could be detected by SEM, which consisted of small areas of cell with unevenness of their surface, occasional cellular ruptures, and diminution of cilia and microvilli. After 1--2 hours of exposure small, solitary corneal opacifications appeared. In these eyes more severe morphological changes affecting larger areas were observed, with additional cellular blebbing, excariocytosis, cellular rupture, disintegration, and disappearance seen in SEM. Thin sections revealed swelling of mitochondria, disorganisation of endoplasmic reticulum, and the existence of myelin bodies. In eyes exposed for 3 and 4 hours to high IOP corneal haziness, implying stromal oedema, appeared. In these eyes the areas affected were larger, the extent of damage being more severe. Many areas were bare of endothelium, surrounded by scattered cellular debris, and showed cells with ballooning surfaces and multiple ruptures. Even in severe cellular damage cellular junctions appeared intact. It is assumed that endothelial cells are more sensitive to IOP elevation than the cellular junctions and that injury to the active pump system due to morphological damage is responsible for the resultant corneal oedema.

[Proof of glycosaminoglycans in the corneal endothelium]

The corneal endothelium of three species (man, dog, and guinea pig) was examined for glycosaminoglycans by using TEM, REM, and EDAX (energy-dispersive analysis of X-rays). Since no clear results can be determined by using ruthenium red (RR) alone, a testicular hyaluronidase was used to digest the glycocalyx. After incubation with hyaluronidase, the dark RR borderline of the outer leaflet of the cell membrane became more fluffy when examined by using TEM and REM. When EDAX was applied, the Ru peak became smaller compared with the S peak. These impressive results had not been found when the material was fixed in glutaraldehyde for 24 h before incubation. Considering the individual results, it can be concluded that RR, even wouthout OsO4, marks the acid groups of glycosaminoglycans that might be in the cell membrane, as well as other acid groups. There was no difference in the results among the three species used in this experiment. Some pictures showed that RR/OsO4 penetrates through the intercellular space into the Descemet's membrane.

Ultrastructure and distribution of intercellular junctions in corneal endothelium

The intercellular junctions of the corneal endothelium has been studied in rabbit, monkey and human eyes. A union of apposing outer leaflets (tight junction) is usually found near the apical end of the intercellular clefts. However, evidence from lanthanum tracer studies indicates that the tight junctions do not seal the intercellular clefts completely. In addition to tight junctions, the presence of gap junctions or nexuses is demonstrated in all three species studied.

Contributions of electron microscopy to the study of corneal pathology

Most of the electron microscopic studies of pathological corneas have been done only recently. Keratoplasty has been the most important source of specimens for ultramicroscopic investigation. With the introduction of electron microscopic techniques, we have been able to confirm many light microscopic studies in pathological corneas. This contribution has been most valuable in the identification of the sites and types of pathological changes in corneal dystrophies and degenerations. This review of electron microscopic studies describes the present concepts on the nature of the histological changes in dystrophies and degenerations of the anterior and posterior corneal layers and corneal stroma. It also includes a review of some corneal inflammatory conditions as well as metabolic disorders affecting its transparency.

Uptake of horseradish peroxidase (HRP) by the endothelium of the short-term stored rabbit cornea

In vitro study of the uptake of horseradish peroxidase (HRP) by the corneal endothelium was carried out on rabbit corneae which were stored in a preserving medium for various periods. The stored corneae were incubated in HRP solution with or without adenosine triphosphate (ATP). The number of pinocytotic vesicles decreased as the storage time was prolonged. Addition of ATP to the HRP solution led to an increase in the pinocytotic vesicles in all storage periods. The pinocytosis in the 12-day stored corneal endothelium was only observed when the tissue was incubated in an HRP solution containing ATP. From these results, it was concluded that the viability of the stored corneae can be evaluated with uptake of HRP by the endothelial cell, and that the addition of ATP leads to an enhancement of pinocytotic activity in the endothelial cell.

The blood-aqueous barrier in five species

The in vivo sites of the blood-aqueous barrier were determined in five animal species, using acriflavine neutral (224 mol. wt.), ultrafreezing and drying, fluorescence microscopy, and fluorometry. Not toxic, acriflavine had specific in vivo affinity for nuclear DNA and RNA, produced intense fluorescence, and did not pass through hematic barriers. Acriflavine was given in doses to produce the same concentrations in the systemic blood or in the aqueous humor. The exact sites of the blood-aqueous barrier, demonstrated by tracing the acriflavine fluorescence through tissues and individual cells, were in the basal cell membranes of inner ciliary and iridial epithelia and apical cell membranes of iridial and corneal endothelia. Acriflavine passed freely from the aqueous humor into the blood stream. It also followed an aqueous flow into the vitreous body and optic nerve head vessels. Interruption or reversal of this flow may cause open-angle glaucoma.

Vascular leakage through intraendothelial channels induced by cholera toxin in the skin of guinea pigs

Guinea pigs were injected intravenously with Evans blue solution (1/2), 1, 5 and 22 hours after intradermal injection of Vibrio cholerae 569B toxin. Then they were fixed by perfusion with diluted Karnovsky's aldehyde mixture 1(1/2), 5, 10, 15 and 90 minutes later. In some cases, horseradish peroxidase was mixed in the dye solution, and chopped tissue slices were incubated histochemically. Dermal blood vessels from these samples were examined under the electron microscope. The fine structure of the endothelial junctions remained unchanged as compared with controls. However, extended rough surfaced endoplasmic reticulum was observed in the endothelial cells at 30 minutes after the toxin injection, when cutaneous bluing was not marked yet. At 22 hours after toxin injection, endothelial perikarya were filled with prominent Golgi complexes, multivesicular bodies and systems of caveolae and vesicles which coalesced, forming intraendothelial channels after diaphragms disappeared. Endothelial fenestrae were numerous in the attenuated portions.

An enzyme histochemical study of mesothelial cells in rodents

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Differentiation of the surface epithelium of the hamster ovary. An electron microscopic study

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Fluid transport in the rabbit gallbladder. A combined physiological and electron microscopic study

The fine structure of the rabbit gallbladder has been studied in specimens whose functional state was undetermined, which were fixed either in situ or directly after removal from the animal; in specimens whose rate of fluid absorption was determined, either in vivo or in vitro, immediately prior to fixation; and in specimens from bladders whose absorptive function was experimentally altered in vitro. Considerable variation was found in the width of the epithelial intercellular spaces in the bladders whose functional state was undefined. In bladders known to be transporting fluid, either in vivo or in vitro, the intercellular spaces were always distended, as were the subepithelial capillaries. This distension was greatest in bladders which had been functioning in vitro. When either Na(+) or Cl(-) was omitted from the bathing media, there was no fluid transport across the wall of the gallbladder studied in vitro. The epithelial intercellular spaces of biopsies taken from several bladders under these conditions were of approximately 200 A width except for minor distension at the crests of mucosal folds. The addition of the missing ion rapidly led to the reestablishment of fluid transport and the distension of the intercellular spaces throughout most of the epithelium of these bladders. Studies of sodium localization (by fixation with a pyroantimonate-OsO(4) mixture) showed high concentrations of this ion in the distended intercellular spaces. Histochemical studies of ATPase activity showed that this enzyme was localized along the lateral plasma membrane of the epithelial cells. The analogy is drawn between the structure of the gallbladder mucosa and a serial membrane model proposed by Curran to account for coupled solute-solvent transport across epithelia. It is concluded that the intercellular compartment fulfills the conditions for the middle compartment of the Curran model and that active transport of solute across the lateral plasma membrane into the intercellular space may be responsible for fluid absorption by the gall bladder.

Electron microscopy: sodium localization in normal and ouabain-treated transporting cells

When rabbit corneas are fixed in an osmic acid fixativeto which potassium pyroantimonate has been added, an electron-opaque precipitate of sodiium pyroantimonate is localized along the cellular side of the lateral plasmla membrane of the endothelial cells. This heavy precipitate represents areas of high Na(+) concentration and may represent the site of a saturated, high-affinity Na(+) carrier. Significantly heavier precipitate occurs at the same site when ouabain is injected into the aqueous humor prior to fixation of the cornea.

AN ELECTRON MICROSCOPIC STUDY OF RETICULOENDOTHELIAL SYSTEM BLOCKADE

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