A descriptive and quantitative study of the keratocytes of the corneal stroma of albino rabbits using transmission electron microscopy

Unveiling Cellular Diversity in the Buffalo Corneal Stroma: Insights into Telocytes and Keratocytes Using Light Microscope, Transmission Electron Microscope, and Immunofluorescence Analysis

Telocytes and keratocytes are important cells that maintain the structure and function of the cornea. The buffalo cornea, known for its resilience in harsh conditions, has not been extensively studied regarding the presence and role of telocytes and keratocytes. We used light microscopy, transmission electron microscopy (TEM), and immunofluorescence assays with platelet-derived growth factor receptor alpha (PDGFRα), CD34, and Vimentin markers to investigate their expression and localization in the cornea. TEM analysis confirmed the presence of spindle-shaped keratocytes with intercellular connections, while telocytes exhibited small spindle-shaped bodies with long, thin branches connecting to corneal keratocytes. Immunofluorescence findings showed that CD34 was more abundant near the endothelium, Vimentin was prominently expressed near the epithelium, and PDGFRα was uniformly distributed throughout the corneal stroma. Co-expression of CD34 and Vimentin, PDGFRα and Vimentin, as well as CD34 and PDGFRα, was observed in keratocytes and telocytes within the stroma, indicating the potential presence of mesenchymal cells. These results suggest the involvement of telocytes and keratocytes in corneal wound healing, transparency maintenance, and homeostasis. The co-expression of these markers highlights the critical role of telocytes and keratocytes in regulating corneal physiological functions, further enhancing our understanding of corneal biology in the buffalo model.

Microstructured collagen films for 3D corneal stroma modelling

PURPOSE/AIM

Corneal injury is a major cause of impaired vision around the globe. The fine structure of the corneal stroma plays a pivotal role in the phenotype and behavior of the embedded cells during homeostasis and healing after trauma or infection. In order to study healing processes in the cornea, it is important to create culture systems that functionally mimic the natural environment.

MATERIALS AND METHODS

Collagen solution was vitrified on top of a grated film to achieve thin collagen films with parallel microgrooves. Keratocytes (corneal stromal cells) were cultured on the films either as a single layer or as stacked layers of films and cells. SEM and F-actin staining were used to analyze the pattern transference onto the collagen and the cell orientation on the films. Cell viability was analyzed with MTS and live/dead staining. Keratocytes, fibroblasts, and myofibroblasts were cultured to study the pattern's effect on phenotype.

RESULTS

A microstructured collagen film-based culture system that guides keratocytes (stromal cells) to their native, layerwise perpendicular orientation in 3D and that can support fibroblasts and myofibroblasts was created. The films are thin and transparent enough to observe cells at least three layers deep. The cells maintain viability in 2D and 3D cultures and the films can support fibroblast and myofibroblast phenotypes.

CONCLUSIONS

The films provide an easily reproducible stroma model that maintains high cell viability and improves the preservation of the keratocyte phenotype in keratocytes that are differentiated to fibroblasts.

Collagen fibril characteristics at the corneo‐scleral boundary and rabbit corneal stromal swelling

BACKGROUND

The aim of this investigation was to reassess the impact of the scleral rim on the swelling of the mammalian corneal stroma and to investigate the ultrastructural features of the scleral rim and corneal stromal tissues.

METHODS

The epithelium and endothelium were removed from corneas excised from three-month-old female rabbits. The resulting preparations consisted of the corneal stroma plus a surrounding scleral rim, excised corneal stroma or a nine-millimetre button of central corneal stroma. These preparations were immersed in a 35 mM bicarbonate-buffered mixed salt solution (equilibrated with five per cent CO2-air, pH 7.54 at 37 degrees Centigrade for nine hours. Some sclero-corneal preparations were fixed for light or transmission electron microscopy.

RESULTS

The initial rate of swelling of corneal stromal buttons was greatest at 127 +/- 8 per cent per hour, less for complete stromal preparations (118 +/- 9 per cent per hour) and least for sclero-stromal preparations (76 +/- 12 per cent per hour). The swelling continued over three to nine hours but sclera preparations swelled up to 40 per cent with no further swelling. Light microscopy demonstrated that the sclero-corneal rim tissue limited the swelling of the posterior corneal stroma. TEM sections of the episclera and sclera indicate that most fibril bundles show a radial orientation to the cornea. There are marked anterio-posterior differences in the collagen fibrils of the scleral surround that are distinctly different from previous reports. Average fibril diameters were 62.8 +/- 7.9 nm in the episclera, 122.4 +/- 18.9 nm, 133.5 +/- 51.9 nm and 56.5 +/- 11.2 nm in the anterior, mid- and posterior scleral stroma, compared to an average fibril diameter of 33.5 +/- 3.5 nm for the posterior corneal stroma.

CONCLUSIONS

When there is a scleral rim in place, the swelling of the corneal stroma is substantially less than for isolated corneal stroma. The effect can be attributed to the absence of a cut-edge effect for the sclero-corneal stromal preparation but the unique and largely radial arrangement of the collagen fibrils in the scleral rim plays a part in limiting the swelling of the adjacent corneal stroma. The heterogeneous nature of this sclero-corneal interface requires further investigation to define the mechanism of the effect.

Toward understanding the purely geometric effects of silver nanoparticles on potential application as ocular therapeutics via treatment of bacterial keratitis

Understanding a complex interaction between therapeutic nanoparticles and biological entities is crucially important for the development of effective disease treatments in the modern nanopharmaceuticals and nanomedicines. Herein, we present a strategy to thoroughly assess geometrical impacts of silver nanoparticles (AgNPs, one of the most promising nanotherapeutic agents) on their biological activities toward treatment of Staphylococcus aureus (S. aureus)-induced keratitis. Specifically, three types of differently shaped AgNPs including silver nanorods (R-Ag), silver nanotriangles (T-Ag), and silver nanospheres (SAg) are employed and interferences of particle surface area and functionality are eliminated to reflect purely geometric effects. Ocular biocompatibility studies on rabbit corneal keratocytes reveal that SAg is the least cytotoxic agent while R-Ag, because of its strongest cellular uptake, induces highest cytotoxic levels. Moreover, SAg is demonstrated to outperform R-Ag and T-Ag in killing S. aureus, possibly due to a predominance of specific particle density and high-atom-density {111} facets of the SAg when interacting with the bacteria. In contrast, owing to its predominance of sharp-tip effects on vascular endothelial cells, R-Ag can suppress blood vessel development in cornea at a greatest extent. In a rabbit model of S. aureus-induced keratitis, intrastromal administration of the differently shaped AgNPs exhibits critical roles of the particle geometry at comparable conditions (i.e., total surface area and functionality) in attenuating progression of S. aureus-induced keratitis. As a compromise among ocular biocompatibility, anti-bacterial activity, and anti-angiogenic capability, SAg shows as the most effective agent that could repair infectious corneal tissues 1.2 and 4-fold greater than the anisotropic counterparts (R-Ag and T-Ag). These findings therefore suggest a promising strategy for a clear-cut evaluation on geometric effects of therapeutic nanoparticles toward preclinical treatment of eye-related microbial infections.

Multimodal imaging quality control of epithelia regenerated with cultured human donor corneal limbal epithelial stem cells

Current imaging techniques for the characterization of differentiated corneal limbal stem cells are destructive and cannot be used in eye bank for monitoring the regenerated epithelium in culture. We presented a minimally invasive, multimodal, marker-free imaging method for the investigation of epithelia regenerated with cultured human donor corneal limbal epithelial stem cells. Two-photon fluorescence and harmonic generation signals were collected from specimens in culture and used for evaluating the structure and morphology of epithelia cultured on two different bio-scaffolds; in addition, donor human corneal tissues were used as controls. The method provided reliable information on the organization of cellular and extracellular components of biomaterial substrates and was highly sensitive to determine differences between the density packing arrangement of epithelial cells of different biomaterials without relying on inferences from exogenous labels. The present minimally invasive standardized quality control methodology can be reliably translated to eye banks and used for monitoring harvested corneal limbal stem cells growth and differentiation in bioengineered materials.

Measuring the Refractive Index of Bovine Corneal Stromal Cells Using Quantitative Phase Imaging

The cornea is the primary refractive lens in the eye and transmits >90% of incident visible light. It has been suggested that the development of postoperative corneal haze could be due to an increase in light scattering from activated corneal stromal cells. Quiescent keratocytes are thought to produce crystallins that match the refractive index of their cytoplasm to the surrounding extracellular material, reducing the amount of light scattering. To test this, we measured the refractive index (RI) of bovine corneal stromal cells, using quantitative phase imaging of live cells in vitro, together with confocal microscopy. The RI of quiescent keratocytes (RI = 1.381 ± 0.004) matched the surrounding matrix, thus supporting the hypothesis that keratocyte cytoplasm does not scatter light in the normal cornea. We also observed that the RI drops after keratocyte activation (RI = 1.365 ± 0.003), leading to a mismatch with the surrounding intercellular matrix. Theoretical scattering models showed that this mismatch would reduce light transmission in the cornea. We conclude that corneal transparency depends on the matching of refractive indices between quiescent keratocytes and the surrounding tissue, and that after surgery or wounding, the resulting RI mismatch between the activated cells and their surrounds significantly contributes to light scattering.

A comparison of glycosaminoglycan distributions, keratan sulphate sulphation patterns and collagen fibril architecture from central to peripheral regions of the bovine cornea

This study investigated changes in collagen fibril architecture and the sulphation status of keratan sulphate (KS) glycosaminoglycan (GAG) epitopes from central to peripheral corneal regions. Freshly excised adult bovine corneal tissue was examined as a function of radial position from the centre of the cornea outwards. Corneal thickness, tissue hydration, hydroxyproline content, and the total amount of sulphated GAG were all measured. High and low-sulphated epitopes of keratan sulphate were studied by immunohistochemistry and quantified by ELISA. Chondroitin sulphate (CS) and dermatan sulphate (DS) distributions were observed by immunohistochemistry following specific enzyme digestions. Electron microscopy and X-ray fibre diffraction were used to ascertain collagen fibril architecture. The bovine cornea was 1021 ± 5.42 μm thick at its outer periphery, defined as 9–12 mm from the corneal centre, compared to 844 ± 8.10 μm at the centre. The outer periphery of the cornea was marginally, but not significantly, more hydrated than the centre (H = 4.3 vs. H = 3.7), and was more abundant in hydroxyproline (0.12 vs. 0.06 mg/mg dry weight of cornea). DMMB assays indicated no change in the total amount of sulphated GAG across the cornea. Immunohistochemistry revealed the presence of both high- and low-sulphated epitopes of KS, as well as DS, throughout the cornea, and CS only in the peripheral cornea before the limbus. Quantification by ELISA, disclosed that although both high- and low-sulphated KS remained constant throughout stromal depth at different radial positions, high-sulphated epitopes remained constant from the corneal centre to outer-periphery, whereas low-sulphated epitopes increased significantly. Both small angle X-ray diffraction and TEM analysis revealed that collagen fibril diameter remained relatively constant until the outer periphery was reached, after which fibrils became more widely spaced (from small angle x-ray diffraction analysis) and of larger diameter as they approached the sclera. Depth-profiled synchrotron microbeam analyses showed that, at different radial positions from the corneal centre outwards, fibril diameter was greater superficially than in deeper stromal regions. The interfibrillar spacing was also higher at mid-depth in the stroma than it was in anterior and posterior stromal regions. Collagen fibrils in the bovine cornea exhibited a fairly consistent spacing and diameter from the corneal centre to the 12 mm radial position, after which a significant increase was seen. While the constancy of the overall sulphation levels of proteoglycans in the cornea may correlate with the fibrillar architecture, there was no correlation between the latter and the distribution of low-sulphated KS.

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Proteoglycans (KS, DS, CS) and collagen were correlated with corneal radial position.

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Total sulfate levels on glycosaminoglycans remained constant across the cornea.

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KS and DS were ubiquitous; CS was found towards the edge of the cornea onwards.

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High-sulfated KS remained constant; low-sulfated KS increased peripherally.

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There was no correlation between fibrillar architecture and sulfation levels of KS.

Adhesion, phenotypic expression, and biosynthetic capacity of corneal keratocytes on surfaces coated with hyaluronic acid of different molecular weights

In ophthalmology, hyaluronic acid (HA) is an important extracellular matrix (ECM) component and is appropriate for use in generating a microenvironment for cell cultivation. The aim of this work was to evaluate the rabbit corneal keratocyte (RCK) growth in response to HA coatings under serum-free conditions. After modification with HA of varying molecular weights (MWs: 35-1500kDa), the surfaces were characterized by atomic force microscopy and contact angle measurements, and were used for cell culture studies. Our data indicated that the substrates coated with higher negatively charged HA become rougher and are more hydrophilic, resulting in the decrease of cell adhesion and cell-matrix interaction. This early cellular event was likely responsible for the determination of keratocyte configuration. Additionally, for the growth of RCKs on dry HA coatings with surface roughness of 1.1-1.7 nm, a strong cell-cell interaction was observed, which may facilitate the formation of multicellular spheroid aggregates and maintenance of mitotically quiescent state. At each culture time point from 1 to 5 days, a better biosynthetic capacity associated with a higher prevalence of elevated ECM production was found for the cells in a spherical configuration. Irrespective of polysaccharide MW of surface coatings, the RCKs presented good viability without hypoxia-induced death. As compared with a monolayer of adherent keratocytes on tissue culture polystyrene plates and low MW HA-modified samples, the cell spheroids (76-110 μm in diameter) showed significantly higher expressions of keratocan and lumican and lower expressions of biglycan, similar to those of keratocytes in vivo. Moreover, the expression levels of corneal crystallin aldehyde dehydrogenase (7-9-fold increase) and nestin (10-16-fold increase) were greater in larger-sized spheroids, indicating higher ability to maintain cellular transparency and self-renewal potential. It is concluded that the cultured RCKs on surfaces coated with HA of different MWs can sense ECM cues, and the multicellular spheroids may potentially be used for corneal stromal tissue engineering applications.

Nanoscale modification of porous gelatin scaffolds with chondroitin sulfate for corneal stromal tissue engineering

Recent studies reflect the importance of using naturally occurring biopolymers as three-dimensional corneal keratocyte scaffolds and suggest that the porous structure of gelatin materials may play an important role in controlling nutrient uptake. In the current study, the authors further consider the application of carbodiimide cross-linked porous gelatin as an alternative to collagen for corneal stromal tissue engineering. The authors developed corneal keratocyte scaffolds by nanoscale modification of porous gelatin materials with chondroitin sulfate (CS) using carbodiimide chemistry. Scanning electron microscopy/energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy showed that the amount of covalently incorporated polysaccharide was significantly increased when the CS concentration was increased from 0% to 1.25% (w/v). In addition, as demonstrated by dimethylmethylene blue assays, the CS content in these samples was in the range of 0.078–0.149 nmol per 10 mg scaffold. When compared with their counterparts without CS treatment, various CS-modified porous gelatin membranes exhibited higher levels of water content, light transmittance, and amount of permeated nutrients but possessed lower Young’s modulus and resistance against protease digestion. The hydrophilic and mechanical properties of scaffolds modified with 0.25% CS were comparable with those of native corneas. The samples from this group were biocompatible with the rabbit corneal keratocytes and showed enhanced proliferative and biosynthetic capacity of cultured cells. In summary, the authors found that the nanoscale-level modification has influence on the characteristics and cell-material interactions of CS-containing gelatin hydrogels. Porous membranes with a CS content of 0.112 ± 0.003 nmol per 10 mg scaffold may hold potential for use in corneal stromal tissue engineering.

Assessment of corneal thickness and keratocyte density in a rabbit model of laser in situ keratomileusis using scanning laser confocal microscopy

PURPOSE

To determine the repeatability of corneal thickness and keratocyte density using in vivo confocal scanning laser microscopy in a rabbit model of laser in situ keratomileusis.

DESIGN

Prospective, experimental animal study.

METHODS

En face tomographic images of corneal tissue were captured from 5 New Zealand white rabbits. Central corneal thickness was compared with conventional ultrasonic pachymetry. Keratocyte density was measured as a function of stromal depth at baseline and 6 weeks after a 130-μm lamellar incision in the following regions: first countable stromal image (30 to 39 μm), anterior stroma (40 to 75 μm), incision zone (76 to 150 μm), mid stroma (151 to 250 μm), and deep stroma (251 to 400 μm).

RESULTS

The mean residual difference between ultrasonic and confocal corneal thickness measurements was 2.1 μm (95% confidence interval [CI], -7.0 to 11.2 μm; P = .61). Before the lamellar incision, keratocyte density was highest in the first countable frame of the anterior stroma, 53 800 cells/mm(3) (95% CI, 35 000 to 72 000 cells/mm(3)) and was least in deep stroma, 27 100 cells/mm(3) (95% CI, 22 400 to 32 000 cells/mm(3)). Six weeks after stromal lamellar incision, keratocyte density was unchanged in the first countable frame of the anterior stroma, 43 700 cells/mm(3) (95% CI, 31 800 to 55 500 cells/mm(3); P = .29). There were no changes in cell density in deeper stromal regions.

CONCLUSIONS

There was excellent agreement between ultrasonic and confocal microscopy measurements of corneal thickness. In vivo repeatability of keratocyte density estimation using scanning laser confocal microscopy is comparable with the results of previous reports using tandem-scanning confocal microscopy. Keratocyte density was more varied, but not significantly different, in the anterior-most corneal stroma 6 weeks after a lamellar incision.

Assessment of collagen fibril spacing in relation to selected region of interest (ROI) on electron micrographs--application to the mammalian corneal stroma

AIMS

To evaluate measurements of collagen fibril spacing using different shaped regions of interest (ROI) on transmission electron micrograph (TEM) images of rabbit corneal stroma.

METHODS

Following glutaraldehyde fixation and phosphotungstic acid staining, TEM images of collagen fibrils in cross section were projected at a final magnification close to 250,000 × to obtain overlays. Interfibril distances (IFDs; center-to-center spacing) were measured within different ROIs of the same nominal area (0.25 μm(2) ) but different shape (with the length to width, L:W, ratio from 1:1 to 6:1). The IFD distribution was analyzed, and the 2D organization assessed using a radial distribution analysis.

RESULTS

The fibrils had an average diameter of 35.3 ± 3.8 (SD) nm, packing density of 393 ± 4 fibrils / μm(2) and a fibril volume fraction of 0.39 ± 0.02. IFDs ranged from 29 to 1400 nm depending on the shape of the ROI, with average values ranging from 263 to 443 nm. By artificially selecting IFD data only to a radial distance of 250 nm, the average IFDs were just 145-157 nm. The radial distributions, to 250 nm, all showed a nearest neighbors first peak which shifted slightly from predominantly at 45-54 nm with more rectangular ROIs. The radial distribution profiles could be shown to be statistically different if the ROI L:W ratio was 2:1 or greater.

CONCLUSION

Selection of an ROI for assessment of packing density and interfibril distances should be standardized for comparative assessments of TEMs of collagen fibrils.

Dimensions and morphology of the cornea in three strains of mice

PURPOSE

To use a histologic approach to obtain dimensional and morphologic information on the cornea in three commonly used strains of mice.

METHODS

Adult mice (three each of 129/SVJ, C57BL/6, and BALB/c) were euthanatized, and the eyes were enucleated, immersed in 2% glutaraldehyde fixative, and prepared for light and transmission electron microscopy. The full corneal, epithelial, stromal, and posterior limiting lamina (PLL) with endothelium thicknesses were measured at the same location centrally and peripherally.

RESULTS

All three strains showed a statistically significant (P < 0.001) decrease in overall thickness in the peripheral compared with the central cornea. The decrease was due to a reduced thickness of both the epithelium and the stroma. The stroma and epithelium contributed to approximately two thirds and one third of the total corneal thickness, respectively. The epithelium had the classic stratified layout and consisted of 13.00 +/- 1.41 layers centrally versus 10.33 +/- 1.37 peripherally. Some adaptation of stromal tissue was found immediately adjacent to the epithelial basement membrane, but a clearly defined anterior limiting lamina did not exist. The stroma was organized into lamellae but lacked the anterior branching and interweaving reported in humans and had unmyelinated nerve fibers within micrometers of the endothelium. The PLL was 2.17 +/- 0.3 microm thick and was divided into pre- and postnatal layers, with striated bodies in the postnatal portion.

CONCLUSIONS

This study demonstrated that in the three strains of mice examined, the cornea becomes significantly thinner toward the periphery. Dimensionally, proportionally, and anatomically the three strains used appeared to be similar. However, morphologic differences were observed compared with other mammals, and awareness of these differences is important when using the mouse as an animal model applicable to the human.

Neutrophil migration in the wounded cornea: the role of the keratocyte

Neutrophil migration into the corneal stroma following epithelial injury is an early event in the non-adaptive immune response. Little is known regarding the molecular and cellular mechanisms that regulate neutrophil migration within the injured cornea. In this article, evidence is presented supporting the idea that migrating neutrophils form integrin-dependent adhesive contacts not only with extracellular matrix molecules, but also with interstitial cells known as keratocytes. Since keratocytes express adhesion molecules and secrete leukotactic factors, and because they are joined to one another to form a cellular network, we suggest that the keratocyte network plays a prominent role in neutrophil migration by serving as a source of contact guidance and chemoattraction for migrating neutrophils. The proposed neutrophil interactions with keratocytes constitute the beginning of a new experimental paradigm for understanding leukocyte migration within the injured cornea.

Development of a hemicornea from human primary cell cultures for pharmacotoxicology testing

We report the reconstruction and characterization of a hemicornea (epithelialized stroma), using primary human cells, for use in research and as an alternative to the use of animals in pharmacotoxicology testing. To create a stromal equivalent, keratocytes from human corneas were cultured in collagen-glycosaminoglycan-chitosan foams. Limbal stem cell-derived epithelial cells were seeded on top of these, giving rise to hemi-corneas. The epithelium appeared morphologically similar to its physiological counterpart, as shown by the basal cell expression of p63 isoforms including, in some cases, the stem cell marker p63DeltaNalpha, and the expression of keratin 3 and 14-3-3sigma in the upper cell layers. In addition, the cuboidal basal epithelial cells were anchored to a basement membrane containing collagen IV, laminin 5, and hemidesmosomes. In the stromal part, the keratocytes colonized the porous scaffold, formed a network of interconnecting cells, and synthesized an ultrastructurally organized extracellular matrix (ECM) containing collagen types I, V, and VI. Electron microscopy showed the newly synthesized collagen fibrils to have characteristic periodic striations, with diameters and interfibril spacings similar to those found in natural corneas. Compared to existing models for corneal pharmacotoxicology testing, this new model more closely approaches physiological conditions by including the inducing effects of mesenchyme and cell-matrix interactions on epithelial cell morphogenesis.

Assessment of the apparent intra- and inter-sample variability in the collagen fibril diameter in the posterior corneal stroma of rabbits. A transmission electron microscopy study

PURPOSE

To assess the natural variability in the diameter of small collagen fibrils in the lamellae of corneal stroma of healthy young rabbits.

MATERIALS AND METHODS

The corneas of 6 young adult female New Zealand White rabbits (2.1 kg) were prepared for transmission electron microscopy (TEM) by fixation with a cacodylate-buffered 2% glutaraldehyde (pH 7.2-7.4, 320-340 mosm/kg). The corneas were embedded in Epon-Araldite, thin sections prepared from the central region of the posterior corneal stroma and stained with acidic uranyl acetate followed by acidic lead citrate. High magnification (x 33,000) micrographs were taken, and fibril diameters (FDs) measured at a final magnification of x 275,000 to a resolution of 2 nm.

RESULTS

Assessment of sampling-related errors indicated that the average diameter of the fibrils within any particular micrograph could be estimated to within 1% or better by measures of 100 fibrils. Assessments of the intra-sample variance (6 micrographs taken from the same cornea) indicated a group mean FD of 32.4 +/- 3.6 nm, whilst the inter-sample variance (6 micrographs taken from 6 different corneas) yielded an average of 33.1 +/- 4.5 nm (n = 100 fibrils/micrograph, +/-SD). However, group-averaged data sets of FDs, while unimodal, were not normally distributed, and cumulative averaging indicated a fixed range of FDs across the data sets.

CONCLUSIONS

Intra- and inter-sample variability in collagen FDs is very similar, but the analysis indicates that the collagen fibrils are not homogeneous and that closely adjacent lamellae can have subtle differences in average FD.

Assessment of the number of lamellae in the central region of the normal human corneal stroma at the resolution of the transmission electron microscope

PURPOSE

To determine the total number of lamellae within the central region of the human corneal stroma by using a continuous section through the corneal stroma and at the ultrastructural resolution of the electron microscope.

METHODS

Six human eye bank corneas, from individuals aged between 42 and 82 years, were received in corneal storage medium (Optisol GS) and then processed for transmission electron microscopy with buffered 2% glutaraldehyde fixation. Thin sections were mounted on parallel bar copper grids for an uninterrupted full corneal thickness observation. A sequence of overlapping micrographs were taken at 2,600x magnification, printed at a final magnification of 3,640x, and assembled as a montage. The number of lamellae were counted across the corneal stromal strip by three observers, independently, by using a preagreed set of criteria for identifying individual lamella.

RESULTS

The average number of lamellae per stroma was determined to be 242+/-4 (range, 234-247). In the more anterior region of the stroma, the density of the lamellae was 50% greater than in the posterior stroma (mean, 57+/-12 per 100 microm anteriorly vs. 38+/-5 per 100 microm posteriorly). Interobserver differences were modest and generally less than the intersample variance.

CONCLUSIONS

When assessed at the resolution of the transmission electron microscope, the number of stromal lamellae in the central region of the human cornea are higher (at approximately 240 per cornea) than most previous estimates reported from light microscopy studies.

Quantitative analysis of corneal microstructure in keratoconus utilising in vivo confocal microscopy

PURPOSE

To establish and quantify the in vivo confocal microscopic features of moderate to advanced keratoconus.

METHODS

Nineteen keratoconus subjects were catergorised using Orbscan-derived corneal apex power and pachymetry as exhibiting moderate (n=7) and advanced (n=12) keratoconus. Control subjects included 23 noncontact lens wearers (Group A) and 15 contact lens wearers (Group B). All subjects underwent Confoscan slit scanning in vivo confocal microscopy.

RESULTS

Compared with Group A (4912+/-434 cells/mm(2)), basal epithelial density was significantly lower in both moderate (4592+/-414 cells/mm(2), P<0.05) and advanced keratoconus (4530+/-596 cells/mm(2), P=0.01). In comparison to Group A (761+/-118 cells/mm(2)), anterior stroma keratocyte density was significantly greater in both moderate keratoconus (883+/-111 cells/mm(2), P=0.001) and advanced keratoconus (952+/-122 cells/mm(2), P<0.001). Compared to Group A (504+/-80 cells/mm(2)) posterior stroma keratocyte density was also significantly greater in advanced keratoconus (599+/-97 cells/mm(2), P<0.001) and posterior stromal keratocyte density appeared to increase with increasing severity of keratoconus (P<0.05). However, comparing control Groups A and B, contact lens wear per se, was associated with significantly reduced (P=0.000) keratocyte density in the anterior stroma (609+/-66 cells/mm(2)) and demonstrated a trend (P=0.056) in the posterior stroma (470+/-63 cells/mm(2)). Keratoconic corneas (429+/-72 microm) were significantly thinner than control Groups A (508+/-77 mm) and B (495+/-80 microm). The presence of keratoconus did not affect the endothelial cell density (P=0.54).

CONCLUSION

In vivo confocal microscopy can provide insight into the microstructural changes that occur in keratoconus.

Resolution and reproducibility of measures of the diameter of small collagen fibrils by transmission electron microscopy--application to the rabbit corneal stroma

AIM

To assess the impact of measurements of different numbers of small collagen fibrils at different final magnification values on the resultant mean values for fibril diameter (FD).

METHODS

Very high magnification (33,000x) electron micrographs were taken of the posterior-central zone of the corneal stroma from young adult rabbits (2 kg), printed at 46,000 or 50,000x, scanned at 400 d.p.i. and 35 mm slides prepared. These were projected to give final magnifications between 150,000x and 450,000x. An overlay of fibril outlines was prepared from the projected images and the fibril diameters (FD's) measured to within 0.5 mm.

RESULTS

On the overlays, at different projection magnifications, the average FD's ranged from 4 to 13.5 mm to allow measures to be made at a real resolution of between 3.5 and 1 nm. Using a fixed sized region of interest (ROI) on the overlays, the average values of FD's ranged between 24.9 and 31.7 nm, and could vary (for any particular micrograph) by up to 3.6 nm according to the number of FD's measured (n=34-384/ROI). Using a fixed number of FD measures (n=100) at different projection magnifications yielded average FD values from different corneas of between 25.1 and 35.2 nm, which could vary by up to 4.3 nm depending on the magnification used.

CONCLUSIONS

The results indicate that different average values for measures of fibril diameter of small collagen fibrils can easily be obtained according to the final magnification used and the number of fibrils measured, and that the overall averages can depend on whether the data sets are averaged or pooled. These aspects of the morphometry of small collagen fibrils therefore need to be carefully specified.

Morphological Characteristics and Intercellular Connections of Corneal Keratocytes

PURPOSE

To investigate the morphological characteristics of keratocytes and the interconnection of keratocytes with adjacent keratocytes using the flat preparation method and scanning electron microscopy with a frontal section of the human corneal stroma.

METHODS

The thin, corneal collagen lamellae were carefully dissected from the cornea (n=7), which had been stained by the flat preparation method. The remaining tissue was fixed in 3% glutaraldehyde and observed by transmission electron microscopy following the frontal section.

RESULTS

The flat preparation revealed the corneal fibroblasts between the lamellae of the collagen fibers and showed that the ramifying cellular processes of the keratocytes were in contact with the cytoplasmic processes or cell bodies of neighboring fibroblasts. Two types of discrete subpopulations of keratocytes were identified: a smaller, cellular type of keratocyte with spindle-shaped nucleus with heterochromatin, and a larger, cellular type with a large indented nucleus with relatively scanty cytoplasm. Collagen fibers ran parallel to each other toward the fenestration of the cytoplasmic wall of the keratocyte.

CONCLUSIONS

These flat preparation method results showed that the keratocytes within the corneal stroma are interconnected with the adjacent keratocytes, which indicates the presence of a functional communicating network through the keratocyte circuits within the stroma. A smaller, cellular type of keratocyte with spindle-shaped nucleus was morphologically differentiated from a larger, cellular type with a large, indented nucleus by flat preparation and transmission electron microscopy.

A physiological perspective on the swelling properties of the mammalian corneal stroma

PURPOSE

The present studies were designed to assess whether measurement of corneal stroma swelling in the laboratory, especially in non-physiological solutions, was associated with a measurable effect on the keratocytes.

METHODS

Complete corneal stroma preparations were made from quality- and age-selected recent post-mortem cattle eyes. These were either assessed immediately or incubated in three different solutions, namely a balanced salts solution with glucose (BSSG), isotonic phosphate buffered saline (PBS) or pure water. Incubations were carried out at 37 degrees C for 9h, and repeated measures of wet mass made so that the rates and extent of swelling could be determined. After incubation, an aqueous extract was made of the stroma for measurements of the levels the enzymes lactate dehydrogenase (LDH), aldehyde dehydrogenase (ALDH) and N-acetyl-beta-glucosaminidase.

RESULTS

The initial rates of swelling were lowest in BSSG, marginally faster in PBS and much faster in water. The secondary rates of swelling showed the same sequence being 10.0%/h in BSSG, 14.8%/h in PBS and 34.2%/h in water. Compared to non-incubated preparations, reductions in all three enzyme activities occurred. For LDH, these were 15% with BSSG, 40% in PBS and 80% with water. Similar results were seen with ALDH activity when comparing the three incubation solutions, while incubation in BSSG also resulted in a substantial (40%) reduction in N-acetyl-glucosaminidase activity.

CONCLUSIONS

When immersed in an isotonic BSSG with added glucose at 37 degrees C, the swelling of a complete bovine corneal stroma is much less than smaller pieces of stroma, and also slightly less than if isotonic PBS was used. With the use of BSSG, little or no change in cytoplasmic enzyme activities occurred, but measurable decreases were noted with PBS and very substantial decreases when water was used, indicating a toxic effect on the keratocytes. The observation that substantial decreases in a lysosomal enzyme activity could occur even with the use of BSSG indicate substantial stress is imposed on the stroma during these types of experiments. Notwithstanding, the data collectively indicate that the keratocyte cells within the collagen matrix of the stroma can be substantially damaged and this needs to be taken into account in future experiments on the true physiology of the corneal stroma.