Models for Meibomian gland dysfunction: In vivo and in vitro

Meibomian gland dysfunction (MGD) is a chronic abnormality of the Meibomian glands (MGs) that is recognized as the leading cause of evaporative dry eye worldwide. Despite its prevalence, however, the pathophysiology of MGD remains elusive, and effective disease management continues to be a challenge. In the past 50 years, different models have been developed to illustrate the pathophysiological nature of MGD and the underlying disease mechanisms. An understanding of these models is crucial if researchers are to select an appropriate model to address specific questions related to MGD and to develop new treatments. Here, we summarize the various models of MGD, discuss their applications and limitations, and provide perspectives for future studies in the field.

Cell-Cell and Cell-Matrix Interactions at the Presumptive Stem Cell Niche of the Chick Corneal Limbus

(1) Background: Owing to its ready availability and ease of acquisition, developing chick corneal tissue has long been used for research purposes. Here, we seek to ascertain the three-dimensional microanatomy and spatiotemporal interrelationships of the cells (epithelial and stromal), extracellular matrix, and vasculature at the corneo-scleral limbus as the site of the corneal stem cell niche of the chicken eye. (2) Methods: The limbus of developing (i.e., embryonic days (E) 16 and 18, just prior to hatch) and mature chicken eyes was imaged using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the volume electron microscopy technique, serial-block face SEM (SBF-SEM), the latter technique allowing us to generate three-dimensional reconstructions from data sets of up to 1000 serial images; (3) Results: Data revealed that miniature limbal undulations of the embryonic basement membrane, akin to Palisades of Vogt (PoV), matured into distinct invaginations of epithelial cells that extended proximally into a vascularized limbal stroma. Basal limbal epithelial cells, moreover, occasionally exhibited a high nuclear:cytoplasmic ratio, which is a characteristic feature of stem cells. SBF-SEM identified direct cell-cell associations between corneal epithelial and stromal cells at the base of structures akin to limbal crypts (LCs), with cord-like projections of extracellular matrix extending from the basal epithelial lamina into the subjacent stroma, where they made direct contact with stomal cells in the immature limbus. (4) Conclusion: Similarities with human tissue suggest that the corneal limbus of the mature chicken eye is likely the site of a corneal stem cell niche. The ability to study embryonic corneas pre-hatch, where we see characteristic niche-like features emerge, thus provides an opportunity to chart the development of the limbal stem cell niche of the cornea.

Developmental Changes in Patterns of Distribution of Fibronectin and Tenascin-C in the Chicken Cornea: Evidence for Distinct and Independent Functions during Corneal Development and Morphogenesis

The cornea forms the tough and transparent anterior part of the eye and by accurate shaping forms the major refractive element for vision. Its largest component is the stroma, a dense collagenous connective tissue positioned between the epithelium and the endothelium. In chicken embryos, the stroma initially develops as the primary stroma secreted by the epithelium, which is then invaded by migratory neural crest cells. These cells secrete an organised multi-lamellar collagenous extracellular matrix (ECM), becoming keratocytes. Within individual lamellae, collagen fibrils are parallel and orientated approximately orthogonally in adjacent lamellae. In addition to collagens and associated small proteoglycans, the ECM contains the multifunctional adhesive glycoproteins fibronectin and tenascin-C. We show in embryonic chicken corneas that fibronectin is present but is essentially unstructured in the primary stroma before cell migration and develops as strands linking migrating cells as they enter, maintaining their relative positions as they populate the stroma. Fibronectin also becomes prominent in the epithelial basement membrane, from which fibronectin strings penetrate into the stromal lamellar ECM at right angles. These are present throughout embryonic development but are absent in adults. Stromal cells associate with the strings. Since the epithelial basement membrane is the anterior stromal boundary, strings may be used by stromal cells to determine their relative anterior-posterior positions. Tenascin-C is organised differently, initially as an amorphous layer above the endothelium and subsequently extending anteriorly and organising into a 3D mesh when the stromal cells arrive, enclosing them. It continues to shift anteriorly in development, disappearing posteriorly, and finally becoming prominent in Bowman's layer beneath the epithelium. The similarity of tenascin-C and collagen organisation suggests that it may link cells to collagen, allowing cells to control and organise the developing ECM architecture. Fibronectin and tenascin-C have complementary roles in cell migration, with the former being adhesive and the latter being antiadhesive and able to displace cells from their adhesion to fibronectin. Thus, in addition to the potential for associations between cells and the ECM, the two could be involved in controlling migration and adhesion and subsequent keratocyte differentiation. Despite the similarities in structure and binding capabilities of the two glycoproteins and the fact that they occupy similar regions of the developing stroma, there is little colocalisation, demonstrating their distinctive roles.

Clinical Trial of Autologous Cultivated Limbal Epithelial Cell Sheet Transplantation for Patients with Limbal Stem Cell Deficiency

OBJECTIVE OR PURPOSE

To confirm the efficacy and safety of Good Manufacturing Practice (GMP)-compliant autologous cultivated limbal epithelial cell sheets in government-controlled clinical trials that adhered to Good Clinical Practice stipulations for patients with unilateral limbal stem cell deficiency (LSCD).

DESIGN

A prospective, multi-center, open-label, uncontrolled, single-arm clinical trial.

SUBJECTS, PARTICIPANTS OR CONTROLS

Ten consecutive eyes of 10 patients with unilateral LSCD were followed for two years after surgery. Preoperative LSCD stage was IIB in four eyes and III in six eyes.

METHODS

A limbal tissue biopsy was obtained from the healthy eye, after which limbal stem cells were dissociated and cultivated on temperature-responsive culture surfaces. All cell sheets were fabricated in a GMP-grade facility under established standard operating procedures. Cell sheets were evaluated using defined shipment criteria before transplantation, and only those that met the criteria were used. The cell sheet was transplanted onto each of the patients' diseased eye after removing the conjunctival scar tissue that covered the corneal surface. The severity of LSCD was determined according to a staging method agreed upon by global consensus, with eyes evaluated as being in stages IA-C representing successful corneal epithelial reconstruction. LSCD diagnosis and staging were determined by the trial's Eligibility Judgment Committee and Effect Assessment Committee using slit-lamp photographs including fluorescein staining. Both committees comprised two or three third-party cornea specialists, who were provided with information anonymously and randomly.

MAIN OUTCOME MEASURE

Corneal epithelial reconstruction rate was the primary endpoint.

RESULTS

Corneal epithelial reconstruction was successful in six of 10 eyes (60%) one year postoperatively and was significantly higher than the 15% clinically significant efficacy rate achieved by allogeneic limbal transplantation. The reconstruction rate was 70% of eyes two years postoperatively. Additionally, improvements in visual acuity were noted in 50% and 60% of eyes at one and two years, respectively. No clinically significant transplantation-related adverse events were observed.

CONCLUSION

The efficacy and safety of cultivated limbal epithelial cell sheet transplantation were thus confirmed, and the cell sheet, named Nepic, is now approved as a Cellular and Tissue-Based Product in Japan.

Chondroitin Sulphate/Dermatan Sulphate Proteoglycans: Potential Regulators of Corneal Stem/Progenitor Cell Phenotype In Vitro

Chondroitin sulphate (CS) proteoglycans with variable sulphation-motifs along their glycosaminoglycan (GAG) chains are closely associated with the stem cell niche of articular cartilage, where they are believed to influence the characteristics of the resident stem cells. Here, we investigated the immunohistochemical distribution of hybrid CS/dermatan sulphate (DS) GAGs in the periphery of the adult chicken cornea, which is the location of the cornea's stem cell niche in a number of species, using a monoclonal antibody, 6C3, that recognises a sulphation motif-specific CS/DS GAG epitope. This revealed positive labelling that was restricted to the subepithelial corneal stroma, as well as nearby bony structures within the sclera, called ossicles. When cultivated on cell culture dishes coated with 6C3-rich CS/DS, corneal stromal cells (keratocytes) that had been isolated from embryonic chicken corneas formed circular colonies, which took several days to reach confluency. A flow cytometric analysis of these keratocytes revealed changes in their expression levels of the indicative stem cell markers, Connexin 43 (Cx43), Paired Box 6 (PAX6), B-lymphoma Moloney murine leukemia virus insertion region-1 (Bmi-1), and C-X-C Chemokine Receptor 4 (CXCR4) suggestive of a less-differentiated phenotype compared with expression levels in cells not exposed to CS/DS. These findings support the view that CS/DS promotes the retention of a stem cell phenotype in corneal cells, much as it has been proposed to do in other connective tissues.

Generation of 3D lacrimal gland organoids from human pluripotent stem cells

Lacrimal glands are the main exocrine glands of the eyes. Situated within the orbit, behind the upper eyelid and towards the temporal side of each eye, they secrete lacrimal fluid as a major component of the tear film. Here we identify cells with characteristics of lacrimal gland primordia that emerge in two-dimensional eye-like organoids cultured from human pluripotent stem cells¹. When isolated by cell sorting and grown under defined conditions, the cells form a three-dimensional lacrimal-gland-like tissue organoid with ducts and acini, enabled by budding and branching. Clonal colony analyses indicate that the organoids originate from multipotent ocular surface epithelial stem cells. The organoids exhibit notable similarities to native lacrimal glands on the basis of their morphology, immunolabelling characteristics and gene expression patterns, and undergo functional maturation when transplanted adjacent to the eyes of recipient rats, developing lumina and producing tear-film proteins.

Obstructive Sleep Apnea Affects Lacrimal Gland Function

Purpose

To determine the effect of obstructive sleep apnea syndrome (OSA) on lacrimal gland function and its mechanism.

Methods

Male mice aged seven to eight weeks were housed in cages with cyclic intermittent hypoxia to mimic OSA, and the control group was kept in a normal environment. Slit-lamp observation, fluorescein staining, and corneal sensitivity detection are used to assess cornea changes. Tear secretion was detected by phenol red cotton thread, and the pathological changes of lacrimal gland were observed by hematoxylin and eosin staining, oil red O staining, cholesterol and triglyceride kits, immunofluorescence staining, immunohistochemical staining, real-time polymerase chain reaction, transmission electron microscopy, and Western blot.

Results

Studies revealed a decreased tear secretion, corneal epithelial defects and corneal hypersensitivity. Myoepithelial cell damage, abnormal lipid accumulation, reduced cell proliferation, increased apoptosis and inflammatory cell infiltration in the lacrimal gland were also seen. Hifα and NF-κB signaling pathways, moreover, were activated, while Pparα was downregulated, in the lacrimal glands of OSA mice. Fenofibrate treatment significantly alleviated pathological changes of the lacrimal gland induced by OSA.

Conclusion

OSA disturbs the Hifα/Pparα/NF-κB signaling axis, which affects lacrimal gland structure and function and induces dry eye.

Diagnosis of Choroidal Disease With Deep Learning-Based Image Enhancement and Volumetric Quantification of Optical Coherence Tomography

Purpose

The purpose of this study was to quantify choroidal vessels (CVs) in pathological eyes in three dimensions (3D) using optical coherence tomography (OCT) and a deep-learning analysis.

Methods

A single-center retrospective study including 34 eyes of 34 patients (7 women and 27 men) with treatment-naïve central serous chorioretinopathy (CSC) and 33 eyes of 17 patients (7 women and 10 men) with Vogt-Koyanagi-Harada disease (VKH) or sympathetic ophthalmitis (SO) were imaged consecutively between October 2012 and May 2019 with a swept source OCT. Seventy-seven eyes of 39 age-matched volunteers (26 women and 13 men) with no sign of ocular pathology were imaged for comparison. Deep-learning-based image enhancement pipeline enabled CV segmentation and visualization in 3D, after which quantitative vessel volume maps were acquired to compare normal and diseased eyes and to track the clinical course of eyes in the disease group. Region-based vessel volumes and vessel indices were utilized for disease diagnosis.

Results

OCT-based CV volume maps disclose regional CV changes in patients with CSC, VKH, or SO. Three metrics, (i) choroidal volume, (ii) CV volume, and (iii) CV index, exhibit high sensitivity and specificity in discriminating pathological choroids from healthy ones.

Conclusions

The deep-learning analysis of OCT images described here provides a 3D visualization of the choroid, and allows quantification of features in the datasets to identify choroidal disease and distinguish between different diseases.

Translational Relevance

This novel analysis can be applied retrospectively to existing OCT datasets, and it represents a significant advance toward the automated diagnosis of choroidal pathologies based on observations and quantifications of the vasculature.

Mitochondrial DNA as a Biomarker for Acute Central Serous Chorioretinopathy: A Case-Control Study

Background

The literature suggests that stress may play a pivotal role in the precipitation of acute central serous chorioretinopathy (CSC) because chorioretinal integrity can be affected by the psychosocial state of the patient, indicating the need for a biomarker. Not only physical stress but also psychological stress causes many types of physical disorders. However, little is known about the pathophysiology of stress-induced disease. The objective of this study was to investigate whether serum factors might be involved in the development of stress-induced ocular diseases.

Methods

This observational case series included 33 eyes of 33 consecutive patients with treatment-naïve acute CSC. Fifty eyes of 50 age-matched healthy volunteers were included in this study as non-CSC controls. Serum samples were collected from all participants, and the levels of mitochondrial DNA (mtDNA) were measured by quantitative real-time (RT)-PCR. Serum levels of high-mobility group box (HMGB) 1 and 8-hydroxy-2'-deoxyguanosine (8-OHdG), biological markers of acute/chronic inflammation and oxidative stress, were also measured. The relationships between serum mtDNA, 8-OHdG, and HMGB1 concentrations were investigated by multivariate regression analysis, alongside an assessment of clinical data.

Results

In the treatment-naïve acute CSC group, the serum mtDNA levels (36.5 ± 32.4 ng/mL) were significantly higher than the levels in the control group (7.4 ± 5.9 ng/mL; p < 0.001). Serum levels of 8-OHdG and HMGB1 in treatment-naïve acute CSC patients measured 0.12 ± 0.08 ng/mL and 18.1 ± 35.0 ng/mL, respectively, indicating that HMGB1 levels were elevated in CSC compared with the control group. Multivariable regression analysis demonstrated that increased serum mtDNA levels were significantly associated with the height of serous retinal detachment.

Conclusion

We showed serum mtDNA and HMGB1 level elevation and its relation to the clinical activities of CSC, indicating that serum mtDNA and HMGB1 could serve as biomarkers for the acute phase of the disease. The use of these biomarkers makes it possible to predict disease onset and determine disease severity.

Barium-induced toxic anterior segment syndrome

Toxic anterior segment syndrome (TASS) is a rapid-onset inflammation of the eye following uneventful ocular surgery. We report a case of TASS following Baerveldt glaucoma implant (BGI) surgery. Inductively coupled plasma-mass spectrometry (ICP-MS) identified barium in the eye and in the eluate from the bleb of the BGI. We attribute TASS in our patient to the dissolution of barium from the BGI and its entry into the eye, where it causes severe inflammation.

PAX6-positive microglia evolve locally in hiPSC-derived ocular organoids

Microglia are the resident immune cells of the central nervous system (CNS). They govern the immunogenicity of the retina, which is considered to be part of the CNS; however, it is not known how microglia develop in the eye. Here, we studied human-induced pluripotent stem cells (hiPSCs) that had been expanded into a self-formed ectodermal autonomous multi-zone (SEAM) of cells that partially mimics human eye development. Our results indicated that microglia-like cells, which have characteristics of yolk-sac-like linage cells, naturally develop in 2D eye-like SEAM organoids, which lack any vascular components. These cells are unique in that they are paired box protein 6 (PAX6)-positive, yet they possess some characteristics of mesoderm. Collectively, the data support the notion of the existence of an isolated, locally developing immune system in the eye, which is independent of the body’s vasculature and general immune system.

•

Eye-like organoids from hiPSCs contained immune cells similar to microglia cells

•

RNA-sequencing shows they have specific characteristic of microglia

•

Single-cell analyses indicate they resemble yol-sac-derived myeloid progenitors

•

These cells are unique in that they are PAX6- (eye master gene) positive immune cells

Response to Letter to Editor "Comments on 'Cell regulation of collagen fibril macrostructure during corneal morphogenesis' by Koudouna et al."

STATEMENT OF SIGNIFICANCE:  .

Contrast-Enhanced Tissue Processing of Fibrillin-Rich Elastic Fibres for 3D Visualization by Volume Scanning Electron Microscopy

Elastic fibres constitute an important component of the extracellular matrix and currently are the subject of intensive study in order to elucidate their assembly, function and involvement in cell-matrix interactions and disease. However, few studies to date have investigated the 3D architecture of the elastic fibre system in bulk tissue. We describe a protocol for preparation of tissue samples, including primary fixation and backscatter electron contrast-enhancement steps, through dehydration into stable resin-embedded blocks for volume electron microscopy. The use of low molecular weight tannic acid and alcoholic lead staining are critical stages in this procedure. Block preparation by ultramicrotomy and evaporative metal coating prior to microscopical examination are also described. We present images acquired from serial block face scanning electron microscopy of cornea and aorta showing target structures clearly differentiated from cells and other matrix components. The processing method imparts high contrast to fibrillin-containing elastic fibres, thus facilitating their segmentation and rendering into 3D reconstructions by image analysis software from large serial image datasets.

Human iPS cells engender corneal epithelial stem cells with holoclone-forming capabilities

Human induced pluripotent stem cells (hiPSCs) can generate a multiplicity of organoids, yet no compelling evidence currently exists as to whether or not these contain tissue-specific, holoclone-forming stem cells. Here, we show that a subpopulation of cells in a hiPSC-derived corneal epithelial cell sheet is positive for ABCB5 (ATP-binding cassette, sub-family B, member 5), a functional marker of adult corneal epithelial stem cells. These cells possess remarkable holoclone-forming capabilities, which can be suppressed by an antibody-mediated ABCB5 blockade. The cell sheets are generated from ABCB5⁺ hiPSCs that first emerge in 2D eye-like organoids around six weeks of differentiation and display corneal epithelial immunostaining characteristics and gene expression patterns, including sustained expression of ABCB5. The findings highlight the translational potential of ABCB5-enriched, hiPSC-derived corneal epithelial cell sheets to recover vision in stem cell-deficient human eyes and represent the first report of holoclone-forming stem cells being directly identified in an hiPSC-derived organoid.

•

Human iPS cell-derived corneal epithelia contain ABCB5-positive stem cells

•

The ABCB5-positive cells possess holoclone-forming capabilities

•

An antibody-mediated ABCB5 blockade suppresses holoclone formation

•

Holoclone-forming stem cells are present in a human iPS cell-derived tissue construct

Generation of functional conjunctival epithelium, including goblet cells, from human iPSCs

The conjunctival epithelium, which covers the sclera (the white of the eye) and lines the inside of the eyelids, is essential for mucin secretion and the establishment of a healthy tear film. Here, we describe human conjunctival development in a self-formed ectodermal autonomous multi-zone (SEAM) of cells that were derived from human-induced pluripotent stem cells (hiPSCs) and mimic whole-eye development. Our data indicate that epidermal growth factor (EGF) drives the generation of cells with a conjunctival epithelial lineage. We also show that individual conjunctival cells can be sorted and reconstituted by cultivation into a functional conjunctival epithelium that includes mucin-producing goblet cells. Keratinocyte growth factor (KGF), moreover, is necessary for the maturation of hiPSC-derived conjunctival epithelium-particularly the goblet cells-indicating key complementary roles of EGF and KGF in directing the differentiation and maturation, respectively, of the human conjunctival epithelium.

Chondroitin Sulfate as a Potential Modulator of the Stem Cell Niche in Cornea

Chondroitin sulfate (CS) is an important component of the extracellular matrix in multiple biological tissues. In cornea, the CS glycosaminoglycan (GAG) exists in hybrid form, whereby some of the repeating disaccharides are dermatan sulfate (DS). These CS/DS GAGs in cornea, through their presence on the proteoglycans, decorin and biglycan, help control collagen fibrillogenesis and organization. CS also acts as a regulatory ligand for a spectrum of signaling molecules, including morphogens, cytokines, chemokines, and enzymes during corneal growth and development. There is a growing body of evidence that precise expression of CS or CS/DS with specific sulfation motifs helps define the local extracellular compartment that contributes to maintenance of the stem cell phenotype. Indeed, recent evidence shows that CS sulfation motifs recognized by antibodies 4C3, 7D4, and 3B3 identify stem cell populations and their niches, along with activated progenitor cells and transitional areas of tissue development in the fetal human elbow. Various sulfation motifs identified by some CS antibodies are also specifically located in the limbal region at the edge of the mature cornea, which is widely accepted to represent the corneal epithelial stem cell niche. Emerging data also implicate developmental changes in the distribution of CS during corneal morphogenesis. This article will reflect upon the potential roles of CS and CS/DS in maintenance of the stem cell niche in cornea, and will contemplate the possible involvement of CS in the generation of eye-like tissues from human iPS (induced pluripotent stem) cells.

The generation of fluorometholone nanocrystal eye drops, their metabolization to dihydrofluorometholone and penetration into rabbit eyes

Fluorometholone is a widely used anti-inflammatory ophthalmic formulation, which elicits a lower ocular hypertensive response than other glucocorticoid medications. This serves to mitigate against the risk of steroid-induced glaucoma. Based on the hypothesis that an improved corneal permeability can increase the bioavailability of a drug, we sought to obtain fluorometholone in suspension with a small particle size. Accordingly, we describe the formulation of fluorometholone nanocrystal eye drops, which have a mean particle size of 201.2 ± 14.1 nm (standard deviation (s.d.)) when measured by dynamic light scattering. Scanning electron microscopy further indicates that fluorometholone nanocrystals are predominantly rectangular in shape. Fluorometholone microcrystals, on the other hand, with a mean particle size of 9.24 ± 4.51 µm (s.d.), tend to have a rod-like morphology. Powder x-ray diffraction revealed that fluorometholone microcrystal and nanocrystal formulations have the same crystal structure, with the main diffraction peaks at 2θ = 10.4 and 15.3°. The nanocrystal formulation was found to be stable, long-term, when stored at 10 °C for up to 6-months. High pressure liquid chromatography (HPLC) of the aqueous humor of rabbit eyes 15-240 mins after the in vivo application of fluorometholone eye drops to the ocular surface revealed that the molecule had been converted to 20α-dihydrofluorometholone (with no evidence of a 20β-dihydrofluorometholone fraction), and that penetration was 2-6 fold higher and longer lasting with the nanocrystal, rather than the microcrystal, formulation. In current study we show how newly generated fluorometholone nanocrystals when administered as eye drops enter the anterior chamber of the eye and become metabolized to dihydrofluorometholone.

High-Fat Diet-Induced Functional and Pathologic Changes in Lacrimal Gland

The lacrimal gland is critical for maintaining the homeostasis of the ocular surface microenvironment through secreting aqueous tears in mammals. Many systemic diseases such as Sjögren syndrome, rheumatoid arthritis, and diabetes can alter the lacrimal gland function, eventually resulting in aqueous tear-deficient dry eye. Here, a high-fat diet (HFD) experimental mouse model was used to clarify how hyperlipidemia affects lacrimal gland function. Aqueous tear secretion fell about 50% after 1 month on a HFD. Lipid droplets accumulated in the matrix and acinar cells of the lacrimal gland after this period, along with changes in the lipid metabolism, changes in gene expression levels, and disruption of fatty acid oxidative activity. Immune cell infiltration and rises in the gene expression levels of the inflammation-related cytokines Il1β, Tnfα, Tsg6, Il10, Mmp2, and Mmp9 were found. HFD also induced mitochondrial hypermegasoma, increased apoptosis, and decreased lacrimal gland acinar cell proliferation. Replacement of the HFD with the standard diet partially reversed pathologic changes in the lacrimal gland. Similarly, supplementing the HFD with fenofibrate also partially reversed the inhibited tear secretion and reduced lipid accumulation, inflammation, and oxidative stress levels. The authors conclude that a HFD induces pathophysiological changes and functional decompensation of the lacrimal gland. Therefore, ingestion of a HFD may be a causative factor of dry eye disease.

Recapitulation of normal collagen architecture in embryonic wounded corneas

Wound healing is characterized by cell and extracellular matrix changes mediating cell migration, fibrosis, remodeling and regeneration. We previously demonstrated that chick fetal wound healing shows a regenerative phenotype regarding the cellular and molecular organization of the cornea. However, the chick corneal stromal structure is remarkably complex in the collagen fiber/lamellar organization, involving branching and anastomosing of collagen bundles. It is unknown whether the chick fetal wound healing is capable of recapitulating this developmentally regulated organization pattern. The purpose of this study was to examine the three-dimensional collagen architecture of wounded embryonic corneas, whilst identifying temporal and spatial changes in collagen organization during wound healing. Linear corneal wounds that traversed the epithelial layer, Bowman´s layer, and anterior stroma were generated in chick corneas on embryonic day 7. Irregular thin collagen fibers are present in the wounded cornea during the early phases of wound healing. As wound healing progresses, the collagen organization dramatically changes, acquiring an orthogonal arrangement. Fourier transform analysis affirmed this observation and revealed that adjacent collagen lamellae display an angular displacement progressing from the epithelium layer towards the endothelium. These data indicate that the collagen organization of the wounded embryonic cornea recapitulate the native macrostructure.

Assessment of a self-assembling peptide gel, SPG-178, in providing a clear operative field for trabeculectomy surgery for glaucoma in an animal model

The presence of blood during ophthalmic surgery is problematic, as it can obstruct a surgeon’s view of the operative field. This is particularly true when performing trabeculectomy surgery to enhance ocular fluid outflow and reduce intraocular pressure as a treatment for glaucoma, one of the most common vision loss conditions worldwide. In this study, we investigated the performance of a transparent, self-assembling peptide gel (SPG-178) and its ability to maintain visibility during trabeculectomy surgery. Unlike the hyaluronic acid gel commonly used in ophthalmic surgery, SPG-178 did not permit the ingress of blood into the gel itself. Rather, it forced blood to flow peripherally to the gel. Moreover, if bleeding occurred under the SPG-178 gel, perfusion with saline was able to effectively flush the blood away along the interface between the SPG-178 and the ocular tissue (in this case scleral) to clear the surgical field of view. In experimental trabeculectomy surgeries with mitomycin C used as an adjuvant, there were no differences in the postoperative recovery of intraocular pressure or bleb morphology with or without the use of SPG-178. SPG-178, therefore, when used in a gel formulation, represents a new material for use in intraocular surgery to ensure a clear operative field with likely beneficial treatment outcomes.

Ocular surface ectoderm instigated by WNT inhibition and BMP4

We sought to elucidate how and when the ocular surface ectoderm commits to its differentiation into the corneal epithelium in eye development from human induced pluripotent stem cells (hiPSCs) under the influence of WNT signaling and the actions of BMP4. These signals are key drivers ocular surface ectodermal cell fate determination. It was discovered that secreted frizzled related protein-2 (SFRP2) and Dickkopf1 (DKK1), which are expressed in neural ectoderm, are both influential in the differentiation of hiPSCs, where they act as canonical WNT antagonists. BMP4, moreover, was found to simultaneously initiate non-neural ectodermal differentiation into a corneal epithelial lineage. Combined treatment of hiPSCs with exogenous BMP4 aligned to WNT inhibition for the initial four days of differentiation increased the ocular surface ectodermal cell population and induced a corneal epithelial phenotype. Specification of a surface ectodermal lineage and its fate is thus determined by a fine balance of BMP4 exposure and WNT inhibition in the very earliest stages of human eye development.

Cell jamming, stratification and p63 expression in cultivated human corneal epithelial cell sheets

Corneal limbal epithelial stem cell transplantation using cultivated human corneal epithelial cell sheets has been used successfully to treat limbal stem cell deficiencies. Here we report an investigation into the quality of cultivated human corneal epithelial cell sheets using time-lapse imaging of the cell culture process every 20 minutes over 14 days to ascertain the level of cell jamming, a phenomenon in which cells become smaller, more rounded and less actively expansive. In parallel, we also assessed the expression of p63, an important corneal epithelial stem cell marker. The occurrence of cell jamming was variable and transient, but was invariably associated with a thickening and stratification of the cell sheet. p63 was present in all expanding cell sheets in the first 9 days of culture, but it's presence did not always correlate with stratification of the cell sheet. Nor did p63 expression necessarily persist in stratified cell sheets. An assessment of cell jamming, therefore, can shed significant light on the quality and regenerative potential of cultivated human corneal epithelial cell sheets.

In Vitro Topical Delivery of Chlorhexidine to the Cornea: Enhancement Using Drug-Loaded Contact Lenses and β-Cyclodextrin Complexation, and the Importance of Simulating Tear Irrigation

Microbial keratitis is a severe, sight-threatening condition caused by various pathogens. Eyedrops are the standard delivery modality for treating these disorders; however, blinking reflex, elevated tear production, and nasolacrimal drainage eliminate much of the instilled dose within a few seconds. Therefore, eyedrops must be applied repeatedly for prolonged periods. The present study aimed to probe more effective ocular delivery of chlorhexidine based upon drug-loaded hydrogel contact lenses and β-cyclodextrin (β-CD), while also determining the effect of constant irrigation with simulated tear fluid (STF) in in vitro experiments. Chlorhexidine digluconate (as 0.2 and 2% solutions, β-CD inclusion complexes, and loaded hydrogel contact lenses) were applied to enucleated porcine eyes as single or multiple 10 μL doses, or as drug-loaded contact lenses, with and without β-CD. The corneas were then excised and drug-extracted quantified by high-performance liquid chromatography (HPLC). The effect of constant irrigation by STF was evaluated to test the effect of increased tear production on corneal delivery. Potential antimicrobial activity of the delivered drug was also assessed. Results showed that drug-loaded contact lenses delivered the greatest amount of chlorhexidine into the cornea over a 24 h period, while the eyedrop solution comparator delivered the least. The β-CD significantly enhanced chlorhexidine delivery to the cornea from eyedrop solution, although contact lenses loaded with chlorhexidine-β-CD failed to enhance delivery. β-CD within the hydrogel matrix impeded drug release. Constant irrigation with STF significantly reduced the amount of drug delivered to the cornea in all cases. Chlorhexidine retained antimicrobial activity in all delivery methods. Hydrogel contact lenses loaded with chlorhexidine delivered significantly higher levels to the cornea compared to eyedrops, either multiple hourly doses or a single dose. They also offer reduced application, in particular, to a nonulcerated corneal infection. Finally, the importance of fully accounting for tear production in in vitro ocular delivery experiments was highlighted.

X-Ray Diffraction Imaging of Corneal Ultrastructure

X-ray scattering enables the structure of collagen-rich tissues, such as the cornea, to be examined at both the molecular and fibrillar level. The high-intensity X-rays available at synchrotron radiation sources, coupled with minimal sample preparation requirements, facilitates the rapid generation of high-quality X-ray scattering data from corneal tissue at a close-to-physiological state of hydration. Analysis of resulting X-ray scatter patterns allows one to quantify numerous structural parameters relating to the average diameter, lateral arrangement and alignment of collagen fibrils within the cornea, as well as the axial and lateral arrangements of collagen molecules within the fibrils. Here we describe the typical experimental setup and considerations involved in the collection of X-ray scattering data from corneal tissue.

Ultrastructural variability of the juxtacanalicular tissue along the inner wall of Schlemm's canal

Purpose

Increased resistance of aqueous humor drainage from the eye through Schlemm's canal (SC) is the basis for elevated intraocular pressure in glaucoma. Experimental evidence suggests that the bulk of outflow resistance lies in the vicinity of the inner wall endothelial lining of SC and the adjacent juxtacanalicular tissue (JCT). However, there is little understanding of how this resistance is generated, and a detailed understanding of the structure-function relationship of the outflow pathway has not been established yet. In the present study, regional variations in the ultrastructure of the JCT and the inner wall of SC were investigated in three dimensions.

Methods

With the use of serial block face scanning electron microscopy (SBF-SEM), the volume occupied by the electron lucent spaces of the JCT compared to that occupied by the cellular and extracellular matrix was investigated and quantified. The distribution of giant vacuoles (GVs) and pores in the inner wall endothelium of SC was further examined.

Results

With increasing distance from the inner wall of SC, the volume of the electron lucent spaces increased above 30%. In contrast, the volume of these spaces in immediate contact with the inner wall endothelium was minimal (<10%). Circumferential variability in the type and distribution of GVs was observed, and the percentage of GVs with pores varied between 3% and 27%.

Conclusions

These studies provide a detailed quantitative analysis of the ultrastructure of JCT and the distribution of GVs along the circumference of SC in three dimensions, supporting the non-uniform or segmental aqueous outflow.

Recovering vision in corneal epithelial stem cell deficient eyes

•

Corneal limbal epithelial stem cells deficiencies cause severe ocular surface instability and visual impairment.

•

These conditions, caused by injury or disease, are very difficult to treat.

•

Laboratory-grown epithelial cell sheets expanded from healthy limbal tissue can be used to reconstruct the ocular surface.

•

Other epithelia, such as the oral mucosa, can be used to generate the therapeutic cell sheets.

A healthy corneal epithelium, which is essential for proper vision and protection from external pathogens, is continuously replenished throughout life by stem cells located at the limbus. In diseased or injured eyes, however, in which stem cells are deficient, severe ocular problems manifest themselves. These are notoriously difficult to manage and as a result the last 20 or so years has seen a number of therapeutic strategies emerge that aim to recover the ocular surface and restore vision in limbal stem cell deficient eyes. The dominant concept involves the generation of laboratory cultivated epithelial cell sheets expanded from small biopsies of the epithelial limbus (for patient or donors) or another non-corneal epithelial tissue such as the oral mucosa. Typically, cells are grown on sterilised human amniotic membrane as a substrate, which then forms part of the graft, or specially formulated plastic culture dishes from which cells sheets can be released by lowering the temperature, and thus the adherence of the plastic to the cells. Overall, clinical results are promising, as is discussed, with new cultivation methodologies and different cell lineages currently being investigated to augment the treatment options for visual disturbance caused by a corneal epithelial limbal stem cell deficiency.

CD200 facilitates the isolation of corneal epithelial cells derived from human pluripotent stem cells

The in vitro induction of corneal epithelial cells (CECs) from human induced pluripotent stem cells (iPSCs) represents a new strategy for obtaining CE stem/progenitor cells for the surgical reconstruction of a diseased or injured ocular surface. The clinical promise of this strategy is considerable, but if the approaches’ potential is to be realised, robust methods for the purification of iPSC-derived CE lineage cells need to be developed to avoid contamination with other cells that may carry the risk of unwanted side effects, such as tumorigenesis. Experiments conducted here revealed that during CEC isolation, CD200-negative selection using a cell sorter considerably reduced the contamination of the cell population with various non-CECs compared with what could be achieved using TRA-1-60, a conventional negative marker for CECs. Furthermore, CD200-negative sorting did not affect the yield of CECs nor that of their stem/progenitor cells. Single-cell gene expression analysis for CEC sheets obtained using CD200-negative sorting showed that all analysed cells were CE-lineage cells, expressing PAX6, delta-N p63, and E-cadherin. Non-CECs, on the other hand, expressed non-CEC genes such as FGFR1 and RPE65. CD200, thus, represents a robust negative marker for purification of induced CE lineage cells, which is expressed by undifferentiated iPSCs and non-CECs, including iPSC-derived neural and retinal cells.

Cell regulation of collagen fibril macrostructure during corneal morphogenesis

While tissue form and function is highly dependent upon tissue-specific collagen composition and organization, little is known of the mechanisms controlling the bundling of collagen fibrils into fibers and larger structural designs that lead to the formation of bones, tendons and other tissues. Using the cornea as a model system, our previous 3 dimensional mapping of collagen fiber organization has demonstrated that macrostructural organization of collagen fibers involving interweaving, branching and anastomosing plays a critical role in controlling mechanical stiffness, corneal shape and refractive power. In this work, the cellular and mechanical mechanisms regulating critical events in the assembly of collagen macrostructure are analysed in the developing chicken cornea. We elucidated the temporal events leading to adult corneal structure and determined the effects of intraocular pressure (IOP) on the organization of the collagen macrostructure. Our findings indicate that the complex adult collagen organization begins to appear on embryonic day 10 (E10) after deposition of the primary stroma and full invasion of keratocytes. Importantly, organizational changes in keratocytes appearing at E9 preceded and predicted later changes in collagen organization. Corneal collagen organization remained unaffected when the development of IOP was blocked at E4. These findings support a primary role for keratocytes in controlling stromal organization, mechanical stiffness and corneal shape that are not regulated by the IOP. Our findings also suggest that the avian cornea represents an excellent experimental model for elucidating key regulatory steps and mechanisms controlling the collagen fiber organization that is critical to determining tissue form and function.

STATEMENT OF SIGNIFICANCE

This work by using an ex ovo model system, begins to investigate the potential mechanisms controlling collagen fibril macrostructure. In particular, this work highlights a convergent role for the corneal keratocytes in organizing the complex collagen macrostructure, necessary to support high visual acuity. Our data supports that the intraocular pressure does not influence collagen fibril macrostructure and suggest that the avian cornea represents an excellent experimental model for elucidating key regulatory steps and mechanisms controlling the collagen fiber organization that is critical to determining tissue form and function. Clearly understanding the cellular and molecular mechanisms that underlie collagen fibril macrostructure will be highly beneficial for future tissue engineering and regenerative medicine applications.

Keratan Sulfate Phenotype in the β-1,3-N-Acetylglucosaminyltransferase-7–Null Mouse Cornea

Purpose

Methods

Results

Conclusions

Generation of a TALEN-mediated, p63 knock-in in human induced pluripotent stem cells

The expression of p63 in surface ectodermal cells during development of the cornea, skin, oral mucosa and olfactory placodes is integral to the process of cellular self-renewal and the maintenance of the epithelial stem cell status. Here, we used TALEN technology to generate a p63 knock-in (KI) human induced pluripotent stem (hiPS) cell line in which p63 expression can be visualized via enhanced green fluorescent protein (EGFP) expression. The KI-hiPS cells maintained pluripotency and expressed the stem cell marker gene, ΔNp63α. They were also able to successfully differentiate into functional corneal epithelial cells as assessed by p63 expression in reconstructed corneal epithelium. This approach enables the tracing of p63-expressing cell lineages throughout epithelial development, and represents a promising application in the field of stem cell research.

Sustained Activation of the Unfolded Protein Response Induces Cell Death in Fuchs' Endothelial Corneal Dystrophy

Purpose

The unfolded protein response (UPR) is believed to play a role in the pathogenesis of Fuchs' endothelial corneal dystrophy (FECD). The purpose of this study was to investigate whether unfolded proteins accumulate in the corneal endothelium in FECD and if they are involved in triggering cell death.

Methods

Descemet's membranes with corneal endothelial cells (CECs) were obtained during keratoplasty, and expression of aggresomes, type 1 collagen, fibronectin, and agrin was evaluated. Endoplasmic reticulum (ER) stress of immortalized human CECs from non-FECD subjects and from FECD patients (iHCEC and iFECD, respectively) were evaluated. The effect of MG132-mediated aggresome formation on the UPR and intrinsic pathway and the effect of mitochondrial damage on UPR were also examined. The effect of CHOP knockdown on the ER stress-mediated intrinsic pathway was also evaluated.

Results

Aggresome formation was higher in iFECD than in iHCEC and was colocalized with type 1 collagen, fibronectin, and agrin. GRP78, phosphorylated IRE1, PERK, and CHOP showed higher activation in iFECD than in iHCEC. MG132-mediated aggresome formation upregulated ER stress sensors, the mitochondrial membrane potential drop, cytochrome c release to the cytoplasm, and activation of caspase-9 and -3. By contrast, staurosporine-mediated mitochondrial damage did not induce ER stress. Knockdown of CHOP attenuated the ER stress-induced cleavage of caspase-9, which is caused by intrinsic pathway activation.

Conclusions

Excessive synthesis of extracellular matrix proteins induced unfolded protein accumulation in FECD. Prolonged ER stress-mediated cell death, occurring via the intrinsic apoptotic signaling pathway, therefore might be associated with the pathogenesis of FECD.

Involvement of ZEB1 and Snail1 in excessive production of extracellular matrix in Fuchs endothelial corneal dystrophy

Fuchs endothelial corneal dystrophy (FECD) due to corneal endothelial cell degeneration is a major cause of corneal transplantation. It is characterized by abnormal deposition of extracellular matrix (ECM), such as corneal guttae, accompanied by a loss of endothelial cells. Although recent studies have revealed several genomic factors, the molecular pathophysiology of FECD has not yet been revealed. In this study, we establish a cellular in vitro model by using immortalized corneal endothelial cells obtained from late-onset FECD and control patients and examined the involvement of epithelial mesenchymal transition (EMT) on excessive ECM production. We demonstrate that the EMT-inducing genes ZEB1 and SNAI1 were highly expressed in corneal endothelial cells in FECD and were involved in excessive production of ECM proteins, such as type I collagen and fibronectin through the transforming growth factor (TGF)-β signaling pathway. Furthermore, we found that SB431542, a specific inhibitor of TGF-β type I ALK receptors, suppressed the expression of ZEB1 and Snail1 followed by reduced production of ECM. These findings suggest that increased expression levels of ZEB1 and Snail1 in FECD cells were responsible for an increased responsiveness to TGF-β present in the aqueous humor and excessive production of ECM. In addition, these results suggest that the regulation of EMT-related genes by blocking the TGF-β signaling pathway may be a feasible therapeutic strategy for FECD.

From nano to macro: studying the hierarchical structure of the corneal extracellular matrix

In this review, we discuss current methods for studying ocular extracellular matrix (ECM) assembly from the 'nano' to the 'macro' levels of hierarchical organization. Since collagen is the major structural protein in the eye, providing mechanical strength and controlling ocular shape, the methods presented focus on understanding the molecular assembly of collagen at the nanometre level using X-ray scattering through to the millimetre to centimetre level using non-linear optical (NLO) imaging of second harmonic generated (SHG) signals. Three-dimensional analysis of ECM structure is also discussed, including electron tomography, serial block face scanning electron microscopy (SBF-SEM) and digital image reconstruction. Techniques to detect non-collagenous structural components of the ECM are also presented, and these include immunoelectron microscopy and staining with cationic dyes. Together, these various approaches are providing new insights into the structural blueprint of the ocular ECM, and in particular that of the cornea, which impacts upon our current understanding of the control of corneal shape, pathogenic mechanisms underlying ectatic disorders of the cornea and the potential for corneal tissue engineering.

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.

•

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

•

Total sulfate levels on glycosaminoglycans remained constant across the cornea.

•

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

•

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

•

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

Three-dimensional architecture of collagen type VI in the human trabecular meshwork

PURPOSE

Type VI collagen is a primary component of the extracellular matrix of many connective tissues. It can form distinct aggregates depending on tissue structure, chemical environment, and physiology. In the current study we examine the ultrastructure and mode of aggregation of type VI collagen molecules in the human trabecular meshwork.

METHODS

Trabecular meshwork was dissected from donor human eyes, and three-dimensional transmission electron microscopy of type VI collagen aggregates was performed.

RESULTS

Electron-dense collagen structures were detected in the human trabecular meshwork and identified as collagen type VI assemblies based on the three-dimensional spatial arrangement of the type VI collagen molecules, the 105-nm axial periodicity of the assemblies themselves, and their characteristic double bands, which arose from the globular domains of the type VI collagen molecules. Sulfated proteoglycans were also seen to associate with the assemblies either with the globular domain or the inner rod-like segments of the tetramers.

CONCLUSIONS

No extended structural regularity in the organization of type VI collagen assemblies within the trabecular meshwork was evident, and the lateral separation of the tetramers forming the assemblies varied, as did the angle formed by the main axes of adjacent tetramers. This is potentially reflective of the specific nature of the trabecular meshwork environment, which facilitates aqueous outflow from the eye, and we speculate that extracellular matrix ions and proteins might prevent a more tight packing of type VI collagen tetramers that form the assemblies.

Fibroblast growth factor 2 and transforming growth factor β1 induce precocious maturation of articular cartilage

OBJECTIVE

We have discovered that a combination of fibroblast growth factor 2 and transforming growth factor β1 induce profound morphologic changes in immature articular cartilage. The purpose of this study was to test the hypothesis that these changes represent accelerated postnatal maturation.

METHODS

Histochemical and biochemical assays were used to confirm the nature of the morphologic changes that accompany growth factor stimulation of immature bovine articular cartilage explants in serum-free culture medium. Growth factor-induced apoptosis, cellular proliferation, and changes in the collagen network were also quantitatively analyzed.

RESULTS

Growth factor stimulation resulted in rapid resorption from the basal aspect of immature cartilage explants that was simultaneously opposed by cellular proliferation from the apical aspect driven from a pool of chondroprogenitor cells we have previously described. Maturation-dependent changes in tissue stiffness, collagen crosslinking, and collagen fibril architecture as well as differentiation of the extracellular matrix into distinct pericellular, territorial, and interterritorial domains were all present in growth factor-stimulated cartilage samples and absent in control samples.

CONCLUSION

Our data demonstrate that it is possible to significantly enhance the maturation of cartilage tissue using specific growth factor stimulation. This may have applications in transplantation therapy or in the treatment of diseased cartilage, through phenotype modulation of osteoarthritic chondrocytes in order to stimulate growth and maturation of cartilage repair tissue.

The effect of riboflavin/UVA collagen cross-linking therapy on the structure and hydrodynamic behaviour of the ungulate and rabbit corneal stroma

Purpose

To examine the effect of riboflavin/UVA corneal crosslinking on stromal ultrastructure and hydrodynamic behaviour.

Methods

One hundred and seventeen enucleated ungulate eyes (112 pig and 5 sheep) and 3 pairs of rabbit eyes, with corneal epithelium removed, were divided into four treatment groups: Group 1 (28 pig, 2 sheep and 3 rabbits) were untreated; Group 2 (24 pig) were exposed to UVA light (3.04 mW/cm²) for 30 minutes and Group 3 (29 pig) and Group 4 (31 pig, 3 sheep and 3 rabbits) had riboflavin eye drops applied to the corneal surface every 5 minutes for 35 minutes. Five minutes after the initial riboflavin instillation, the corneas in Group 4 experienced a 30 minute exposure to UVA light (3.04 mW/cm²). X-ray scattering was used to obtain measurements of collagen interfibrillar spacing, spatial order, fibril diameter, D-periodicity and intermolecular spacing throughout the whole tissue thickness and as a function of tissue depth in the treated and untreated corneas. The effect of each treatment on the hydrodynamic behaviour of the cornea (its ability to swell in saline solution) and its resistance to enzymatic digestion were assessed using in vitro laboratory techniques.

Results

Corneal thickness decreased significantly following riboflavin application (p<0.01) and also to a lesser extent after UVA exposure (p<0.05). With the exception of the spatial order factor, which was higher in Group 4 than Group 1 (p<0.01), all other measured collagen parameters were unaltered by cross-linking, even within the most anterior 300 microns of the cornea. The cross-linking treatment had no effect on the hydrodynamic behaviour of the cornea but did cause a significant increase in its resistance to enzymatic digestion.

Conclusions

It seems likely that cross-links formed during riboflavin/UVA therapy occur predominantly at the collagen fibril surface and in the protein network surrounding the collagen.

Chemical composition and sulfur speciation in bulk tissue by x-ray spectroscopy and x-ray microscopy: corneal development during embryogenesis

The chemical composition and sulfur (S) speciation of developing chick corneas at embryonic days 12, 14, and 16 were investigated using synchrotron scanning x-ray fluorescence microscopy and x-ray absorption near-edge structure spectroscopy. The aim was to develop techniques for the analysis of bulk tissue and identify critical physicochemical variations that correlate with changes in corneal structure-function relationships. Derived data were subjected to principal component analysis and linear discriminant analysis, which highlighted differences in the elemental and S species composition at different stages of embryonic growth. Notably, distinct elemental compositions of chlorine, potassium, calcium, phosphorus, and S altered with development during the transition of the immature opaque cornea to a mature transparent tissue. S structure spectroscopy revealed developmentally regulated alterations in thiols, organic monosulfides, ester sulfate, and inorganic sulfate species. The transient molecular structures and compositional changes reported here provide a deeper understanding of the underlying basis of corneal development during the acquisition of transparency. The experimental and analytical approach is new, to our knowledge, and has wide potential applicability in the life sciences.

A selective inhibitor of the Rho kinase pathway, Y-27632, and its influence on wound healing in the corneal stroma

PURPOSE

Our study examined the effect of a selective Rho kinase inhibitor, Y-27632, on corneal wound healing and potential stromal scarring after superficial keratectomy.

METHODS

Rabbit keratocytes were induced into myofibroblasts by transforming growth factor β1 (TGFβ1) either with or without Y-27632. Then α-smooth muscle actin (α-SMA) was examined by immunohistochemistry and western blotting, and the contractility of the seeded collagen gels was measured. Y-27632 eye drops (or vehicle only) were administered to eyes after a superficial keratectomy, and the tissue was examined by immunohistochemistry for α-SMA, collagen types I, II, and III, and keratan sulfate. Electron microscopy was conducted with and without histochemical contrasting of sulfated proteoglycans.

RESULTS

Spindle-like cells in culture constituted 99.5±1.1% with TGFβ1 stimulation, but 3.5±1.0% after TGFβ1 and Y-27632 treatment (p<0.01, n=6). α-SMA was seen in 4% of TGFβ1-treated cells, but in only 0.3% of cells with Y-27632 added (p<0.01, n=6), which was confirmed by western blotting. Y-27632 also inhibited the TGFβ1-induced contraction of seeded collagen gels. After superficial keratectomies, collagen type I and keratan sulfate were unchanged by Y-27632 application. Collagen type II was not detected in Y-27632 or vehicle-only corneas. With Y-27632 treatment, α-SMA expression increased and the collagen type III signal became in the weaker subepithelial area. Interestingly, bundles of aligned and uniformly spaced collagen fibrils were more prevalent in keratocytes in Y-27632-treated corneas, which is reminiscent of fibripositor-like structures that have been proposed as a mechanism of matrix deposition in embryonic connective tissues.

CONCLUSIONS

Y-27632 inhibits keratocyte-to-myofibroblast transition, and its topical application after a superficial lamellar keratectomy elicits an altered wound healing response, with evidence of an embryonic-type deposition of collagen fibrils.

Ultraviolet light transmission through the human corneal stroma is reduced in the periphery

This article investigates in vitro light transmission through the human cornea in the ultraviolet (UV) portion of the electromagnetic spectrum as a function of position across the cornea from center to periphery. Spectrophotometry was used to measure UV transmission in the wavelength range 310-400 nm, from the central cornea to its periphery. UV transmission decreases away from the center, and this is attributed to scattering and absorbance. Corneal endothelial cells, which line the back of the cornea and are more numerous in the periphery, therefore receive a lower dose of UV than do those in the central cornea. This is consistent with the recent observation that endothelial cells in the corneal periphery exhibit less nuclear oxidative DNA damage than those in the central cornea.

Alterations in the biomolecular signatures of developing chick corneas as determined by biospectroscopy and multivariate analysis

PURPOSE

Biospectroscopy tools are increasingly being recognized as novel approaches toward interrogating complex biological structures in a nondestructive fashion. This study was conducted to apply these tools to interrogate alterations in the molecular signatures of developing chick corneas during the onset and development of transparency.

METHODS

Embryonic chick corneas (n = 46) were obtained at 2-day intervals from embryonic day (E)10 to E18 of incubation and investigated with attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy and Raman microspectroscopy. Resultant spectra were analyzed for variance by using principal component analysis and linear discriminant analysis (PCA-LDA).

RESULTS

Mean spectra after ATR-FTIR spectroscopy or Raman microspectroscopy derived from corneas at each developmental stage showed some overlap; however, in PCA-LDA scores plots, a clear segregation of spectra was evident, and two-category discrimination indicated that significant molecular alterations occur during tissue morphogenesis. Notable by both techniques was the increasing intensity of DNA signal (1080 cm⁻¹) from E10 onward. Major segregating biomarkers identified by ATR-FTIR spectroscopy between E10 and E18 were in the DNA/RNA (1126 cm⁻¹), glycogen (1045 cm⁻¹), protein (1470 cm⁻¹), and amide II (1512 cm⁻¹ and 1524 cm⁻¹) spectral regions. Raman spectroscopy also identified major distinguishing vibrational modes that included proteins, amino acids (tyrosine, proline phenylalanine, and valine), and secondary structures of proteins (amide I and amide II).

CONCLUSIONS

The developing chick cornea undergoes significant changes in its biomolecular composition in the E10 to E18 developmental period, with the major changes occurring in the spectral regions associated with DNA/RNA, proteins, glycogen, and secondary protein structures.

Large proteoglycan complexes and disturbed collagen architecture in the corneal extracellular matrix of mucopolysaccharidosis type VII (Sly syndrome)

PURPOSE

Deficiencies in enzymes involved in proteoglycan (PG) turnover underlie a number of rare mucopolysaccharidoses (MPS), investigations of which can considerably aid understanding of the roles of PGs in corneal matrix biology. Here, the authors analyze novel pathologic changes in MPS VII (Sly syndrome) to determine the nature of PG-collagen associations in stromal ultrastructure.

METHODS

Transmission electron microscopy and electron tomography were used to investigate PG-collagen architectures and interactions in a cornea obtained at keratoplasty from a 22-year-old man with MPS VII, which was caused by a compound heterozygous mutation in the GUSB gene.

RESULTS

Transmission electron microscopy showed atypical morphology of the epithelial basement membrane and Bowman's layer in MPS VII. Keratocytes were packed with cytoplasmic vacuoles containing abnormal glycosaminoglycan (GAG) material, and collagen fibrils were thinner than in normal cornea and varied considerably throughout anterior (14-32 nm), mid (13-42 nm), and posterior (17-39 nm) regions of the MPS VII stroma. PGs viewed in three dimensions were striking in appearance in that they were significantly larger than PGs in normal cornea and formed highly extended linkages with multiple collagen fibrils.

CONCLUSIONS

Cellular changes in the MPS VII cornea resemble those in other MPS. However, the wide range of collagen fibril diameters throughout the stroma and the extensive matrix presence of supranormal-sized PG structures appear to be unique features of this disorder. The findings suggest that the accumulation of stromal chondroitin-, dermatan-, and heparan-sulfate glycosaminoglycans in the absence of β-glucuronidase-mediated degradation can modulate collagen fibrillogenesis.