A synchrotron X-ray diffraction study of bovine cornea stained with cupromeronic blue

The effect of bacteriochlorophyll derivative WST-D and near infrared light on the molecular and fibrillar architecture of the corneal stroma

A cross-linking technique involving application of Bacteriochlorophyll Derivative WST-11 mixed with dextran (WST-D) to the epithelium-debrided cornea and illumination with Near Infrared (NIR), has been identified as a promising therapy for stiffening pathologically weakened corneas. To investigate its effect on corneal collagen architecture, x-ray scattering and electron microscopy data were collected from paired WST-D/NIR treated and untreated rabbit corneas. The treated eye received 2.5 mg/mL WST-D and was illuminated by a NIR diode laser (755 nm, 10 mW/cm²). An increase in corneal thickness (caused by corneal oedema) occurred at 1-day post-treatment but resolved in the majority of cases within 4 days. The epithelium was fully healed after 6–8 days. X-ray scattering revealed no difference in average collagen interfibrillar spacing, fibril diameter, D-periodicity or intermolecular spacing between treated and untreated specimens. Similarly, electron microscopy images of the anterior and posterior stroma in healed WST-D/NIR corneas and untreated controls revealed no obvious differences in collagen organisation or fibril diameter. As the size and organisation of stromal collagen is closely associated with the optical properties of the cornea, the absence of any large-scale changes following treatment confirms the potential of WST-D/NIR therapy as a means of safely stiffening the cornea.

Intrastromal Injection of Hyaluronidase Alters the Structural and Biomechanical Properties of the Corneal Stroma

Purpose

Glycosaminoglycans (GAGs) are important components of the corneal stroma, and their spatiotemporal arrangement regulates the organization of collagen fibrils and maintains corneal transparency. This study was undertaken to determine the consequences of hyaluronidase (HAse) injected into the corneal stroma on stromal stiffness and ultrastructure.

Methods

Equal volumes of HAse or balanced salt solution (vehicle) were injected intrastromally into the corneas of New Zealand white rabbits. Ophthalmic examination and multimodal imaging techniques, including Fourier-domain optical coherence tomography and in vivo confocal microscopy (IVCM), were performed at multiple time points to evaluate the impact of HAse treatment in vivo. Atomic force microscopy and transmission electron microscopy (TEM) were used to measure corneal stiffness and collagen's interfibrillar spacing, respectively.

Results

Central corneal thickness progressively decreased after HAse injection, reaching its lowest value at day 7, and then returned to normal by day 42. The HAse did not impact the corneal endothelium but transiently altered keratocyte morphology at days 1 and 7, as measured by IVCM. HAse-injected corneas became stiffer by day 1 postinjection, were stiffest at day 7, and returned to preinjection values by day 90. Changes in stromal stiffness correlated with decreased interfibrillar spacing as measured by TEM.

Conclusions

Degradation of GAGs by HAse decreases the corneal thickness and increases stromal stiffness through increased packing of the collagen fibrils in a time-dependent manner.

Translational Relevance

Intrastromal HAse injection appears relatively safe in the normal cornea, but its impact on corneal biomechanics and structure under pathologic conditions requires further study.

Role of Decorin Core Protein in Collagen Organisation in Congenital Stromal Corneal Dystrophy (CSCD)

The role of Decorin in organising the extracellular matrix was examined in normal human corneas and in corneas from patients with Congenital Stromal Corneal Dystrophy (CSCD). In CSCD, corneal clouding occurs due to a truncating mutation (c.967delT) in the decorin (DCN) gene. Normal human Decorin protein and the truncated one were reconstructed in silico using homology modelling techniques to explore structural changes in the diseased protein. Corneal CSCD specimens were also examined using 3-D electron tomography and Small Angle X-ray diffraction (SAXS), to image the collagen-proteoglycan arrangement and to quantify fibrillar diameters, respectively. Homology modelling showed that truncated Decorin had a different spatial geometry to the normal one, with the truncation removing a major part of the site that interacts with collagen, compromising its ability to bind effectively. Electron tomography showed regions of abnormal stroma, where collagen fibrils came together to form thicker fibrillar structures, showing that Decorin plays a key role in the maintenance of the order in the normal corneal extracellular matrix. Average diameter of individual fibrils throughout the thickness of the cornea however remained normal.

The structural and optical properties of type III human collagen biosynthetic corneal substitutes

The structural and optical properties of clinically biocompatible, cell-free hydrogels comprised of synthetically cross-linked and moulded recombinant human collagen type III (RHCIII) with and without the incorporation of 2-methacryloyloxyethyl phosphorylcholine (MPC) were assessed using transmission electron microscopy (TEM), X-ray scattering, spectroscopy and refractometry. These findings were examined alongside similarly obtained data from 21 human donor corneas. TEM demonstrated the presence of loosely bundled aggregates of fine collagen filaments within both RHCIII and RHCIII-MPC implants, which X-ray scattering showed to lack D-banding and be preferentially aligned in a uniaxial orientation throughout. This arrangement differs from the predominantly biaxial alignment of collagen fibrils that exists in the human cornea. By virtue of their high water content (90%), very fine collagen filaments (2–9 nm) and lack of cells, the collagen hydrogels were found to transmit almost all incident light in the visible spectrum. They also transmitted a large proportion of UV light compared to the cornea which acts as an effective UV filter. Patients implanted with these hydrogels should be cautious about UV exposure prior to regrowth of the epithelium and in-growth of corneal cells into the implants.

Corneal structure and transparency

The corneal stroma plays several pivotal roles within the eye. Optically, it is the main refracting lens and thus has to combine almost perfect transmission of visible light with precise shape, in order to focus incoming light. Furthermore, mechanically it has to be extremely tough to protect the inner contents of the eye. These functions are governed by its structure at all hierarchical levels. The basic principles of corneal structure and transparency have been known for some time, but in recent years X-ray scattering and other methods have revealed that the details of this structure are far more complex than previously thought and that the intricacy of the arrangement of the collagenous lamellae provides the shape and the mechanical properties of the tissue. At the molecular level, modern technologies and theoretical modelling have started to explain exactly how the collagen fibrils are arranged within the stromal lamellae and how proteoglycans maintain this ultrastructure. In this review we describe the current state of knowledge about the three-dimensional stromal architecture at the microscopic level, and about the control mechanisms at the nanoscopic level that lead to optical transparency.

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.

Electron tomography reveals multiple self-association of chondroitin sulphate/dermatan sulphate proteoglycans in Chst5-null mouse corneas

The spatial distribution of collagen fibrils in the corneal stroma is essential for corneal transparency and is primarily regulated by extrafibrillar proteoglycans, which are multi-functional polymers that interact with hybrid type I/V collagen fibrils. In order to understand more about proteoglycan organisation and collagen associations in the cornea, three-dimensional electron microscopy reconstructions of collagen-proteoglycan interactions in the anterior, mid and posterior stroma from a Chst5 knockout mouse, which lacks a keratan sulphate sulphotransferase, were obtained. Both longitudinal and transverse section show sinuous, oversized proteoglycans with near-periodic, orthogonal off-shoots. In many cases, these proteoglycans traverse over 400nm of interfibrillar space interconnecting over 10 collagen fibrils. The reconstructions suggest that multiple chondroitin sulphate/dermatan sulphate proteoglycans have aggregated laterally and, possibly, end-to-end, with orthogonal extensions protruding from the main electron-dense stained filament. We suggest possible mechanisms as to how sulphation differences may lead to this increase in aggregation of proteoglycans in the Chst5-null mouse corneal stroma and how this relates to proteoglycan packing in healthy corneas.

Structural collagen alterations in macular corneal dystrophy occur mainly in the posterior stroma

PURPOSE

Collagen fibrils in the corneal stroma in macular corneal dystrophy, on average, are more closely spaced than in the normal cornea. This study was conducted to investigate if this occurs uniformly across the stroma or is more prevalent at certain stromal depths.

METHODS

Microbeam synchrotron X-ray fiber diffraction patterns were obtained in 25 microm steps across the whole thickness of a thin strip of a macular corneal dystrophy cornea obtained at keratoplasty. Data were analyzed for mean collagen interfibrillar spacing at all positions. Serum was analyzed immunochemically to determine immunophenotype, and transmission electron microscopy was carried out to visualize stromal ultrastructure.

RESULTS

Keratan sulphate was not detectable in blood serum, classifying the disease as macular corneal dystrophy type I. Collagen interfibrillar spacing dropped linearly with stromal depth from the anterior to posterior cornea, measuring 5-10% less in the posterior 100 microm of the MCD stroma compared to the anterior 100 microm (p < 0.001). Isolated pockets of collagen fibrils with unusually large diameters were identified in the deep stroma.

CONCLUSIONS

Collagen fibril spacing is reduced and large-diameter collagen fibrils are seen in macular corneal dystrophy type I, with the deep stroma affected more. We speculate that the ultrastructural abnormalities are more prevalent in the posterior stroma because the structural influence of sulphated keratan sulphate glycosaminoglycans/proteoglycans is high in this region of the cornea.

Structural interactions between collagen and proteoglycans are elucidated by three-dimensional electron tomography of bovine cornea

Interactions between collagens and proteoglycans help define the structure and function of extracellular matrices. The cornea, which contains proteoglycans with keratan sulphate or chondroitin/dermatan sulphate glycosaminoglycan chains, is an excellent model system in which to study collagen-proteoglycan structures and interactions. Here, we present the first three-dimensional electron microscopic reconstructions of the cornea, and these include corneas from which glycosaminoglycans have been selectively removed by enzymatic digestion. Our reconstructions show that narrow collagen fibrils associate with sulphated proteoglycans that appear as extended, variable-length linear structures. The proteoglycan network appears to tether two or more collagen fibrils, and thus organize the matrix with enough spatial specificity to fulfill the requirements for corneal transparency. Based on the data, we propose that the characteristic pseudohexagonal fibril arrangement in cornea is controlled by the balance of a repulsive force arising from osmotic pressure and an attractive force due to the thermal motion of the proteoglycans.

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.

Corneal collagen-its role in maintaining corneal shape and transparency

Corneal collagen has a number of properties that allow it to fulfil its role as the main structural component within the tissue. Fibrils are narrow, uniform in diameter and precisely organised. These properties are vital to maintain transparency and to provide the biomechanical prerequisites necessary to sustain shape and provide strength. This review describes the structure and arrangement of corneal collagen from the nanoscopic to the macroscopic level, and how this relates to the maintenance of the form and transparency of the cornea.

Proteoglycan-collagen interactions

Among the most important events in connective tissue physiology are the nucleation, growth and calcification of collagen fibrils. It has been speculated that all are associated with, or even controlled by collagen-proteoglycan interactions. We therefore developed methods for investigating these associations in tissues, particularly for understanding their significance for type I collagen, the commonest form of collagen in the body, especially predominant in bone. Using an electron-dense dye, Cupromeronic blue, in the 'critical electrolyte concentration' mode, and digestion by hyaluronidase, chondroitinase ABC or keratanase, supported by biochemical analyses, we found that dermatan sulphate proteoglycan of soft connective tissue (skin, tendon, cornea) was regularly and orthogonally arrayed at the fibril surface, at the d or e band. Keratan sulphate proteoglycan in the cornea associates orthogonally at the a and c bands. Bone, demineralized by a non-aqueous technique which retains proteoglycans in the tissue, does not contain orthogonal arrays; the interfibrillar proteoglycan filaments are oriented parallel to the fibril axis. The main proteoglycan in bone is chondroitin sulphate-rich. There are thus four separate specific binding sites on type I collagen fibrils, each one associating with one particular proteoglycan, and apparently no other. This implies that there are two corresponding binding sites in each proteoglycan. Available evidence shows that there are two species of small dermatan sulphate and keratan sulphate proteoglycans. It is suggested that each species is specific for its own band (a, c, d or e). Hyaluronate and chondroitin sulphate are probably mainly interfibrillar, acting in a space-filling capacity.

Differential expression of the keratan sulphate proteoglycan, keratocan, during chick corneal embryogenesis

Keratan sulphate (KS) proteoglycans (PGs) are key molecules in the connective tissue matrix of the cornea of the eye, where they are believed to have functional roles in tissue organisation and transparency. Keratocan, is one of the three KS PGs expressed in cornea, and is the only one that is primarily cornea-specific. Work with the developing chick has shown that mRNA for keratocan is present in early corneal embryogenesis, but there is no evidence of protein synthesis and matrix deposition. Here, we investigate the tissue distribution of keratocan in the developing chick cornea as it becomes compacted and transparent in the later stages of development. Indirect immunofluorescence using a new monoclonal antibody (KER-1) which recognises a protein epitope on the keratocan core protein demonstrated that keratocan was present at all stages investigated (E10-E18), with distinct differences in localisation and organisation observed between early and later stages. Until E13, keratocan appeared both cell-associated and in the stromal extracellular matrix, and was particularly concentrated in superficial tissue regions. By E14 when the cornea begins to become transparent, keratocan was located in elongate arrays, presumably associated along collagen fibrils in the stroma. This fibrillar label was still concentrated in the anterior stroma, and persisted through E15-E18. Presumptive Bowman's layer was evident as an unlabelled subepithelial zone at all stages. Thus, in embryonic chick cornea, keratocan, in common with sulphated KS chains in the E12-E14 developmental period, exhibits a preferential distribution in the anterior stroma. It undergoes a striking reorganisation of structure and distribution consistent with a role in relation to stromal compaction and corneal transparency.

Skewness angle of interfibrillar proteoglycans increases with applied load on mitral valve chordae tendineae

In highly aligned connective tissues, such as tendon, collagen fibrils are linked together by proteoglycans (PGs). Recent mechanical and theoretical studies on tendon micromechanics have implied that PGs mediate mechanical interactions between adjacent collagen fibrils. We used transmission electron microscopy to observe the collagen fibril-PG interactions in porcine mitral valve chordae under variable loading conditions and found that PGs attached to collagen fibrils perpendicularly in the load-free situation, and became skewed when the chordae were loaded. The average skewness angle of PGs increased with the applied load, and hence the strain in the chordae. The observation of PG skewing with the application of load demonstrates that, in mitral valve chordae, interfibrillar slippage occurs and that PGs play a role in fibril-to-fibril interaction and likely transfer force. The results of this study provide new insights into the mechanical role of PGs and support some recent theoretical models.

The organization of collagen in the corneal stroma

The cornea has evolved to fulfil the dual functions of enclosing and protecting the inner contents of the eye, and focussing light onto the retina with minimum scatter and optical degradation. It does this by means of the arrangement of the constituent collagen fibrils, an arrangement that is unique in connective tissues. This article reviews our current knowledge about the detailed organization of collagen in the corneal stroma, and presents new data suggesting that a significant proportion of collagen fibrils running across the cornea, change direction near the limbus and fuse with the circumferential limbal collagen.

An X-ray scattering investigation of corneal structure in keratocan-deficient mice

The transparency of the cornea has been closely linked with the characteristic size and arrangement of its constituent collagen fibrils. This arrangement, in turn, is thought to depend on interactions with intervening matrix proteoglycans. The purpose of this investigation was to examine fibrillar collagen organisation in the corneas of mice homozygous for a null mutation in keratocan, a keratan sulfate-containing proteoglycan. Low-angle synchrotron X-ray scattering techniques were used. We found that keratocan-deficient mice had corneal collagen fibrils with significantly larger diameters than those in wild-type littermates. Furthermore, there was an increase in the centre-to-centre spacing of the collagen fibrils that was accompanied by a decrease in nearest-neighbour fibrillar order. We hypothesise that a lack of keratocan might lower the number of keratan sulfate proteoglycans that associate with collagen, leading to alterations in their diameters and spatial arrangements. Alternatively, it might change the osmotic balance between the inside and outside of fibrils, causing them to swell and move further apart.

Corneal and scleral collagens--a microscopist's perspective

This paper reviews our existing understanding of the distribution and organisation of collagen types within the corneal and scleral stroma from a microscopical perspective. The contribution of various types of light microscopy, electron microscopy and atomic force microscopy to this field are separately discussed. Light microscopy was used in the earliest studies of the cornea and lead to the first description of the lamellar structure of the stroma. More recently polarised light microscopy has been used to obtain specific information on fibril orientation within individual lamellae. Light microscope immunolabelling techniques have been utilised to determine the distribution of several collagen types within the cornea and sclera, while recent developments in confocal microscopy have allowed detailed observations to be made on live cornea. Scanning electron microscopy has proved useful in determining the 3D organisation of lamellae within both corneal and scleral stroma. The transmission electron microscope was responsible for first revealing the regular diameter and high degree of order of the collagen fibrils present in the corneal stroma and contrasting this with the irregular diameter of fibrils present in sclera. This finding lead directly to the formulation of a theory of corneal transparency based on the uniformity of fibril diameter and packing. The use of specialised stains such as cuprolinic blue allowed direct observation of the glycosaminoglycan chains on proteoglycan molecules in cornea and sclera. These images allowed the binding sites of the proteoglycans along the collagen fibrils to be identified and provided convincing evidence for the importance of the proteoglycan molecules in collagen fibril organisation. Immunogold labelling has been used to map the distribution of several collagen types within the corneal and scleral stroma at the ultrastructural level and provided critical evidence for the role of type V collagen in the regulation of fibril diameter within the cornea. Specialised freezing-etching techniques have revealed the surface features of the collagen fibrils in corneal stroma, indicating clearly the presence of crossbridge structures between fibrils. The technique of rotary shadowing has been used to determine the conformation of several collagen types. In more recent years atomic force microscopy has been applied to the study of the corneal stroma. It has largely confirmed the observations made by the transmission electron microscope and provided independent evidence of crossbridge structures between the collagen fibres in cornea and sclera. The full potential of this technique has yet to be realised.

The use of X-ray scattering techniques to determine corneal ultrastructure

The manner in which X-rays are scattered or diffracted by the cornea provides us with valuable insights into the fine structure of the corneal stroma. This is because when X-rays pass through a cornea a diffraction pattern is formed due to scattering from regularly arranged collagen molecules and fibrils that comprise the bulk of the stromal matrix. Collagen provides the cornea with most of its strength, and its proper organisation is believed to be important for tissue transparency. Ever since 1978, when the first X-ray diffraction patterns were obtained from the cornea using radiation from a powerful synchrotron source, biophysicists have recorded and analysed a huge number of X-ray diffraction patterns from many different corneas. This article aims to explain the ideas that underpin our use of X-ray diffraction to investigate corneal ultrastructure, and show how the knowledge gained to date has far-reaching implications for tissue biomechanics, disease changes and transparency.

Human corneal keratan sulfates

The keratan sulfate-containing proteoglycans were isolated from fourteen pooled human corneas (thirteen from 61- to 86-year-olds, plus one from a 12-year-old). These proteoglycans were subjected to digestion with the enzyme keratanase II, and the released oligosaccharides, which included nonreducing termini and repeat region oligosaccharides but not linkage regions, were reduced with alkaline borohydride and identified on two separate ion-exchange columns. Both of the latter had been calibrated with samples, most of which had been derived from bovine corneal keratan sulfate (Tai, G.-H., Huckerby, T. N., and Nieduszynski, I. A. (1996) J. Biol. Chem. 271, 23535-23546) and all of which had been fully characterized by NMR spectroscopic analysis. The capping structures identified in human corneal keratan sulfates occurred in the relative proportions: NeuAcalpha(2-6)- >NeuAcalpha(2-3)- >GalNAc(S)beta(1-3)-. The other groups of capping structures which had been identified in bovine corneal keratan sulfate, i.e. NeuGcalpha(2-3)-, NeuGcalpha(2-6)-, GlcNAc(S)beta(1-3)- were absent, although the possibility of the presence of some Galalpha(1-3)- structures could not be excluded. In addition, the human sample showed significantly higher levels of alpha(1-3)-fucosylated repeat region structures than did the bovine sample, and it is not clear whether this reflects a species or age dependence as the bovine corneas were from young animals, whereas the human corneas were predominantly from an older group. The charge densities and keratan sulfate chain sizes of the human and bovine keratan sulfate-containing proteoglycans were seen to be similar.

Proteoglycans contain a 4.6 A repeat in muscular dystrophy corneas: x-ray diffraction evidence

Synchrotron x-ray diffraction patterns from macular corneal dystrophy (MCD) corneas contain an unusual reflection that arises because of an undefined ultrastructure with a periodic repeat in the region of 4.6 A. In this study, we compared with wide-angle x-ray diffraction patterns obtained from four normal human corneas and four MCD corneas. Moreover, portions of two of the MCD corneas were pretreated with a specific glycosidase to shed light on the origin of the 4.6 A reflection. None of the normal corneas produced an x-ray reflection in the region of 4.6 A, whereas all four of the MCD corneas did (MCD type I at 4.65 A and 4.63 A, MCD type II at 4.63 A and 4.67 A). This reflection was diminished after incubation of the MCD tissues with either chondroitinase ABC or N-glycanase. The findings indicate that glycosaminoglycans or proteoglycans contribute to the unusual MCD x-ray reflection and hence most likely contain a periodic 4.6 A ultrastructure. Furthermore, the results imply that periodic 4.6 A MCD ultrastructures reside in either intact, unsulfated lumican molecules and regions of the CS/DS-containing molecules or in a region of a hybrid macromolecular aggregate formed by the interaction of the two molecules.

Application of cationic probes for the ultrastructural localization of proteoglycans in basement membranes

The application of cationic probes for the ultrastructural detection of proteoglycans in basement membranes is reviewed. Proteoglycans are highly negatively charged macromolecules due to their glycosaminoglycan side chains. The interaction of cationic probes with proteoglycans is of an electrostatic nature. Methods are discussed to increase the specificity of probes for proteoglycans. The use of phthalocyanin-like dyes such as Cuprolinic blue, according to the critical electrolyte concentration method, results in a selective staining of proteoglycans. Enzymatic or chemical digestions, however, should be done to validate the proteoglycan nature of the dye-positive granules/filaments, and to establish the class of proteoglycan. The value of cationic probes in basement membrane research on development and pathology is discussed. The potential for deducting molecular information from the ultrastructural appearance of stained proteoglycans is indicated.

Ultrastructure of the corneal stroma: a comparative study

Using a high intensity synchrotron x-ray source, we have recorded diffraction over a range of angles from the corneas of a wide variety of species. The results show that the interfibrillar Bragg spacing varies from 39 nm to 67 nm, the fibril diameter varies from 24 nm to 43 nm, but in the species studied intermolecular Bragg spacing is constant (1.58 +/- 0.03 nm). Using these data, a number of other structural parameters were calculated including the interfibrillar volume, V, and the surface-to-surface fibril separation, S. Large variations were found, particularly between aquatic and terrestrial animals. We found that the parameter which appears to be most constant throughout the species was the volume fraction, that is, the proportion of the tissue occupied by the hydrated fibrils. Ignoring the volume of the stroma occupied by cells, the tissue fibril volume fraction was (28 +/- 3)% for both aquatic and land animals. The observation of a constant volume fraction led us to propose a simple model in which collagen molecules and interfibrillar glycosaminoglycans occur in a fixed ratio in all the species--thus species with narrow fibrils have fewer interfibrillar glycosaminoglycans and the fibrils are thus more closely spaced, and vice versa. This model agrees with many of the experimental data on corneal composition and on the physical properties of the tissue reported in the literature.

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.

Alteration of the stromal architecture and depletion of keratan sulphate proteoglycans in oedematous human corneas: histological, immunochemical and X-ray diffraction evidence

The structure and content of the extracellular stromal matrix of several oedematous human corneas was investigated using electron microscopy, X-ray diffraction and biochemical techniques. Electron microscopy revealed the presence of wavy lamellae and various sized collagen-free 'lakes' within the stroma of the oedematous corneas, with their posterior sections containing by far the largest 'lakes'. The existence of 'lakes' was supported by the equatorial X-ray diffraction evidence. Staining the oedematous corneas with Cuprolinic blue prior to electron microscopical and meridional X-ray diffraction studies demonstrated a loss of stromal proteoglycans normally associated with collagen. Immunochemical evidence demonstrated reduced levels of antigenic keratan sulphate in the oedematous corneas while biochemical techniques revealed constant chondroitin sulphate levels in the same corneas.

Macular corneal dystrophy: the macromolecular structure of the stroma observed using electron microscopy and synchrotron X-ray diffraction

The distribution of sulphated proteoglycans within the stromas of three patients (A,B,C) suffering from macular corneal dystrophy was studied using the specific dye Cuprolinic Blue in a 'critical electrolyte concentration' method. The corneas were examined using transmission electron microscopy and A and C were further studied by low-angle synchroton X-ray diffraction. Sera from all three patients were analyzed for the presence of keratan sulphate using a monoclonal antibody in an enzyme-linked immunosorbent assay. The serum from Patient A contained keratan sulphate, but the chains were thought to be shorter or less sulphate in their sera. Electron microscopy showed many electron-transparent lacunae randomly distributed throughout the specimens. The average collagen fibril diameter was normal but there were differences in packing between the specimens. Specimen A was closely-packed with most collagen fibrils in contact with their neighbours. Specimens B and C showed fewer regions of close packing; in most of the tissue the interfibrillar spacing appeared normal. Staining with Cuprolinic Blue revealed an unusual distribution of proteoglycans in some parts of the interfibrillar matrix, particularly in A, with 'small' proteoglycans running exclusively parallel to the collagen fibrils. Furthermore in A, and to a lesser extent in B and C, some lacunae were filled with clusters of abnormal sulphated proteoglycan filaments (of various sizes) which were chondroitinase ABC susceptible. Clearly defined regions, both within the lacunae and elsewhere, failed to stain with Cuprolinic Blue; this suggests an absence of sulphated proteoglycans within these areas. Equatorial X-ray diffraction of the wet tissues (A and C) gave values for the mean interfibrillar centre-to-centre separation of 43 +/- 2 nm in Specimen A and 52 +/- 3 nm in Specimen C. The differences observed in the serum keratan sulphate levels, the packing of the collagen fibrils and the distribution of chondroitin/dermatan sulphate proteoglycans confirm the heterogeneity that exists within the macular corneal dystrophies.

Axial electron density of human scleral collagen. Location of proteoglycans by x-ray diffraction

The low angle meridional x-ray diffraction pattern from fresh human sclera was analyzed to ascertain if collagen-bound proteoglycans affect the axially-projected electron density distribution to the same extent as appears to occur in the cornea. The results showed that, unlike cornea, the electron density of the sclera is similar to that seen in rat tail tendon collagen. The proteoglycans were specifically stained using either Cuprolinic blue or Cupromeronic blue, both under critical electrolyte conditions. The tissue was then examined by electron microscopy and by low angle x-ray diffraction. The electron-optical observations suggested that proteoglycans associate with collagen near the d/e staining bands in the gap zone. A difference Fourier analysis from the x-ray results confirmed that these observations were not e.m. preparative artefacts and allowed a quantitative estimate to be made of the axial extent of the proteglycans in the wet tissue.

Proteoglycan-fibrillar collagen interactions

Images

Bovine corneal stroma contains a structural glycoprotein located in the gap region of the collagen fibrils

Treatment of bovine corneal stroma using SDS-containing extracting solutions removes a 135,000 MW glycoprotein from the main collagen framework of the tissue. Low-angle synchrotron X-ray diffraction patterns obtained from corneas extracted in this way indicate that the glycoprotein has been removed from the gap regions of the collagen fibrils and is thus an important structural component of the corneal stroma. The glycoprotein (GP 135) shares a number of properties with one of the subunits of type VI collagen, but tests have so far failed to establish their identity.