Corneal scar formation

Inhibition of enhancer of zeste homolog 2 prevents corneal myofibroblast transformation in vitro

PURPOSE

Corneal fibroblast can be transformed into corneal myofibroblasts by TGF-β1. Enhancer of zeste homolog 2 (EZH2) upregulation has been observed in the occurrence of other fibrotic disorders. We investigated the role of EZH2 in the progression of corneal fibrosis and the antifibrotic effect of EZH2 inhibition in corneal fibroblasts (CFs).

METHODS

Primary CFs were isolated from corneal limbi and the CFs were treated with TGF-β1 to induce fibrosis. EPZ-6438 and EZH2 siRNA were used to inhibit EZH2 expression. Myofibroblast activation and extracellular matrix (ECM) protein synthesis was detected by quantitative real-time PCR, western blotting, and immunofluorescence staining assay. The functions of myofibroblast were evaluated by cell migration and collagen gel contraction assays. Molecular mechanisms involved in EZH2 inhibition were investigated by RNA sequencing.

RESULTS

TGF-β1 activated EZH2 expression in CFs. Treatment with EPZ-6438 (5 μM) and EZH2 siRNA considerably suppressed corneal myofibroblast activation and ECM protein synthesis in CFs induced by TGF-β1 when compared to the control group. EPZ-6438 (5 μM) suppressed cell migration and gel contraction in CFs. RNA sequencing results revealed that antifibrotic genes were activated after EZH2 inhibition to suppress corneal myofibroblast activation.

CONCLUSION

Inhibition of EZH2 suppresses corneal myofibroblast activation and ECM protein synthesis, and could serve as a novel therapeutic target for preventing corneal scarring.

Understanding cornea epithelial stem cells and stem cell deficiency: Lessons learned using vertebrate model systems

Animal models have contributed greatly to our understanding of human diseases. Here, we focus on cornea epithelial stem cell (CESC) deficiency (commonly called limbal stem cell deficiency, LSCD). Corneal development, homeostasis and wound healing are supported by specific stem cells, that include the CESCs. Damage to or loss of these cells results in blindness and other debilitating ocular conditions. Here we describe the contributions from several vertebrate models toward understanding CESCs and LSCD treatments. These include both mammalian models, as well as two aquatic models, Zebrafish and the amphibian, Xenopus. Pioneering developments have been made using stem cell transplants to restore normal vision in patients with LSCD, but questions still remain about the basic biology of CESCs, including their precise cell lineages and behavior in the cornea. We describe various cell lineage tracing studies to follow their patterns of division, and the fates of their progeny during development, homeostasis, and wound healing. In addition, we present some preliminary results using the Xenopus model system. Ultimately, a more thorough understanding of these cornea cells will advance our knowledge of stem cell biology and lead to better cornea disease therapeutics.

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.

Stem Cells in the Cornea

The cornea is the tough, transparent tissue through which light first enters the eye and functions as a barrier to debris and infection as well as two-thirds of the refractive power of the eye. Corneal damage that is not promptly treated will often lead to scarring and vision impairment. Due to the limited options currently available to treat corneal scars, the identification and isolation of stem cells in the cornea has received much attention, as they may have potential for autologous, cell-based approaches to the treatment of damaged corneal tissue.

Nanomedicine approaches for corneal diseases

Corneal diseases are the third leading cause of blindness globally. Topical nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, antibiotics and tissue transplantation are currently used to treat corneal pathological conditions. However, barrier properties of the ocular surface necessitate high concentration of the drugs applied in the eye repeatedly. This often results in poor efficacy and several side-effects. Nanoparticle-based molecular medicine seeks to overcome these limitations by enhancing the permeability and pharmacological properties of the drugs. The promise of nanomedicine approaches for treating corneal defects and restoring vision without side effects in preclinical animal studies has been demonstrated. Numerous polymeric, metallic and hybrid nanoparticles capable of transporting genes into desired corneal cells to intercept pathologic pathways and processes leading to blindness have been identified. This review provides an overview of corneal diseases, nanovector properties and their applications in drug-delivery and corneal disease management.

Quantification of collagen ultrastructure after penetrating keratoplasty - implications for corneal biomechanics

PURPOSE

To quantify long-term changes in stromal collagen ultrastructure following penetrating keratoplasty (PK), and evaluate their possible implications for corneal biomechanics.

METHODS

A pair of 16 mm post-mortem corneo-scleral buttons was obtained from a patient receiving bilateral penetrating keratoplasty 12 (left)/28 (right) years previously. Small-angle x-ray scattering quantified collagen fibril spacing, diameter and spatial order at 0.5 mm or 0.25 mm intervals along linear scans across the graft margin. Corresponding control data was collected from two corneo-scleral buttons with no history of refractive surgery. Wide-angle x-ray scattering quantified collagen fibril orientation at 0.25 mm (horizontal)×0.25 mm (vertical) intervals across both PK specimens. Quantification of orientation changes in the graft margin were verified by equivalent analysis of data from a 13 year post-operative right PK specimen obtained from a second patient in a previous study, and comparison made with new and published data from normal corneas.

RESULTS

Marked changes to normal fibril alignment, in favour of tangentially oriented collagen, were observed around the entire graft margin in all PK specimens. The total number of meridional fibrils in the wound margin was observed to decrease by up to 40%, with the number of tangentially oriented fibrils increasing by up to 46%. As a result, in some locations the number of fibrils aligned parallel to the wound outnumbered those spanning it by up to five times. Localised increases in fibril spacing and diameter, with an accompanying reduction in matrix order, were also evident.

CONCLUSIONS

Abnormal collagen fibril size and spatial order within the PK graft margin are indicative of incomplete stromal wound remodelling and the long term persistence of fibrotic scar tissue. Lasting changes in collagen fibril orientation in and around PK wounds may alter corneal biomechanics and compromise the integrity of the graft-host interface in the long term.

Impaired autophagy and delayed autophagic clearance of transforming growth factor β-induced protein (TGFBI) in granular corneal dystrophy type 2

Granular corneal dystrophy type 2 (GCD2) is an autosomal dominant disease characterized by a progressive age-dependent extracellular accumulation of transforming growth factor β-induced protein (TGFBI). Corneal fibroblasts from GCD2 patients also have progressive degenerative features, but the mechanism underlying this degeneration remains unknown. Here we observed that TGFBI was degraded by autophagy, but not by the ubiquitin/proteasome-dependent pathway. We also found that GCD2 homozygous corneal fibroblasts displayed a greater number of fragmented mitochondria. Most notably, mutant TGFBI (mut-TGFBI) extensively colocalized with microtubule-associated protein 1 light chain 3β (MAP1LC3B, hereafter referred to as LC3)-enriched cytosolic vesicles and CTSD in primary cultured GCD2 corneal fibroblasts. Levels of LC3-II, a marker of autophagy activation, were significantly increased in GCD2 corneal fibroblasts. Nevertheless, levels of SQSTM1/p62 and of polyubiquitinated protein were also significantly increased in GCD2 corneal fibroblasts compared with wild-type (WT) cells. However, LC3-II levels did not differ significantly between WT and GCD2 cells, as assessed by the presence of bafilomycin A 1, the fusion blocker of autophagosomes and lysosomes. Likewise, bafilomycin A 1 caused a similar change in levels of SQSTM1. Thus, the increase in autophagosomes containing mut-TGFBI may be due to inefficient fusion between autophagosomes and lysosomes. Rapamycin, an autophagy activator, decreased mut-TGFBI, whereas inhibition of autophagy increased active caspase-3, poly (ADP-ribose) polymerase 1 (PARP1) and reduced the viability of GCD2 corneal fibroblasts compared with WT controls. These data suggest that defective autophagy may play a critical role in the pathogenesis of GCD2.

Quantitative assessment of ultrastructure and light scatter in mouse corneal debridement wounds

PURPOSE

The mouse has become an important wound healing model with which to study corneal fibrosis, a frequent complication of refractive surgery. The aim of the current study was to quantify changes in stromal ultrastructure and light scatter that characterize fibrosis in mouse corneal debridement wounds.

METHODS

Epithelial debridement wounds, with and without removal of basement membrane, were produced in C57BL/6 mice. Corneal opacity was measured using optical coherence tomography, and collagen diameter and matrix order were quantified by x-ray scattering. Electron microscopy was used to visualize proteoglycans. Quantitative PCR (Q-PCR) measured mRNA transcript levels for several quiescent and fibrotic markers.

RESULTS

Epithelial debridement without basement membrane disruption produced a significant increase in matrix disorder at 8 weeks, but minimal corneal opacity. In contrast, basement membrane penetration led to increases in light scatter, matrix disorder, and collagen diameter, accompanied by the appearance of abnormally large proteoglycans in the subepithelial stroma. This group also demonstrated upregulation of several quiescent and fibrotic markers 2 to 4 weeks after wounding.

CONCLUSIONS

Fibrotic corneal wound healing in mice involves extensive changes to collagen and proteoglycan ultrastructure, consistent with deposition of opaque scar tissue. Epithelial basement membrane penetration is a deciding factor determining the degree of ultrastructural changes and resulting opacity.

Structural and biochemical aspects of keratan sulphate in the cornea

Keratan sulphate (KS) is the predominant glycosaminoglycan (GAG) in the cornea of the eye, where it exists in proteoglycan (PG) form. KS-PGs have long been thought to play a pivotal role in the establishment and maintenance of the array of regularly-spaced and uniformly- thin collagen fibrils which make up the corneal stroma. This characteristic arrangement of fibrils allows light to pass through the cornea. Indeed, perturbations to the synthesis of KS-PG core proteins in genetically altered mice lead to structural matrix alterations and corneal opacification. Similarly, mutations in enzymes responsible for the sulphation of KS-GAG chains are causative for the inherited human disease, macular corneal dystrophy, which is manifested clinically by progressive corneal cloudiness starting in young adulthood.

Human corneal fibrosis: an in vitro model

PURPOSE

Corneal injury may ultimately lead to a scar by way of corneal fibrosis, which is characterized by the presence of myofibroblasts and improper deposition of extracellular matrix (ECM) components. TGF-beta1 is known to stimulate overproduction and deposition of ECM components. Previously, an in vitro three-dimensional (3-D) model of a corneal stroma was developed by using primary human corneal fibroblasts (HCFs) stimulated with stable vitamin C (VitC). This model mimics corneal development. The authors postulate that with the addition of TGF-beta1, a 3-D corneal scar model can be generated.

METHODS

HCFs were grown in four media conditions for 4 or 8 weeks: VitC only; VitC+TGF-beta1 for the entire time; VitC+TGF-beta1 for 1 week, then VitC only for 3 or 7 weeks; and VitC for 4 weeks, then VitC+TGF-beta1 for 4 weeks. Cultures were analyzed with TEM and indirect immunofluorescence.

RESULTS

Compared with the control, addition of TGF-beta1 increased construct thickness significantly, with maximum increase in constructs with TGF-beta1 present for the entire time-2.1- to 3.2-fold at 4 and 8 weeks, respectively. In all TGF-beta-treated cultures, cells became long and flat, numerous filamentous cells were seen, collagen levels increased, and long collagen fibrils were visible. Smooth muscle actin, cellular fibronectin, and type III collagen expression all appeared to increase. Cultures between weeks 4 and 8 showed minimal differences.

CONCLUSIONS

Human corneal fibroblasts stimulated by VitC and TGF-beta1 appear to generate a model that resembles processes observed in human corneal fibrosis. This model should be useful in examining matrix deposition and assembly in a wound-healing situation.

Prelude to corneal tissue engineering - gaining control of collagen organization

By most standard engineering practice principles, it is premature to credibly discuss the "engineering" of a human cornea. A professional design engineer would assert that we still do not know what a cornea is (and correctly so), therefore we cannot possibly build one. The proof resides in the fact that there are no clinically viable corneas based on classical tissue engineering methods available. This is possibly because tissue engineering in the classical sense (seeding a degradable scaffolding with a population synthetically active cells) does not produce conditions which support the generation of organized tissue. Alternative approaches to the problem are in their infancy and include the methods which attempt to recapitulate development or to produce corneal stromal analogs de novo which require minimal remodeling. Nonetheless, tissue engineering efforts, which have been focused on producing the fundamental functional component of a cornea (organized alternating arrays of collagen or "lamellae"), may have already provided valuable new insights and tools relevant to development, growth, remodeling and pathologies associated with connective tissue in general. This is because engineers ask a fundamentally different question (How can that be done?) than do biological scientists (How is that done?). The difference in inquiry has prompted us to closely examine (and to mimic) development as well as investigate collagen physicochemical behavior so that we may exert control over organization both in cell culture (in vitro) and on the benchtop (de novo). Our initial results indicate that reproducing corneal stroma-like local and long-range organization of collagen may be simpler than we anticipated while controlling spacing and fibril morphology remains difficult, but perhaps not impossible in the (reasonably) near term.

Light-scattering and ultrastructure of healed penetrating corneal wounds

PURPOSE

To investigate quantitatively for the first time the relationship between light-scattering and ultrastructure of semitransparent scars resulting from penetrating wounds in rabbit cornea.

METHODS

Penetrating wounds, 2 mm in diameter, were made in the central cornea and allowed to heal for 3.6 to 4.5 years at which time the rabbits were killed. The scar and cornea thickness outside the scar were measured using ultrasonic pachymetry. Corneas were excised immediately and their transmissivity was measured from 400 to 700 nm. The tissue was then prepared for transmission electron microscopy. Transmission electron micrographs (TEMs) were analyzed to determine fibril positions and radii. Scattering was calculated using the direct summation of fields (DSF) METHOD:

RESULTS

Scar thickness averaged 0.26 +/- 0.04 mm, and the scars were flat. Thickness outside the scars averaged 0.40 +/- 0.04 mm. Three scars were moderately transparent, five were less transparent, and one was much less transparent. The wavelength dependence of the measured total scattering cross- section was indicative of the presence of voids (lakes) in the collagen fibril distribution, and lakes were evident in the TEMs. The images showed enlarged fibrils and some showed bimodal distributions of fibril diameters. Calculated scattering was characteristic of that expected from regions containing lakes-a finding consistent with the scattering measurements.

CONCLUSIONS

Despite the long healing time, these scars remained highly scattering. A combination of lakes, disordered fibril distributions, and a significant population of enlarged fibrils can explain the scattering. A possible cellular contribution cannot be ruled out.

Constitutive collagenase-1 synthesis through MAPK pathways is mediated, in part, by endogenous IL-1α during fibrotic repair in corneal stroma

Collagenase-1 is a protease expressed by active fibroblasts that is involved in remodeling of the extracellular matrix (ECM). In this study, we characterize the intracellular signaling mechanism of collagenase-1 production by IL-1alpha in subcultured normal fibroblasts (NF) from uninjured normal corneas, compared to that in repair wound fibroblasts (WF). In NF, collagenase-1 was induced specifically after the exogenous addition of IL-1alpha via activation of ERK and p38MAPK. Collagenase-1 expression was strongly suppressed upon treatment with either a MEK or p38MAPK inhibitor. In contrast, repair WF constitutively synthesized both IL-1alpha and collagenase-1. Combined treatment with both mitogen-activated protein kinase (MAPK) inhibitors dramatically reduced collagenase-1 synthesis, while individual MEK1 or p38 inhibitors weakly modulated the collagenase-1 level. The results indicate that both pathways are crucial in the regulation of collagenase-1 synthesis. Furthermore, an IL-1alpha receptor antagonist (IL-1ra) could not abolish constitutive collagenase-1 synthesis, even at high doses, suggesting that other cytokines/factors are additionally involved in this process. We propose that induction of collagenase-1 by IL-1alpha in both WF and NF depends on a unique combination of cell type-specific signaling pathways.

How the cornea heals: cornea-specific repair mechanisms affecting surgical outcomes

In mammals, penetrating injuries typically heal by deposition of fibrotic "repair tissue" that fills and seals wounds but does not restore normal function. Excessive deposition of fibrotic repair tissue can lead to pathologies involving excessive scarring and contracture. In the cornea, fibrotic repair presents special challenges affecting both clarity and shape of the cornea. With the increasing popularity of surgical techniques that alter corneal refractive errors, understanding of cornea repair mechanisms has acquired new significance. The cornea has unique anatomic, cellular, molecular, and functional features that lead to important mechanistic differences in the process of repair in comparison with what occurs in skin and other organs. Moreover, corneal function calls for special outcomes. This review addresses these features from the viewpoint of the authors' research on factors of importance to understanding and improving surgical outcomes.

Selective reduction of fibrotic markers in repairing corneas of mice deficient in Smad3

The cytokine transforming growth factor-beta (TGF-beta) is a key mediator of fibrosis in all organs. Expression of fibrotic markers in repairing cutaneous wounds is reduced in mice lacking Smad3, a downstream cytoplasmic mediator of TGF-beta signaling (Ashcroft et al., 1999, Nat Cell Biol 1(5):260-266). This is correlated with a reduction in inflammation, and thus in the blood elements thought to be a significant source of TGF-beta at the wound site, the principle form being TGF-beta1. Since the major cellular source of TGF-beta in corneal wounds is the epithelium, and the principal isoform is TGF-beta2, we investigated whether Smad3 deficiency has similar anti-fibrotic effects on corneal repair. In contrast to the situation of cutaneous repair, expression of the fibrotic marker, fibronectin, was equivalent in corneal repair tissue of Smad3-/- mice as compared to their +/- littermates, even though expression of a second fibrotic marker not previously examined in cutaneous wounds, alpha-smooth muscle (sm) actin, was reduced. Also unlike in cutaneous wounds, the inflammatory response was unaffected. These differences between corneal and cutaneous repair correlated with the lack of apparent change in the levels of corneal TGF-beta2. There was a significant reduction of alpha-sm actin expression in stromal cell cultures established from Smad3-/- mice as compared to their +/- littermates, but the rate of cell proliferation stimulated by TGF-beta, as well as expression of fibronectin, was unaffected. Therefore, a deficiency in Smad3 has different effects on corneal and cutaneous repair, probably due to the difference in cellular source and principal isoform of the TGF-beta involved.

Proteomic analysis of the soluble fraction from human corneal fibroblasts with reference to ocular transparency

The transparent corneal stroma contains a population of corneal fibroblasts termed keratocytes, which are interspersed between the collagen lamellae. Under normal conditions, the keratocytes are quiescent and transparent. However, after corneal injury the keratocytes become activated and transform into backscattering wound-healing fibroblasts resulting in corneal opacification. At present, the most popular hypothesis suggests that particular abundant water-soluble proteins called enzyme-crystallins are involved in maintaining corneal cellular transparency. Specifically, corneal haze development is thought to be related to low levels of cytoplasmic enzyme-crystallins in reflective corneal fibroblasts. To further investigate this hypothesis, we have used a proteomic approach to identify the most abundant water-soluble proteins in serum-cultured human corneal fibroblasts that represent an in vitro model of the reflective wound-healing keratocyte phenotype. Densitometry of one-dimensional gels revealed that no single protein isoform exceeded 5% of the total water-soluble protein fraction, which is the qualifying property of a corneal enzyme-crystallin according to the current definition. This result indicates that wound-healing corneal fibroblasts do not contain enzyme-crystallins. A total of 254 protein identifications from two-dimensional gels were performed representing 118 distinct proteins. Proteins protecting against oxidative stress and protein misfolding were prominent, suggesting that these processes may participate in the generation of cytoplasmic light-scattering from corneal fibroblasts.

The Structure and Swelling of Corneal Scar Tissue in Penetrating Full-Thickness Wounds

OBJECTIVE

The purpose of this study was to determine corneal thickness and to examine the collagen and proteoglycans in full-thickness corneal scars up to 16 months following wounding.

METHODS

Ultrasound pachymetry was used to measure the depths of penetrating central scars and their surrounding areas in 16 rabbit corneas. Measurements were taken at regular intervals: 5, 10, 12, and 16 months after healing. Transmission electron microscopy was then used to study the stroma of the resulting scars to observe the collagen organization and the amount, as well as the size, of cuprolinic blue-stained proteoglycan filaments within the stroma. Furthermore, ex vivo swelling of selected wounded contralateral excised corneas was undertaken by the measured addition of distilled water.

RESULTS

In vivo, the thickness of the scar tissue was significantly less than that of the surrounding tissue throughout the period studied. By 12 months the proteoglycan filaments within the scar were of a similar size and number to those within the adjacent tissue, whereas the collagen fibrils within the scar were still disorganized (collagen interweaving, lack of a lamellar structure). Once excised and allowed to swell in water, scar tissue thickness remained relatively unchanged, whereas the surrounding tissue swelled considerably.

CONCLUSION

Disorganized fibril arrangement inhibits the normal swelling of the scar tissue, which remains reduced. Furthermore, even after many months of healing, collagen remodeling in corneal scar tissue is not complete.

Organization of corneal collagen fibrils during the healing of trephined wounds in rabbits

The purpose of this study was to investigate the structure and organization of collagen during the healing of penetrating rabbit corneal wounds. Full-thickness wounds were produced in both corneas of six New Zealand White rabbits with a 2-mm trephine. After 5, 10, and 16 months, wounds were examined using transmission electron microscopy and wide-angle synchrotron X-ray diffraction, which allowed the quantification of the relative amount of collagen and fibril orientation in different parts of the cornea. The intensity of the diffraction patterns taken from the wound edge was much higher than patterns collected at the wound center, indicating a greater amount of collagen near the wound edge. This collagen was shown to have a preferred orientation, forming a circular pattern around the wound. As the wounds healed, the amount of preferentially aligned collagen decreased and lamella formation and ordered fibrillar arrangement increased as shown by electron microscopy. The orientation of collagen fibrils in healing penetrating corneal wounds is not random. Instead, fibrils tend to be circularly arranged within and without the wound, gradually becoming more normal over time. We speculate that collagen is artefactually displaced during trephination, resulting in an abnormal alignment of the fibrils surrounding the wound edge, and that this influences the wound healing process.

Fibroblast growth factor-2 promotes keratan sulfate proteoglycan expression by keratocytes in vitro

Keratocytes of the corneal stroma produce a specialized extracellular matrix responsible for corneal transparency. Corneal keratan sulfate proteoglycans (KSPG) are unique products of keratocytes that are down-regulated in corneal wounds and in vitro. This study used cultures of primary bovine keratocytes to define factors affecting KSPG expression in vitro. KSPG metabolically labeled with [(35)S]sulfate decreased during the initial 2-4 days of culture in quiescent cultures with low serum concentrations (0.1%). Addition of fetal bovine serum, fibroblast growth factor-2 (FGF-2), transforming growth factor beta, or platelet derived growth factor all stimulated cell division, but only FGF-2 stimulated KSPG secretion. Combined with serum, FGF-2 also prevented serum-induced KSPG down-regulation. KSPG secretion was lost during serial subculture with or without FGF-2. Expression of KSPG core proteins (lumican, mimecan, and keratocan) was stimulated by FGF-2, and steady state mRNA pools for these proteins, particularly keratocan, were significantly increased by FGF-2 treatment. KSPG expression therefore is supported by exogenous FGF-2 and eliminated by subculture of the cells in presence of serum. FGF-2 stimulates KSPG core protein expression primarily through an increase in mRNA pools.

Effect of epithelial debridement on glycosaminoglycan synthesis by human corneal explants

The purpose of the present work was to investigate the effects of mechanical epithelial debridement upon glycosaminoglycan synthesis by human corneal explants. Corneal explants were maintained under tissue culture conditions for 2-72 days and the glycosaminoglycans synthesized in 24 h were metabolically labeled by addition of 35S-sulfate to the culture medium. These compounds were isolated from the tissue explants and analyzed by a combination of agarose gel electrophoresis and enzymatic degradation with specific mucopolysaccharidases. The glycosaminoglycans synthesized by isolated epithelial cells and by corneas previously submitted to epithelial cell debridement were compared to controls. Keratan sulfate (26 kDa) and dermatan sulfate (43 kDa) were the main corneal glycosaminoglycans, each one corresponding to about 50% of the total. Nevertheless, the main 35S-labeled glycosaminoglycan was 35S-dermatan sulfate (73%), with smaller amounts of 35S-keratan sulfate (15%) and 35S-heparan sulfate (12%), suggesting a lower synthesis rate for keratan sulfate. The main glycosaminoglycan synthesized by isolated epithelial cells was heparan sulfate. The removal of epithelial layer caused a decrease in heparan sulfate labeling and induced the synthesis of dermatan sulfate by stromal cells. This increased synthesis of dermatan sulfate suggests a relationship between epithelium and stroma and could be related to the corneal opacity that may appear after epithelial cell debridement.

CxGELSIX: a novel preparation of type VI collagen with possible use as a biomaterial

PURPOSE

This study was initiated to evaluate tissue acceptance and stability of a novel type VI collagen preparation (CxGelsix) as a biomaterial in the rabbit corneal stroma. We hypothesized that CxGelsix, embedded intrastromally, does not have any adverse affect on surrounding corneal tissues, and remains intact in the presence of an acute inflammatory reaction during corneal wound healing.

METHODS

Type VI collagen was extracted and purified from rabbit corneal stroma under nondenaturing conditions. This preparation, Gelsix, was concentrated and cross-linked with polyethylene glycol to produce a transparent film (CxGelsix). Discs of CxGelsix, 4.0-mm diameter, 9- to 35-microm thick were implanted intrastromally and clinically examined periodically for 4 months. In another experiment, implantation of CxGelsix, 2.0-mm-diameter, was followed by corneal wounding adjacent to the implant and examined clinically for 30 weeks. At the end of these periods, the tissues from these experiments were processed for light and transmission electron microscopy.

RESULTS

An intralamellar 4.0-mm-diameter disc of CxGelsix does not alter the structure of corneal epithelium above the implant, suggesting normal transport of nutrients through CxGelsix. Moreover, no structural abnormalities were seen in the rest of the cornea, and the cornea remains transparent. Although the cornea accepts the presence of CxGelsix disc as judged by clinical criteria, gradual degradation of the implant is seen ultrastructurally. CxGelsix is remarkably stable despite its exposure to endogenous enzymes during inflammation and wound healing. Partial degradation of the implant occurs only after many months, and it is gradually replaced with bundles of fine collagen fibrils reminiscent of normal cornea.

CONCLUSION

The results of this study suggest that CxGelsix is potentially useful as a biomaterial.

Proteoglycan metabolism during repair of the ruptured medial collateral ligament in skeletally mature rabbits

The metabolism of the chondroitin/dermatan sulfate (CS/DS) proteoglycans (PGs) decorin and biglycan is markedly altered during short-term (3-6 weeks) and long-term (40 weeks-2 years) repair of surgically ruptured medial collateral ligaments from mature rabbits. A PG-rich extracellular matrix accumulates in injury gaps by 3 weeks postsurgery and extends into tissue regions containing the original ligaments, and elevated PG levels remain apparent up to 2 years postinjury. CS/DS PGs were prepared from such ligaments and identified after SDS-polyacrylamide gel electrophoresis by Alcian blue staining or immunoblotting. In normal ligaments, decorin is the most abundant proteoglycan (accounting for approximately 80% of the total); the remainder is biglycan and a large PG, possibly versican. In repairing ligaments, decorin is barely detected, but instead a large proteoglycan and abundant amounts of biglycan accumulate. Biglycan is present in two forms in repairing ligaments, and they can be separated on SDS-PAGE into 200- and 140-kDa forms. The slower migrating species is absent in normal ligaments and may represent a different glycoform (containing either a single or two short chondroitin/dermatan sulfate chains) of biglycan. Alteration in PG expression and posttranslational processing during medial collateral ligament repair are similar to those reported for repair and scar formation of other connective tissues. The accumulation of biglycan observed here may interfere with proper collagen network remodeling and may lead to persistent inflammatory and matrix turnover processes, thus preventing restoration of a long-term functional ligament tissue.

Corneal stromal wound healing in refractive surgery: the role of myofibroblasts

While laser and incisional refractive surgery offer the promise to correct visual refractive errors permanently and predictably, variability and complications continue to hinder wide-spread acceptance. To explain variations, recent studies have focused on the role of corneal wound healing in modulating refractive outcomes. As our understanding of the corneal response to refractive surgery broadens, it has become apparent that the response of one cell, the corneal stromal keratocyte, plays a pivotal role in defining the results of refractive surgery. Studies reviewed herein demonstrate that injury-induced activation and transformation of keratocytes to myofibroblasts control the deposition and organization of extracellular matrix in corneal wounds. Myofibroblasts establish an interconnected meshwork of cells and extracellular matrix that deposits new matrix and contracts wounds using a novel and unexpected "shoe-string-like" mechanism. Transformation of keratocytes to myofibroblasts is induced in culture by transforming growth factor beta (TGFbeta) and blocked in vivo by antibodies to TGFbeta. Overall, myofibroblast appearance in corneal wounds is associated with wound contraction and regression following incisional keratotomy and the development of "haze" or increased scattered light following laser photorefractive keratectomy (PRK). By contrast, absence of myofibroblasts is associated with continued widening of wound gape and progressive corneal flattening after incisional procedures. Based on these studies, we have arrived at the inescapable conclusion that a better understanding of the cellular and molecular biology of this one cell is required if refractive surgery is ever to achieve predictable and safe refractive results.

Mitomycin C reduces scar formation after excimer laser (193 nm) photorefractive keratectomy in rabbits

Sixteen eyes of eight rabbits were randomised to either mitomycin C or Balanced Salt Solution (BSS) application after photorefractive keratectomy (PRK). Regular examinations of wound healing and haze were performed with the slit lamp. The animals were killed between 1 and 26 weeks after treatment, and the corneas examined by light and electron microscopy. While the grade of haze showed no relevant differences between the two groups, scar tissue was found histologically in the mitomycin group in only 1 of 8 corneas compared with 5 of 8 in the BSS group. A marked reduction in keratocytes in all mitomycin-treated corneas and a normal density of keratocytes in the BSS group was observed. Mitomycin reduced the number of keratocytes in the treated corneas, leading to less scar formation but not to a reduction in haze. Since no morphological correlate has been found, haze remains unexplained in the mitomycin-treated corneas.

Localization of type XII collagen in normal and healing rabbit cornea by in situ hybridization

To identify the cell types responsible for type XII collagen synthesis in normal and healing rabbit cornea, a partial cDNA sequence of rabbit type XII collagen, obtained from an adult rabbit cornea cDNA library, was used to develop highly specific oligonucleotide probes for Northern blot analysis and in situ hybridization. Approximately 2000 bases of a type XII collagen 2.2 kb cDNA clone were sequenced. Comparative sequence analysis of the bases showed a 74% identity with chick alpha 1 (XII) chain of type XII collagen. The deduced amino acid sequence indicated a 72% identity with chick type XII collagen. Northern blot analysis showed that cultures of cornea stromal and endothelial cells each contain two RNA species, greater than 10 kb, that hybridize to rabbit type XII collagen oligonucleotide probes. Although normal stromal cells failed to show type XII collagen mRNA, normal endothelial cells contain mRNA for this collagen. These results indicate that endothelium of normal rabbit cornea has a potential to synthesize type XII collagen. During corneal wound healing, both endothelium-derived and stroma-derived cells in the developing scar tissue contained type XII mRNA. In view of the known presence of type XII collagen in corneal stromas from chick and mouse, the distribution of mRNA in normal cornea is puzzling.

Structure of corneal scar tissue: an X-ray diffraction study

Full-thickness corneal wounds (2 mm diameter) were produced in rabbits at the Schepens Eye Research Institute, Boston. These wounds were allowed to heal for periods ranging from 3 weeks to 21 months. The scar tissue was examined using low- and wide-angle x-ray diffraction from which average values were calculated for 1) the center-to-center collagen fibril spacing, 2) the fibril diameter, 3) the collagen axial periodicity D, and 4) the intermolecular spacing within the collagen fibrils. Selected samples were processed for transmission electron microscopy. The results showed that the average spacing between collagen fibrils within the healing tissue remained slightly elevated after 21 months and there was a small increase in the fibril diameter. The collagen D-periodicity was unchanged. There was a significant drop in the intermolecular spacing in the scar tissues up to 6 weeks, but thereafter the spacing returned to normal. The first-order equatorial reflection in the low-angle pattern was visible after 3 weeks and became sharper and more intense with time, suggesting that, as healing progressed, the number of nearest neighbor fibrils increased and the distribution of nearest neighbor spacings reduced. This corresponded to the fibrils becoming more ordered although, even after 21 months, normal packing was not achieved. Ultrastructural changes in collagen fibril density measured from electron micrographs were consistent with the increased order of fibril packing measured by x-ray diffraction. The results suggest that collagen molecules have a normal axial and lateral arrangement within the fibrils of scar tissue. The gradual reduction in the spread of interfibrillar spacings may be related to the progressive decrease in the light scattered from the tissue as the wound heals.

Stromal fibroblasts synthesize collagenase and stromelysin during long-term tissue remodeling

The process of connective tissue remodeling is an important mechanism contributing to tissue morphogenesis in development and homeostasis. Although it has long been known that remodeling tissues actively mediate collagenolysis, little is understood about the molecular mechanisms controlling this cell-regulated process. In this study, we examined the biosynthesis of collagenase and the related metalloproteinase, stromelysin, during remodeling of repair tissue deposited after mechanical injury to the rabbit cornea. Neither enzyme was synthesized by uninjured corneas; however, synthesis and secretion was detectable within one day after injury. Collagenase accumulated in its latent form while stromelysin appeared to be partially activated. Enzymes were synthesized by cells having a fibroblast phenotype. These cells were found within the stroma. New synthesis was correlated with accumulation of enzyme-specific mRNA. Highest levels of enzyme synthesis were observed in the repair tissue. However, stromal cells outside of the repairing area also synthesized both enzymes. The level of synthesis decreased in a gradient radiating from the repair tissue. Total synthetic levels in a given area of cornea were dependent on both the number of cells expressing enzyme and the rate of enzyme synthesis. Synthesis of collagenase was detected in repair tissue as long as nine months after injury. Our findings provide direct support for the hypothesis that new collagenase synthesis by cells in repair tissue is the first step in collagen degradation during long-term tissue remodeling.

A morphological study of rabbit corneas after laser keratectomy

We have examined the morphology of the collagen and proteoglycans in rabbit corneas that have undergone excimer laser photorefractive keratectomy using a clinical, 193 nm excimer laser. The photoablation was carried out to a stromal depth of 100 microns and a diameter of 6 mm. All ablated corneas developed a haze that was most intense between week 4 and week 8 and which showed no improvement after week 16. The corneas were stained with the cationic dye cuprolinic blue to visualise proteoglycans and were then processed for transmission electron microscopy. The ultrastructural location of proteoglycans (keratan sulphate and dermatan sulphate) was observed in the corneal wounds at different time intervals. Corneas that had undergone steroid treatment post-operatively were also examined. In the healing tissue proteoglycan filaments of abnormal size were observed, which became most prominent after 2 weeks. As healing progressed these abnormal filaments decreased but after 45 weeks some were still present, indicating that the proteoglycan content had not returned to normal.

Proteoglycan and collagen morphology in superficially scarred rabbit cornea

We have examined the changes in collagen and proteoglycan morphology in superficial lamellar keratectomy wounds produced in rabbit corneas. The ultrastructural location within the tissue of keratan sulphate and chondroitin sulphate proteoglycans was demonstrated using the cationic dye Cuprolinic Blue under critical electrolyte conditions. Large proteoglycan filaments (up to 500 nm long) appeared in the early stages of wound healing; these were most common after two weeks' wound healing, after which they decreased both in number and size. At these early stages of scar formation, spaces containing proteoglycans were present amongst bundles of collagen fibrils. As proteoglycans play an important role in controlling corneal hydration, the presence of the large proteoglycan-filled spaces would result in an abnormally high water content which is found in early scar tissue.

Differential roles for two gelatinolytic enzymes of the matrix metalloproteinase family in the remodelling cornea

We have documented changes in collagenolytic/gelatinolytic enzymes of the matrix metalloproteinase family (MMP) in remodelling rabbit cornea. MMP-2 (65 kDa gelatinase) in the proenzyme form is synthesized by the cells of the normal corneal stroma. After keratectomy the level of MMP-2 is increased in the stroma and enzyme appears in both pro- and activated forms. In addition, corneal cells synthesize MMP-9 (92 kDa gelatinase) in the proenzyme form after keratectomy; expression occurs in both the epithelial as well as stromal corneal layers. Changes in expression of both enzymes are precisely localized to the repairing portion of cornea, but demonstrate important differences in timing that correlate with the timing of specific events of matrix remodelling. Our data suggest that each of the gelatinases plays a different role in tissue remodelling after injury. We hypothesize that MMP-2 performs a surveillance function in normal cornea, catalyzing degradation of collagen molecules that occasionally become damaged. After wounding, this enzyme appears to participate in the prolonged process of collagen remodelling in the corneal stroma that eventually results in functional regeneration of the tissue. MMP-9 expression does not correlate with stromal remodelling, but we suggest that the enzyme might play a part in controlling resynthesis of the epithelial basement membrane.

Comparison of collagen degradation and synthesis in the peripheral and central areas of experimental corneal grafts

The distribution of collagen degradation and synthesis (turnover) between the central and peripheral areas of clear corneal allografts was determined, following transplantation of 6-mm [3H]proline-labelled corneal buttons from extensively pre-labelled, young rabbits into 13 nonradioactive mature rabbits. Control, contralateral 6-mm corneal buttons of each radioactive pair were frozen at -70 degrees C after being trephined into a central 3-mm button and a peripheral ring of cornea. The grafts were removed 10-100 days after the operation and trephined similarly to the paired controls. The central and peripheral areas of the graft and control corneal buttons were analysed separately for total hydroxyproline and radioactivity. A significant degradation of old collagen (as seen by the loss of total radioactivity) occurred early in both the central (19.4 +/- 6.1%) and peripheral (12.3 +/- 7.5%) areas of the grafted corneas, compared to the corresponding areas of the contralateral controls. A significant increase in new, non-radioactive collagen occurred in both the central (22.0 +/- 7.6%) and peripheral areas (30.6 +/- 6.1%); however, a significant net increase in total collagen occurred only in the peripheral area (18.3 +/- 11.4). Loss of original (old) collagen was significantly greater in the central area than in the peripheral area; whereas, the increase in new collagen was significantly greater in the peripheral area than the central area, accounting for a significant increase in total collagen. There was no correlation in collagen turnover with time after surgery. Collagen turnover occurs acutely throughout the clear corneal graft and thereafter is stable.

Wound healing of rabbit cornea in organ culture

A 2 mm perforating cut was made in the central part of 28 rabbit corneas. The corneas were then kept in organ culture from 3 h to 21 days. The healing process was studied by light microscopy and by transmission and scanning electron microscopy. The swelling of the corneal stroma adjacent to the wound created contact between the opposing wound edges within 3 h. Epithelial and endothelial cell migration into the wound was observed after 23 h. Penetration of cells through the perforation was avoided because the opposing wound edges were touching each other, thereby creating contact inhibition of cell proliferation. Cell cover of both the anterior and the posterior surface was completed within 3 days.

Collagen degradation and synthesis in experimental corneal grafts

Thirty-two weanling New Zealand white rabbits were labelled repeatedly with [2,3-3-H]-proline for 4 weeks. Four weeks after the end of labelling, 32 rabbits underwent bilateral 6 mm trephinations for donor purposes. One control cornea of each pair was frozen at -70 degrees C. The contralateral corneas were used for autografts, allografts, and xenografts. Grafts were observed from 7-200 days, then retrephined out for determination of loss in total radioactivity in hydroxyproline (specific for collagen), increase in newly synthesized collagen, and net change in collagen mass, by comparing with the matched, ungrafted control corneas. For all three transplant groups there was a small, but significant, decrease early in total radioactivity (loss of original collagen) and a significant increase in new collagen. These data also indicated that the loss of original collagen was replaced by an equivalent or greater increase in new collagen in all transplant groups. Significant relationships between graft clarity and collagen turnover were noted in both the auto- and allograft groups. The degradation of old collagen was significantly greater in the cloudy vs. the clear grafts; however, there was no significant difference in the increase in new collagen between these groups. A progressive loss of original collagen over time was noted in the cloudy autografts, but not the allografts. A trend toward a progressive increase in new collagen was noted over time in both the cloudy auto- and allografts. No relation for these variables to time, however, was noted in the clear grafts.

Initial healing of the posterior corneal surface following perforating trauma in guinea pig: a scanning electron microscope study

Using a scanning electron microscope, we have studied the healing of the posterior corneal surface after a small perforating trauma. Both corneas of adult guinea pigs were perforated with a needle and studied at various time intervals during recovery. Thirty minutes after the perforation the wound was sealed with a fibrous plug. Leukocytes adhered to the wounded area from the first day of healing on, but usually most of them disappeared after the third day. Fibroblast-like cells were present during the entire healing process from the first day on. Endothelial cells at the margin of the wound lost their hexagonal shape and began to slide over the plug. In 7 days the wound was completely covered by irregularly arranged endothelium. The endothelial cells did not reach their regular arrangement during the 14-day follow-up period.

Proteoglycan changes during restoration of transparency in corneal scars

Corneal scars generated in rabbits by penetrating wounds are initially opaque but become transparent within a year. Previous studies have shown that the corneal stroma consists of proteoglycans and collagen fibrils spaced at regular intervals and that the interfibrillar spaces, the presumed location of proteoglycans, are abnormally large in opaque scars. In the present study, the size and glycosaminoglycan composition of the corneal stromal proteoglycans were determined in corneal scars during the restoration of transparency. The results showed that initially opaque scars which contained the large interfibrillar spaces also contained unusually large chondroitin sulfate proteoglycans with glycosaminoglycan side chains of normal size. These opaque scars also lacked the keratan sulfate proteoglycan but did contain hyaluronic acid. In the 1-year-old scars there was a restoration of normal interfibrillar spacing, and a return to corneal stromal proteoglycans of normal size and composition. These correlations suggest that the corneal stromal proteoglycans may play a fundamental role in regulating corneal collagen fibril spacing.

Histopathology of corneal leukoma and fibroma in the ACL syndrome

Corneal tissues of four persons and a buccal fibroma from one of these persons with ACL syndrome, an autosomal dominant disorder, were evaluated clinically and microscopically. The corneal lesions appeared as a gray epithelial infiltrate over the cornea, destroying Bowman's membrane. Light and electron microscopic images of both types of lesions showed abnormal accumulation of granular mucopolysaccharide material and extensive aberrant orientation of collagen fibers. The authors postulate that the mucopolysaccharide accumulation is involved in the pathogenesis of the syndrome.

Quantitative analysis of collagen from normal developing corneas and corneal scars

We measured the relative solubility of collagen in acetic acid after pepsin digestion and tentatively identified the types of collagen present in corneas of rabbits of various ages and in corneal scar tissue, using hydroxyproline assays and polyacrylamide gel electrophoretic analyses. More than 80% of the collagen in normal developing rabbit cornea was soluble after pepsin treatment; no more than 45% of that in two-week-old corneal scars was soluble. The predominant collagens in normal cornea and healing tissue were types I and AB. Type AB increased from 6% of the total collagen in fetal cornea to 11% in cornea from young adults. Collagen from two-week-old corneal wounds contained 16% type AB. Corneal type AB collagen was less soluble and more resistant to degradation by mammalian collagenase than was type I collagen. Unlike the normal cornea, in healing tissue the relative rate of synthesis of type I to type AB collagens did not correspond to their deposition. These results suggest a basic alteration in the molecular structure of the corneal scar, which may be instrumental in preventing the healing tissue from producing a normal, functioning organ.

[Histochemical investigations of acid mucopolysaccharides in corneal wound healing following superficial keratectomy (author's transl)]

In rabbit eyes there was made a central superficial defect into the cornea with a 7.2-mm Trepan/0.35 mm depth. To show alterations in the pattern of acid mucopolysaccharides during wound healing histochemical investigation was carried out in a series of toluidine blue preparations of pH 3,8-5,2 of unfixed frozen corneal sections. Table 1 shows the results of metachromasia after superficial keratectomy. On the first postoperative day metachromasia was extremely reduced in the area of keratectomia. Metachromasia in the surrounding cornea became normal after three days while in the area of corneal defect it lasted a long time, until the normal pattern of acid mucopolysaccharides is reconstructed. After three months the metachromasia was still reduced in the superficial parts of the corneal stroma (Fig. 3). After six months cornea was transparent in the area of ketatectomy with no alterations in the pattern of mucopolysaccharides. This showed the long alterations in the pattern of mucopolysaccharides. This showed the long healing procedure in bradytrophic corneal tissue.

Use of platelet-fibrinogen-thrombin mixture to seal experimental penetrating corneal wounds

A platelet-fibrinogen-thrombin mixture utilizing autologous platelets was studied for its potential to seal perforating corneal wounds. In rabbits, the mixture demonstrated sufficient adhesive properties to permit 75% of penetrating keratoplasties to remain in place without the use of sutures. All 12 grafts held initially; after 48 h three grafts extruded. In control eyes only two out of 12 (16%) of the corneal grafts remained in place. The material is simple to prepare and apply and is not toxic to the cornea. It does not cause inflammation or lid irritation. Intraocular complications observed with its use were transient anterior synechiae and retrocorneal membrane formation.