The limbal vasculature

Diabetic Ocular Surface Has Defects in Oxygen Uptake Revealed by Optic Fiber Microsensor

Purpose

Diabetes mellitus causes diabetic keratopathy (DK). This and other ocular surface disorders are underdiagnosed and problematic for affected patients as well as recipients of diabetic donor corneas. Thus, it is important to find noninvasive means to facilitate determination of the potentially vision-threatening DK. It has been reported that diabetic corneas uptake significantly less oxygen (O2) than healthy controls. However, an integral assessment of the ocular surface is missing.

Methods

Using an optic-fiber O2 micro-sensor (optrode) we demonstrated recently that the healthy ocular surface displays a unique spatiotemporal map of O2 consumption. We hypothesize that diabetes impairs the spatiotemporal profile of O2 uptake at the ocular surface.

Results

Using streptozotocin (STZ)-induced diabetic mice, we found diminished O2 uptake and loss of the unique pattern across the ocular surface. A diabetic cornea consumes significantly less O2 at the bulbar conjunctiva and limbus, but not the central and peripheral cornea, compared to controls. Further, we show that, contrary to the healthy cornea, the diabetic cornea does not increase the O2 consumption at the limbus in the evening as the normal control.

Conclusions

Altogether, our measurements reveal a previously unknown impairment in O2 uptake at the diabetic cornea, making it a potential tool to diagnose ocular surface abnormalities and suggesting a new etiology mechanism.

Short-Term UVB Irradiation Leads to Persistent DNA Damage in Limbal Epithelial Stem Cells, Partially Reversed by DNA Repairing Enzymes

The cornea is frequently exposed to ultraviolet (UV) radiation and absorbs a portion of this radiation. UVB in particular is absorbed by the cornea and will principally damage the topmost layer of the cornea, the epithelium. Epidemiological research shows that the UV damage of DNA is a contributing factor to corneal diseases such as pterygium. There are two main DNA photolesions of UV: cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4) photoproducts (6-4PPs). Both involve the abnormal linking of adjacent pyrimide bases. In particular, CPD lesions, which account for the vast majority of UV-induced lesions, are inefficiently repaired by nucleotide excision repair (NER) and are thus mutagenic and linked to cancer development in humans. Here, we apply two exogenous enzymes: CPD photolyase (CPDPL) and T4 endonuclease V (T4N5). The efficacy of these enzymes was assayed by the proteomic and immunofluorescence measurements of UVB-induced CPDs before and after treatment. The results showed that CPDs can be rapidly repaired by T4N5 in cell cultures. The usage of CPDPL and T4N5 in ex vivo eyes revealed that CPD lesions persist in the corneal limbus. The proteomic analysis of the T4N5-treated cells shows increases in the components of the angiogenic and inflammatory systems. We conclude that T4N5 and CPDPL show great promise in the treatment of CPD lesions, but the complete clearance of CPDs from the limbus remains a challenge.

Line-scanning SD-OCT for in-vivo, non-contact, volumetric, cellular resolution imaging of the human cornea and limbus

In-vivo, non-contact, volumetric imaging of the cellular and sub-cellular structure of the human cornea and limbus with optical coherence tomography (OCT) is challenging due to involuntary eye motion that introduces both motion artifacts and blur in the OCT images. Here we present the design of a line-scanning (LS) spectral-domain (SD) optical coherence tomography system that combines 2 × 3 × 1.7 µm (x, y, z) resolution in biological tissue with an image acquisition rate of ∼2,500 fps, and demonstrate its ability to image in-vivo and without contact with the tissue surface, the cellular structure of the human anterior segment tissues. Volumetric LS-SD-OCT images acquired over a field-of-view (FOV) of 0.7 mm × 1.4 mm reveal fine morphological details in the healthy human cornea, such as epithelial and endothelial cells, sub-basal nerves, as well as the cellular structure of the limbal crypts, the palisades of Vogt (POVs) and the blood microvasculature of the human limbus. LS-SD-OCT is a promising technology that can assist ophthalmologists with the early diagnostics and optimal treatment planning of ocular diseases affecting the human anterior eye.

UV Protection in the Cornea: Failure and Rescue

Simple Summary

The sun is a deadly laser, and its damaging rays harm exposed tissues such as our skin and eyes. The skin’s protection and repair mechanisms are well understood and utilized in therapeutic approaches while the eye lacks such complete understanding of its defenses and therefore often lacks therapeutic support in most cases. The aim here was to document the similarities and differences between the two tissues as well as understand where current research stands on ocular, particularly corneal, ultraviolet protection. The objective is to identify what mechanisms may be best suited for future investigation and valuable therapeutic approaches.

Abstract

Ultraviolet (UV) irradiation induces DNA lesions in all directly exposed tissues. In the human body, two tissues are chronically exposed to UV: the skin and the cornea. The most frequent UV-induced DNA lesions are cyclobutane pyrimidine dimers (CPDs) that can lead to apoptosis or induce tumorigenesis. Lacking the protective pigmentation of the skin, the transparent cornea is particularly dependent on nucleotide excision repair (NER) to remove UV-induced DNA lesions. The DNA damage response also triggers intracellular autophagy mechanisms to remove damaged material in the cornea; these mechanisms are poorly understood despite their noted involvement in UV-related diseases. Therapeutic solutions involving xenogenic DNA-repair enzymes such as T4 endonuclease V or photolyases exist and are widely distributed for dermatological use. The corneal field lacks a similar set of tools to address DNA-lesions in photovulnerable patients, such as those with genetic disorders or recently transplanted tissue.

Comparative Analysis of Tear Proteins in Keratoconic Scleral Lens Wearers with Variation in Limbal Clearance

SIGNIFICANCE

Cytokine and protease analysis revealed relative changes in the post-lens tear film of scleral lenses with low and high limbal clearances. Results from this study indicate that midperipheral lens fit is an important fitting feature that can impact the inflammatory response of a keratoconic eye.

PURPOSE

The purpose of this study was to investigate changes in levels of inflammatory mediators in the post-lens tear film of keratoconic scleral lens wearers with varying limbal clearance designs.

METHODS

Twenty-two keratoconic eyes were fitted with two sets of scleral lenses that were consistent in lens diameter and central sagittal depth but varied in limbal clearance by approximately 50 μm. Lenses were worn in a randomly assigned order for a 2-week period each. At each follow-up visit, immediately after lens removal, tear samples were collected with a microcapillary tube (10 μL, 0.5 mm in diameter) from the bowl of the inverted scleral lens. Tear cytokine and protease analysis was performed using a multiplex electrochemiluminescent array (Meso Scale Discovery, Rockville, MD) instrument. Levels of interleukins 1, 6, and 8; tumor necrosis factor α; and matrix metalloproteinases 1 and 9 were compared and analyzed.

RESULTS

Levels of interleukin 1β, tumor necrosis factor α, and matrix metalloproteinase 1 increased with high limbal clearance (P = .01, .006, and .02, respectively). No change in interleukins 6 and 8 levels was found (P > .05). A decrease in matrix metalloproteinase 9 was noted in post-lens tear film of scleral lenses with high limbal clearance (P = .10).

DISCUSSION

Relative changes in the cytokine and protease levels were found when comparing low and high limbal clearance, indicating that the midperipheral lens fit is an important feature that can impact the inflammatory response of the keratoconic eye.

The limbus: Structure and function

Limbal function is a key determinant of corneal epithelial integrity. Lineage tracing studies in mice have highlighted that the centripetal movement of epithelial progenitors from the limbus drives both the steady-state maintenance of the corneal epithelium and its regeneration following injury. It is well established that this is facilitated by a population of limbal epithelial stem cells within the limbus. It is becoming increasingly apparent that the behaviour of these stem cells and their ability to respond to the needs of the tissue are closely linked to their immediate microenvironment - the stem cell niche. Increasing understanding of the structural features of this niche and the signalling networks that they coordinate is required to enhance the therapeutic application of these cells in the treatment of limbal stem cell deficiency. Importantly, an improved characterisation of the hierarchy of limbal epithelial progenitors using both new and old putative markers will enable a greater appreciation for the effects of many of these limbal niche factors on stem cell fate.

Relationship between vessel diameter and depth measurements within the limbus using ultra-high resolution optical coherence tomography

PURPOSE

To establish a relationship between the diameter and depth position of vessels in the superior and inferior corneo-scleral limbus using ultra-high resolution optical coherence tomography (UHR-OCT).

METHODS

Volumetric OCT images of the superior and inferior limbus were acquired from 14 healthy subjects with a research-grade UHR-OCT system. Differences in vessel diameter and depth between superior and inferior limbus were analyzed using repeated measured ANOVA in SPSS and R.

RESULTS

The mean (± SD) superior and inferior diameters were 29±18μm and 24±18μm respectively, and the mean (± SD) superior and inferior depths were 177±109μm and 207±132μm respectively. The superior limbal vessels were larger than the inferior ones (RM-ANOVA, p=0.004), and the inferior limbal vessels were deeper than the superior vessels (RM-ANOVA, p=0.041). There was a positive linear association between limbal vessel depth and size within the superior and inferior limbus with Pearson correlation coefficients of 0.803 and 0.754, respectively.

CONCLUSION

This study demonstrated that the UHR-OCT was capable of imaging morphometric characteristics such as the size and depth of vessels in the limbus. The results of this study suggest a difference in the size and depth of vessels across different positions of the limbus, which may be indicative of adaptations to chronic hypoxia caused by the covering of the superior limbus by the upper eyelid. UHR-OCT may be a useful tool to evaluate the effect of contact lenses on the microvascular properties within the limbus.

In-vivo imaging of the palisades of Vogt and the limbal crypts with sub-micrometer axial resolution optical coherence tomography

A research-grade OCT system was used to image in-vivo and without contact with the tissue, the cellular structure and microvasculature of the healthy human corneo-scleral limbus. The OCT system provided 0.95 µm axial and 4 µm (2 µm) lateral resolution in biological tissue depending on the magnification of the imaging objective. Cross-sectional OCT images acquired tangentially from the inferior limbus showed reflective, loop-like features that correspond to the fibrous folds of the palisades of Vogt (POV). The high OCT resolution allowed for visualization of individual cells inside the limbal crypts, capillaries extending from the inside of the POV's fibrous folds and connecting to a lateral grid of micro-vessels located in the connective tissue directly below the POV, as well as reflections from individual red blood cells inside the capillaries. Difference in the reflective properties of the POV was observed among subjects of various pigmentation levels of the POV. Morphological features observed in the high resolution OCT images correlated well with histology. The ability to visualize the limbal morphology and microvasculature in-vivo at cellular level can aid the diagnostics and treatment of limbal stem cell dysfunction and dystrophies.

Two-Photon Microscopy of the Mouse Peripheral Cornea Ex Vivo

PURPOSE

To investigate the 3-dimensional (3D) cell and extracellular matrix (ECM) structure of mouse peripheral corneas in normal and corneal neovascularization tissues using 2-photon microscopy (TPM) based on both intrinsic and extrinsic moxifloxacin contrasts.

METHODS

Peripheral corneas in freshly enucleated mouse eyes were imaged by TPM based on both intrinsic and extrinsic contrasts. Intrinsic autofluorescence and second harmonic generation were used to image cells and ECM collagen, respectively. Moxifloxacin ophthalmic solution was applied to image cells. The peripheral cornea, limbus, and sclera were imaged in 3D. In addition to normal mice, mouse models of suture-induced corneal neovascularization were imaged to visualize changes in the microstructure.

RESULTS

Complex 3D cell and ECM structures in the cornea, limbus, and sclera were visualized by TPM. TPM images based on intrinsic contrasts visualized both cell and ECM structures, and TPM images based on moxifloxacin visualized cell structures with enhanced contrast. On the limbus side of the mouse peripheral cornea, TPM images visualized the vasculature in the limbus, the trabecular meshwork/Schlemm canal, iris, and ciliary body. On the scleral side, TPM images visualized cell and ECM structures in the sclera and multiple cell layers below the sclera. TPM images of the peripheral cornea in the corneal neovascularization condition visualized the extension of vasculature from the limbus to the cornea.

CONCLUSIONS

TPM imaging based on both intrinsic and external moxifloxacin contrasts visualized detailed 3D cell and ECM microstructures in the mouse peripheral cornea. TPM based on moxifloxacin might be advantageous for studying cell structures by enhancing image contrast.

An integrated approach to quantitative modelling in angiogenesis research

Angiogenesis, the process by which new vessels form from existing ones, plays an important role in many developmental processes and pathological conditions. We study angiogenesis in the context of a highly controllable experimental environment: the cornea micropocket assay. Using a multidisciplinary approach that combines experiments, image processing and analysis, and mathematical modelling, we aim to provide mechanistic insight into the action of two angiogenic factors, vascular endothelial growth factor A (VEGF-A) and basic fibroblast growth factor (bFGF). We use image analysis techniques to extract quantitative data, which are both spatially and temporally resolved, from experimental images, and we develop a mathematical model, in which the corneal vasculature evolves in response to both VEGF-A and bFGF. The experimental data are used for model parametrization, while the mathematical model is used to assess the utility of the cornea micropocket assay and to characterize proposed synergies between VEGF-A and bFGF.

In vivo microstructural and microvascular imaging of the human corneo-scleral limbus using optical coherence tomography

The corneo-scleral limbus contains several biological components, which are important constituents for understanding, diagnosing and managing several ocular pathologies, such as glaucoma and corneal abnormalities. An anterior segment optical coherence tomography (AS-OCT) system integrated with optical microangiography (OMAG) is used in this study to non-invasively visualize the three-dimensional microstructural and microvascular properties of the limbal region. Advantages include first the ability to correct optical distortion of microstructural images enabling quantification of relationships in the anterior chamber angle. Second, microvascular images enable the visualization of the microcirculation in the limbal area without the use of exogenous contrast agents. Third, by combining the microstructural and microvascular information, the aqueous outflow pathway can be identified. The proposed AS-OCT can serve as a useful tool for ophthalmological research to determine normal and pathologic changes in the outflow system. As a clinical tool it has the potential to detect early aqueous outflow system abnormalities that lead to the pressure elevation in glaucoma. Recent surgical innovations and their implementations also rely on an assessment of outflow system structure and function, which can be revealed by AS-OCT.

In vivo volumetric imaging of the human corneo-scleral limbus with spectral domain OCT

The limbus is the structurally rich transitional region of tissue between the cornea on one side, and the sclera and conjunctiva on the other. This zone, among other things, contains nerves passing to the cornea, blood and lymph vasculature for oxygen and nutrient delivery and for waste, CO(2) removal and drainage of the aqueous humour. In addition, the limbus contains stem cells responsible for the existence and healing of the corneal epithelium. Here we present 3D images of the healthy human limbus, acquired in vivo with a spectral domain optical coherence tomography system operating at 1060nm. Cross-sectional and volumetric images were acquired from temporal and nasal locations in the human limbus with ~3µm x 18µm (axial x lateral) resolution in biological tissue at the rate of 92,000 A-scans/s. The imaging enabled detailed mapping of the corneo-scleral tissue morphology, and visualization of structural details such as the Vogt palisades, the blood and lymph vasculature including the Schlemm's canal and the trabecular meshwork, as well as corneal nerve fiber bundles. Non-invasive, volumetric, high resolution imaging reveals fine details of the normal human limbal structure, and promises to provide invaluable information about its changes in health and disease as well as during and after corneal surgery.

Impact of age, systemic glucocorticoids, and progressive knee arthritis on specific mRNA levels in different areas of the rabbit cornea

PURPOSE

To determine the influence of factors such as age, osteoarthritis (OA), and glucocorticoid treatment on total RNA and mRNA regulation in the cornea and how these factors differ between prepupillary and peripheral areas of the cornea.

METHODS

Molecular analyses of corneal tissue were performed using rabbits of different age groups and skeletally mature animals that had undergone anterior cruciate ligament (ACL) transection, an established model of knee OA. Systemic glucocorticoids were administered to cohorts of the osteoarthritic and control animals to determine the influence of distal joint disease on the corneal response. Corneal tissue was analyzed for changes in mRNA levels for several relevant genes: collagen I, collagen III, collagen V, decorin core protein, cyclooxygenase-2 (COX-2), glucocorticoid receptor, and the housekeeping gene beta-actin.

RESULTS

The corneal tissue was found to have diminishing total RNA with age, which is consistent with previous studies in the literature. Interestingly, in skeletally mature animals, distal joint OA was found to affect corneal mRNA levels for several important structural and inflammatory genes (collagen I, decorin core protein, and COX-2) in a manner that progressed with OA progression. Although systemic glucocorticoid treatment did not alter mRNA levels in the normal cornea, it did counteract the changes observed early after OA induction (3 weeks) while having less of an effect in later, more established arthritis (6 weeks).

CONCLUSIONS

This study reveals that distal joint OA can affect mRNA levels for several structural and inflammatory genes of the cornea, changes that seem to be suppressed by systemic glucocorticoid treatment. These findings indicate that OA has associated systemic factors that influence corneal cell metabolism.

Impact of Manufacturing Technology and Material Composition on the Clinical Performance of Hydrogel Lenses

PURPOSE

To establish the clinical impact of three different methods of manufacture used to produce soft contact lenses.

METHODS

Clinical performance of five lens types was investigated by undertaking a prospective, double-masked, randomized, crossover study. Three of the lenses were made from poly(hydroxyethyl methacrylate) (pHEMA) by three different manufacturing processes (lathing, spin casting, and cast molding), and the remaining two lenses were cast molded from different materials-hydroxyethyl methacrylate/methacrylic acid and hydroxyethyl methacrylate/glycerol methacrylate (HEMA/GMA). All lenses were specially fabricated for this work at the same manufacturing plant. Thirty-four soft contact lens wearers wore each lens for 1 month on a daily-wear basis. Several clinical variables, such as ocular response, visual acuity, lens fitting, prelens tear film, lens surface dehydration, subjective response, and protein deposition, were measured.

RESULTS

In general, the spun-cast pHEMA lens performed inferiorly compared with the other pHEMA lenses. This lens induced significantly more limbal and conjunctival hyperemia than the cast-molded lens and provided poorer low contrast visual acuity (LCVA) than the other two lenses. It dehydrated more and had the least on-eye movement. However, the spun-cast lens deposited the least protein of the pHEMA lenses. In general, the HEMA/GMA lens performed inferiorly compared with the other cast-molded lenses. LCVA was worse with this lens, and subjective responses showed that this lens was thought to give the worst visual performance of the cast-molded lenses. It was also thought to be the most difficult lens to handle. Significantly more breakages occurred with this lens than any other.

CONCLUSIONS

Overall, this work has shown that manufacturing method and material composition have a fundamental effect on many clinical properties of a lens. Therefore, method of manufacture is also an important consideration in the overall production of a soft lens.