Analyses of areas and shapes of cells on the corneal surface of the albino rabbit by scanning electron microscopy

Structure and function of corneal surface of mudskipper fishes

Vertebrate corneal epithelium cell plays an important role for imaging, and the cell density, together with the appearance or type of affiliated microstructures, is considered as a result of evolution adapting to alternate terrestrial or aquatic environment. In this paper, we investigated the corneal cells of both larvae and adult amphibious mudskippers Boleophthalmus pectinirostris and Periophthalmus magnuspinnatus, to testify the relationship between morphology and function. The cell density values of the two species were 31,137 and 31,974 cells per mm(2) in larvae and then significantly decreased to 15,826 and 25,954 cells per mm(2) in adult (p < 0.001), respectively, which could be explained as the habitat change from aquatic to different degrees of terrestrial environment. The corneal epithelium cells were ridge type in larvae and differentiated into ridge type and reticular type in adult P. magnuspinnatus and ridge type, reticular type and ridge-reticular type in adult B. pectinirostris. Four kinds of microstructures as microridge, microvilli, microplicae and microhole appeared in both species. The difference of microridge width and its separation indicated that a dense cell connection was requested in a saltier and more terrestrial environment.

Ocular toxicity of benzalkonium chloride homologs compared with their mixtures

This study was performed to assess the in vivo ocular toxicity of benzalkonium chloride (BAK) homologs compared with commercially available BAK (BAK mixture) and to assess the ocular toxicity of BAK homolog after repeated ocular application. Rabbit eyes were examined by ophthalmology and scanning electron microscopy (SEM) after 10 applications of BAK homologs with C12 (C12-BAK) and C14 (C14-BAK) alkyl chain lengths and a BAK mixture at concentrations of 0.001% (w/v), 0.003% (w/v), 0.005% (w/v), 0.01% (w/v) and 0.03% (w/v). The ocular toxicity of C12-BAK to rabbit eyes was examined by ophthalmology and histopathology after repeated ocular application for 39 weeks. In addition, the antimicrobial activities of C12-BAK and C14-BAK against A. niger, S. aureus and P. aeruginosa were assessed. Ocular toxicity of C12-BAK was less than those of the BAK mixture and C14-BAK. No ocular toxicity was noted after ocular application of 0.01% C12-BAK to rabbits for 39 weeks. C12-BAK showed antimicrobial activities at a concentration of 0.003%. These results suggest that the use of C12-BAK to replace BAK mixture as a preservative in ophthalmic solutions should be considered in order to reduce the incidence of the corneal epithelial cell injury induced clinically by BAK.

Further assessment of the size, shape and surface features of superficial cells of the bovine corneal epithelium, using scanning electron microscopy

PURPOSE

To qualitatively and quantitatively assess the size and shape of the surface cells of the bovine corneal epithelium as assessed by scanning electron microscopy.

METHODS

After quality-control measures on recent postmortem bovine eyes obtained from a slaughterhouse, the ocular surface was aggressively washed with 0.97% saline and then fixed for scanning electron microscopy (SEM) by a dropwise application, in 37 degrees C moist chamber, of a 2% glutaraldehyde solution (in 80 mM cacodylate, pH 7.2 to 7.4). A set of images were taken from the central region of the surface of the corneal epithelium. The cell areas, perimeter and dimensions of 25 to 30 cells from 18 different samples were measured with a digitiser pad or rule, and number of bordering cells (sides) counted manually. Cell shape was assessed by comparing the longer and shorter dimensions (LS ratio) and calculation of the figure coefficient (FC, 4PiA/P2 ).

RESULTS

With the washing protocol, surface cells were evident that displayed a range of electron reflexes (light, medium and dark) and that were decorated with microplicae and some crater-like features. These features were generally not evident if the surface was only rinsed with saline. The cell surface areas ranged from 58 to 6444 microm2 (mean 1748 +/- 1169 microm2), and increased as the number of sides increases. Cells with a higher number of sides (n=10) tended to be more rounded (LS ratio averaging 1.24) while cells with small number of sides (n=3) were more elongated (LS ratio 1.94). Shape (FC) values ranged from 0.379 to 0.967, and increased as the number of cell sides increases. Assessment of cell shape in relation to the number of cell sides identified only a small proportion of surface cells with unusually low FC values.

CONCLUSIONS

With an appropriate preparative procedure (aggressive washing with saline), bovine corneal epithelial surface (squamous) cells can be shown to have a similar appearance by SEM to that previously observed for rabbit and human corneas. The cell sizes are however substantially greater than those seen in rabbits, presumably because of the much larger corneal diameter (c. 28 mm vs. 13 mm). Based on detailed analysis of cell size and shape, the results indicate that the squamous cells have reasonably predictable characteristics.

Comparative anatomy of laboratory animal corneas with a new-generation high-resolution in vivo confocal microscope

PURPOSE

The aim of the current study was to compare the corneas of three commonly used laboratory animals with a new in vivo confocal microscope.

METHODS

Six eyes of three adult male New Zealand albino rabbits, six eyes of three adult male Lewis rats, and six eyes of three adult male Swiss mice were used in this study. Corneas were analyzed in vivo using the Rostock Cornea Module of the Heidelberg Retina Tomograph (HRT)-II. For all eyes, 20 confocal microscopic images of each layer, that is, the superficial and basal corneal epithelia, the Bowman layer, the anterior and posterior stroma, and the endothelium, were recorded. The images were then analyzed qualitatively and compared among animals. Cellular densities of anterior and posterior stroma keratocytes of rabbits and endothelium density of the three different animals were also measured and compared.

RESULTS

The Rostock Cornea Module of the HRT II was successfully used to analyze all corneal layers of these three commonly used laboratory animals. Although the cellular patterns of the corneal layers of these three animals, as observed with in vivo confocal microscopy, were quite similar, some differences were seen in terms of endothelial cell density and stroma appearance. Superficial cells were seen as hyper- and hyporeflective polygonal cells. Basal cells had dark cytoplasm without visible nuclei and were closely organized. A Bowman layer was observed in all three animals as an amorphous tissue containing fine subepithelial nerve plexus. In rabbits, the stroma consisted of an amorphous ground substance with hyper-reflective structures corresponding with keratocyte nuclei. In rats and mice, numerous reflective stellate structures with no clearly visible nuclei were observed within the stroma. Besides endothelial cell density, the endothelium was similar among the three animals and was seen as hyper-reflective cells with dark limits organized in a honeycomb pattern.

CONCLUSIONS

The Rostock Cornea Module of the HRT II can provide high-resolution images of all corneal layers of rabbits, rats, and mice without sacrificing animals or preparing tissue. This new device may be useful for evaluating the cornea during experimental animal studies.

The corneal epithelial surface in the eyes of vertebrates: Environmental and evolutionary influences on structure and function

The smooth optical surface of the cornea is maintained by a tear film, which adheres to a variety of microprojections. These microprojections increase the cell surface area and are thought to improve the movement of oxygen, nutrients, and metabolic products across the outer cell membranes. However, little is known of these structural adaptations in vertebrates inhabiting different environments. This field emission scanning electron microscopic study examined the cell density and surface structure of corneal epithelial cells across 51 representative species of all vertebrate classes from a large range of habitats (aquatic, amphibious, terrestrial, and aerial). In particular, we wished to extend the range of vertebrates to include agnathans and some uniquely Australian species, such as the Australian lungfish (Neoceratodus forsteri), the Australian galah (Eolophus roseicapillus), the Australian koala (Phascolarctos cinereus), and the rat-tailed dunnart (Sminthopsis crassicaudata). Epithelial cell densities ranged from 28,860 +/- 9,214 cells mm(-2) in the flathead sole Hippoglossoides elassodon (a marine teleost) to 2,126 +/- 713 cells mm(-2) in the Australian koala (a terrestrial mammal), which may indicate a reduction in osmotic stress across the corneal surface. A similar reduction in cell density occurred from marine to estuarine to freshwater species. The structure and occurrence of microholes, microplicae, microridges, and microvilli are also described with respect to the demands placed on the cornea in different environments. All species that spend significant periods out of an aquatic environment possess microvilli and/or microplicae. These include all of our species of Mammalia, Aves, Reptilia, Amphibia, and even one species of Teleostei (Australian lungfish). Well-developed microridges occur only in teleosts in high osmolarity environments such as marine or estuarine habitats. Clear interspecific differences in corneal surface structure suggest a degree of adaptive plasticity, in addition to some phylogenetic trends.

In Vivo Toxicity of Netilmicin and Ofloxacin on Intact and Mechanically Damaged Eyes of Rabbit

PURPOSE

To evaluate the in vivo toxicity of netilmicin and ofloxacin using both normal and mechanically damaged eyes of rabbit.

METHODS

Male albino New Zealand rabbits were given either 0.3% netilmicin, 0.3% ofloxacin, or 0.9% sodium chloride solution by topical instillation (50 microL) into the conjunctival sac 6 times daily for 5 days. In some animals a 6-mm-diameter epithelial wound was mechanically made to the center of the cornea. Ocular toxicity on normal eyes was evaluated by impression cytology of the conjunctiva, histology of the entire globes, and scanning electron microscopy (SEM) of the cornea. Analysis of toxicity and reepithelialization on wounded corneas was evaluated by SEM with observations being made 48 and 72 hours after induction of the wound.

RESULTS

Cytologic, histopathologic, and SEM analyses of normal healthy eyes following netilmicin treatment revealed no signs of toxicity, whereas those treated with ofloxacin revealed alterations in the cornea (stromal swelling) and conjunctiva (infiltration of polymorphonuclear cells) with reduced goblet cell numbers. Wounded corneas treated with netilmicin exhibited normal morphology and reepithelialization, whereas the administration of ofloxacin resulted in disordered cellular organisation and slower rates of epithelial recovery.

CONCLUSIONS

Netilmicin, an antibacterial aminoglycoside, is well tolerated even in an experimental wound-healing model where the integrity of the ocular surface is compromised, whereas ofloxacin, a fluoroquinolone, appears to provoke an inflammatory response in the normal eye and a clear alteration of reepithelialization in the wounded eye. These findings suggest that netilmicin may offer a superior toxicological profile in both normal eyes and clinical situations where the integrity of the ocular epithelium is suspect.

Viability of porcine corneal epithelium ex vivo and effect of exposure to air: a pilot study for a dry eye model

PURPOSE

To explore the use of an ex vivo, in situ porcine cornea as a model for dry eye (exposure keratitis).

METHODS

Twenty-seven porcine eyes were obtained from freshly killed animals at the local abattoir. The viability of 9 corneas (control-baseline group) was assessed within 5 minutes after enucleation on site. A further 18 eyes were transported to the laboratory, where they were exposed to ambient conditions for 4 hours (experimental group A, 6 eyes), for 6 hours (experimental group B, 6 eyes), and for 4 hours with wetting with Dulbecco Phosphate-Buffered Saline every 5 minutes (exposure control group, 6 eyes). All corneas were assessed by trypan blue exclusion for cell viability.

RESULTS

The number of dead cells in the central region was significantly greater than those in the peripheral region (P < 0.05) in all groups. The number of dead cells in both corneal areas increased significantly (P < 0.05) in the experimental groups with time of exposure, but there was no significant increase in the exposure control group.

CONCLUSION

Preliminary data on the number of dead cells in porcine corneal epithelium after enucleation and the effect of exposure were obtained. It was found that after exposure to air, the corneal cells were maintained well by regular wetting, but there was progressively greater cell damage with exposure without wetting. These baseline data will be useful for the further development of the porcine dry eye model to investigate exposure keratitis.

A comparative SEM study of the vertebrate corneal epithelium

PURPOSE

The anterior surface of the cornea of mammals, including humans, has numerous folds in the anterior epithelial cell membranes in the form of microvilli and microplicae. The role of these surface irregularities may be to increase cell-surface area and therefore aid in intra- and extracellular movement of nutritional and waste products across the cell membranes in addition to stabilizing the corneal tear film. The aim of this study was to investigate and compare the nature of these corneal-surface features in various vertebrate classes residing in different environments.

METHODS

The anterior corneal surfaces of various vertebrates were investigated by using field emission scanning electron microscopy. Cell areas were analyzed by using image-analysis software.

RESULTS

Representative species were examined from all the vertebrate classes, with the exception of the Cephalaspidomorphi. The mean epithelial cell density of aquatic vertebrates (17,602 +/- 9,604 cells/mm2) is greater (p = 0.000018) than that of aerial and terrestrial vertebrate species, including amphibians (3,755 +/- 2,067 cells/ mm2). Similarly, the mean epithelial cell density for the marine vertebrates (22,553 +/- 8,878 cells/mm2) is greater (p = 0.0015) than that of the freshwater and estuarine species (10,529 +/- 5,341 cells/mm2). The anterior corneal surfaces of all species examined were found to show a variety of cell-surface structures. Microvilli are predominant in reptiles, birds, and mammals; microridges appear to be characteristic of the Osteichthyes; and microholes were observed only in the Chondrichthyes.

CONCLUSION

The function of these morphologic variations in surface structure appear to be correlated with the range of ecologic environments (marine, aerial, and terrestrial) occupied by each species, corneal phylogeny, and the demands placed on the cornea to ensure clear vision.

The corneal endothelium in the blowfish (Torquigener pleurogramma)

PURPOSE

In vertebrates, a corneal endothelium is essential for the maintenance of corneal transparency in a variety of environments. Knowledge of the surface structure of the corneal endothelium may assist our understanding of this unique tissue and its evolutionary development. Although there have been many studies of the corneal endothelium of humans and some mammals, there have been few in other vertebrates.

METHODS

The field emission scanning electron microscope was used to study the surface structure of the corneal endothelium in the blowfish, Torquigener pleurogramma (Tetraodontidae, Teleostei), and to examine cell density. Cell areas were measured by using image-analysis software.

RESULTS

The endothelium is composed of a sheet of interdigitating hexagonal and pentagonal cells with a mean area of 154 microm2 and a density of 6,486 cells/mm2. Two types of surface features are identified; primary cilia and microvilli. The cilia are cylindrical, protrude from either a pore or circular indentation in the cell center, and possess a knob-like ending. The microvilli are button-like protrusions with a density of -3.5 x 105 microvilli/mm2 or 54 microvilli/cell in central cornea.

CONCLUSION

The results show that the surface structure of teleost endothelial cells is similar to those described for other vertebrates and indicate that cell density varies across classes, with the presence of cilia a more widespread occurrence than previously believed.

Scanning electron microscopy study of the tarsal and orbital conjunctival surfaces compared to peripheral corneal epithelium in pigmented rabbits

The mammalian palpebral conjunctiva has not been systematically evaluated by scanning electron microscopy (SEM). The upper eyelid of female grey rabbits (2 kg) was fixed in its extended conformation at 15.00 h; some corneas were prepared with the same fixative protocol. The corneal epithelium, within 1 mm of the limbus, is a mosaic of small-to-large (average cell area of 693 m2) squamous cells with light, medium or dark appearance due to different densities of cell surface microplicae. The tarsal conjunctiva was a 1.5 to 3 mm wide mosaic of small (average cell area of 86 microm2) non-desquamating polygonal cells having distinctive light and dark appearances due surface microplicae. The orbital portion of the palpebral conjunctiva is also composed of small (average cell area of 87 microm2) non-desquamating polygonal cells but with a uniform medium-grey appearance due to a relatively consistent density of surface microvilli. Several types of intercellular pore-like openings were also present across the palpebral surface but not the corneal epithelial surface. Very small type 1 pores (1.5 to 5 microm diameter) were present at a density of 480 to 760 / mm2 for the tarsal and 80-160 / mm2 for orbital conjunctiva. Slightly larger (2.5 to 18 microm diameter) type 2 pores with distinct borders were present at 120-200 / mm2 across the orbital conjunctiva. Very large (10-109 microm diameter) type 3 (Henle) pores were present at 40 to 280/mm2 especially at the tarsal/orbital transition zone. Type 4 pores (goblet cell orifices) were oval with a peripheral ring of microvilli and were present at 40 to 160/mm2 for tarsal and 800 to 1600/mm2 for orbital conjunctiva. The rabbit palpebral conjunctival surface is thus distinctly different from the peripheral corneal epithelium across which it slides following eyelid closure.

Evidence for heterogeneity in a small squamous cell type ('light' cells) in the rabbit corneal epithelium--a scanning electron microscope study. Corneal epithelial squamous cells

The rabbit corneal epithelial surface, as viewed by scanning electron microscopy (SEM), is composed of a mosaic of polygonal cells with different appearances (light, medium and dark) with the lighter cells having a characteristically high density of microplicae. From the central zone of the corneal epithelial surface of 16 female New Zealand White rabbits (2 kg), the lighter-appearing cells had an average area of 108 +/- 47 microns2 (n = 567, +/-SD). A subgroup of atypical lighter cells had an average area of 294 +/- 67 microns2 (n = 53) compared to typical light cells of 88 +/- 12 microns (n = 514). These atypical lighter cells had fused microplicae at their periphery (instead of a uniform arrangement of densely packed microplicae), tended to be rounder in shape (as opposed to being angular), could show signs of desquamation and were not decorated with the epithelial craters found on almost all (92%) other light cells. All of these features are suggestive of these atypical lighter cells being the terminal phenotype of the light cells just prior to desquamation from the ocular surface. The observation of the desquamation of lighter cells supports a hypothesis that they constitute a distinct sub-population of cells at the corneal epithelial surface.

The effect of hypoxia on the shedding rate of the corneal epithelium

The reason a cell leaves the corneal epithelial surface at a particular time is not understood. It is likely that a cell must depend on metabolic energy to accomplish this task successfully and with minimal disruption to the epithelial surface and barrier. The hypothesis under test is that the epithelium is directly dependent on atmospheric oxygen to maintain a normal cell shedding rate. Rabbit corneas were excised in pairs and the surfaces bathed with appropriate media for 400 minutes at 304 mOsm/kg and pH 7.4. The epithelial surface of one cornea was bathed with a normoxic solution as a control, while the other cornea was anoxic. Solutions were collected from the epithelial surface at 50 minute intervals. Cell counts of shed corneal epithelial cells were made by staining with acridine orange and viewing with fluorescence microscopy. Stromal thickness was measured at the beginning and at 400 minutes to confirm hypoxia. The results show that hypoxia does reduce the rate at which cells are shed, at least for collections made during the first 50 minutes (P < 0.01). However, by 150 minutes there was no difference between the hypoxic cornea and the control. This result suggests that cell shedding is dependent on oxygen only during the early part of a period of hypoxia. When the hypoxia is prolonged, other mechanisms intervene which preserve the normal shedding rate.

Effect of bicarbonate-free balanced salt solutions on fluid pump and endothelial morphology of rabbit corneas in-vitro

Corneas from young adult albino rabbits were exposed, in-vitro, to various bicarbonate-free balanced salt solutions under an applied hydrostatic pressure of 20 cmH2O at 35 degrees C for up to 4.5 h. The solutions were buffered with a HEPES-MOPS mixture, phosphate salts or acetate-citrate salts (pH 7.1-7.4). All of these solutions support a net fluid pump activity attributed to the corneal endothelium although the net fluid pump rates were less than those measurable with a bicarbonate-buffered, CO2-equilibrated Ringer solution. Evaluation of the endothelia by scanning electron microscopy revealed no evidence of any acute toxic effect of the balanced salt solutions. Morphometric analyses of cell size and shape revealed a normal mosaic for endothelia exposed to commercial balanced salt solution for 5 h. The results further indicate that exogenous bicarbonate is not required for the net fluid pump function of the mammalian corneal endothelium.

Topographical differences in cell area at the surface of the corneal epithelium of the pigmented rabbit

Corneas from female pigmented rabbits were prepared for scanning electron microscopy at 15.00 h. The superior-temporal and inferior-nasal quadrants were evaluated at 500X at-stage magnification, normal to the corneal surface, at central, paracentral and peripheral sites. Non-parametric statistical analyses of the histograms of cell surface areas at each site from seven corneas revealed significant differences between the three sites but not between superior and inferior quadrants.

Concerning the symmetry of the 'hexagonal' cells of the corneal endothelium

Modeling studies are presented to show how the figure coefficient and hexagon deviation index values decrease as a six-sided endothelial cell is systematically distorted from a completely symmetrical geometric shape (a regular hexagon). By use of scanning electron micrographs of rabbit corneal endothelium and an analysis of published specular micrographs of rabbit corneal endothelium, it is shown that the six-sided cells are not necessarily very symmetrical. Such cells should thus be reported as six-sided cells rather than as 'hexagons' which carries the inherent risk of implying regularity (and thus supposed stability in a tesselated mosaic) of such cells.