A new three-dimensional model of the organization of proteoglycans and collagen fibrils in the human corneal stroma

Comparative Outcomes of Single Versus Multiple Cyanoacrylate Tissue Adhesive Applications in the Management of Corneal Thinning and Perforation

PURPOSE

To compare outcomes between patients treated with a single cyanoacrylate tissue adhesive (CTA) patch for corneal thinning or perforation and those requiring multiple CTA applications.

METHODS

We conducted a single-center, comparative cohort study of patients with corneal thinning or perforation treated with either a single or multiple CTA applications in Toronto, Canada, between 2006 and 2024. Primary outcomes were the need for penetrating keratoplasty (PKP) and the best-corrected visual acuity (BCVA) at final follow-up. Associations between the number of CTA applications with baseline characteristics, perforation characteristics, and final outcomes were analyzed using univariable and multivariable logistic regression models.

RESULTS

Overall, 189 patients (median age 69.0 years; 42% female) were included, with 116 (61%) in the single CTA group and 73 (39%) in the multiple-application group (mean 2.3 ± 0.6), over a median follow-up of 4.4 months. Baseline characteristics were similar between groups. Central and paracentral corneal defects were more likely to require multiple glue applications than peripheral defects (OR = 2.92, 95% CI, 1.31-6.51, P = 0.009). No difference was observed in final BCVA between groups (median: 2.0 logarithm of the minimum angle of resolution [single] vs. 2.0 logarithm of the minimum angle of resolution [multiple]; P = 0.838). Patients receiving multiple CTA applications (48%, n = 35/73) were more likely to require PKP (OR = 2.70, 95% CI, 1.42-5.15, P = 0.003) than those treated with a single glue patch (26%, n = 30/116).

CONCLUSIONS

Multiple CTA applications were more frequently needed for central/paracentral defects and were associated with a greater likelihood of PKP. Given no significant difference in final BCVA, a conservative, stepwise management approach can be pursued without compromising long-term visual outcomes, allowing flexibility in surgical planning.

Histological characterization of the human scapholunate ligament

The scapholunate interosseous ligament (SLIL) plays a fundamental role in stabilizing the wrist bones, and its disruption is a frequent cause of wrist arthrosis and disfunction. Traditionally, this structure is considered to be a variety of fibrocartilaginous tissue and consists of three regions: dorsal, membranous and palmar. Despite its functional relevance, the exact composition of the human SLIL is not well understood. In the present work, we have analyzed the human SLIL and control tissues from the human hand using an array of histological, histochemical and immunohistochemical methods to characterize each region of this structure. Results reveal that the SLIL is heterogeneous, and each region can be subdivided in two zones that are histologically different to the other zones. Analysis of collagen and elastic fibers, and several proteoglycans, glycoproteins and glycosaminoglycans confirmed that the different regions can be subdivided in two zones that have their own structure and composition. In general, all parts of the SLIL resemble the histological structure of the control articular cartilage, especially the first part of the membranous region (zone M1). Cells showing a chondrocyte-like phenotype as determined by S100 were more abundant in M1, whereas the zone containing more CD73-positive stem cells was D2. These results confirm the heterogeneity of the human SLIL and could contribute to explain why certain zones of this structure are more prone to structural damage and why other zones have specific regeneration potential. RESEARCH HIGHLIGHTS: Application of an array of histological analysis methods allowed us to demonstrate that the human scapholunate ligament is heterogeneous and consists of at least six different regions sharing similarities with the human cartilage, ligament and other anatomical structures.

Refractive index of human articular cartilage varies with tissue structure and composition

Optical properties of biological tissues, such as refractive index, are fundamental properties, intrinsically linked to a tissue's composition and structure. This study aims to investigate the variation of refractive index (RI) of human articular cartilage along the tissue depth (via collagen fibril orientation and optical density) and integrity (based on Mankin and Osteoarthritis Research Society International (OARSI) scores). The results show the relationship between RI and PG content (p=0.042), collagen orientation (p=0.037), and OARSI score (p=0.072). When taken into account, the outcome of this study suggests that the RI of healthy cartilage differs from that of pathological cartilage (p=0.072). This could potentially provide knowledge on how progressive tissue degeneration, such as osteoarthritis, affects changes in cartilage RI, which can, in turn, be used as a potential optical biomarker of tissue pathology.

Birefringence analysis of collagen supraorganization in cat corneas with tropical keratopathy

OBJECTIVE

To evaluate the birefringent properties of the cornea and examine the supraorganizational aspects of collagen fibers in cats with tropical keratopathy.

PROCEDURE

In this study, 10-micrometer-thick sections of corneal tissue from cats with tropical keratopathy were examined, both in the opaque and transparent areas of the anterior stroma. Control samples were obtained from healthy cat corneas. Polarized light microscopy was employed to evaluate the birefringent properties using two distinct methods. The first method involved measuring the optical retardation associated with corneal birefringence, while the second method assessed the alignment/waviness of the birefringent collagen fibers. Differences were significant when p < .05.

RESULTS

Tropical keratopathy resulted in a significant rise (p < .05) in optical retardation in both opaque and transparent regions of the cat cornea. In the anterior stroma, both the opaque zones and transparent tissue exhibited a higher degree of collagen fiber packing than the control corneas. However, no significant differences (p > .05) in alignment were observed between the transparent tissue of the diseased cornea and the healthy corneas.

CONCLUSION

Supraorganizational changes in collagen fiber packing are not restricted to lesion zones in cat corneas affected by tropical keratopathy. Such alterations also occur in the corneal tissue of the anterior stroma adjoining the lesions. Therefore, it is plausible that the transparent tissue of the anterior stroma in corneas affected by the disease may have functional abnormalities, despite its macroscopic healthy appearance. Additional investigations are required to clarify the implications of these potential defects and their conceivable contribution to tropical keratopathy.

Corneal Wound Healing in the Presence of Antifibrotic Antibody Targeting Collagen Fibrillogenesis: A Pilot Study

Highly organized collagen fibrils interlacing with proteoglycans form the crucial architecture of the cornea and facilitate its transparency. Corneal scarring from accidental injury, surgery, or infection alters this highly organized tissue, causing severe consequences, including blindness. There are no pharmacological or surgical methods to effectively and safely treat excessive corneal scarring. Thus, we tested the anticorneal scarring utility of a rationally designed anticollagen antibody (ACA) whose antifibrotic effects have already been demonstrated in nonocular models. Utilizing a rabbit model with an incisional corneal wound, we analyzed ACA's effects on forming collagen and proteoglycan-rich extracellular matrices in scar neotissue. We used microscopic and spectroscopic techniques to quantify these components and measure crucial parameters characterizing the structure and organization of collagen fibrils. Moreover, we analyzed the spatial distribution of collagen and proteoglycans in normal and healing corneas. Our study demonstrated significant changes in the quality and quantity of the analyzed molecules synthesized in scar neotissue. It showed that these changes extend beyond incision margins. It also showed ACA's positive impact on some crucial parameters defining proper cornea structure. This pilot study provides a stepping stone for future tests of therapeutic approaches that target corneal extracellular scar matrix assembly.

The role of KS GAGs in the microstructure of CXL-treated corneal stroma; a transmission electron microscopy study

The mechanical and physical properties of the cornea originate from the microstructure and composition of its extracellular matrix. It is known that collagen fibrils, with a relatively uniform diameter, are organized in a pseudo-hexagonal array. It has been suggested that proteoglycans and the interaction of their glycosaminoglycan (GAG) side chains with themselves and collagen fibrils are important for collagen fibril organization inside the cornea. There are several diseases such as keratoconus in which the regular collagen fibrillar packing becomes distorted causing corneal optical and mechanical properties to be compromised. The primary purpose of the present work was to investigate the role of GAGs on the microstructure of corneal extracellular matrix before and after corneal crosslinking (CXL) treatment. For this purpose, keratan sulphates (KS) were removed from corneal samples using the keratanase enzyme and the CXL procedure was used to crosslink the specimens. The transmission electron microscopy was then used to characterize the diameter of collagen fibrils and their interfibrillar spacing. It was found that KS GAG depletion increased the collagen interfibrillar spacing while the CXL treatment significantly decreased the interfibrillar spacing. The enzyme and CXL treatments had an insignificant effect on the diameter of collagen fibrils. The underlying mechanisms responsible for these observations were discussed in terms of the assumption that GAG chains form duplexes that behave as tiny ropes holding collagen fibrils in place.

Objective assessment of corneal backscattered light in myopic, hyperopic, and emmetropic children

BACKGROUND

To determine the corneal densitometry(CD) values by using Scheimpflug imaging in myopic and hyperopic children and to compare the results with emmetropic children.

METHODS

The CD measurements of the subject were obtained with Scheimpflug tomography. The values were automatically measured in standardized grayscale units over an area 12mm in diameter, which was subdivided into 4 annular concentric zones(0-2mm, 2-6mm, 6-10mm, 10-12mm) and 3 corneal depths(anterior layer: anterior 120µm; central layer: from 120µm to the last 60µm; posterior layer: last 60µm). In addition, we evaluated the correlation between spherical equivalence and anterior corneal morphological parameters and the CD values.

RESULTS

A total of 211 participants were included in this prospective cross-sectional study. The eyes were divided into three groups related to their spherical equivalent(SE) refractive error values as follows: 88(41.7%) hyperopic eyes, 62(29.4%) myopic eyes, and 61(28.9%) emmetropic control eyes. The hyperopic eyes were found to have lower corneal densitometry values in 4 annular zones and the total 0-12 diameter of all layers except the central layer. However, only the 6-10mm annular zone of the central and posterior layers of the myopic eyes had lower corneal densitometry values than emmetropic eyes. There was also a significant correlation between spherical equivalent and corneal densitometry values in the anterior layer(0-2mm, 2-6mm zones), central layer(0-2mm, 10-12mm zones), posterior layer(6-10mm, 10-12mm zones, and the total 0-12mm diameter), and total corneal thickness(0-2mm zone) of the hyperopic eyes.

CONCLUSIONS

Backward scattering of light was lower in hyperopic eyes and this could indicate better visual quality.

Analysis of Cryopreservation Protocols and Their Harmful Effects on the Endothelial Integrity of Human Corneas

Corneal cryopreservation can partially solve the worldwide concern regarding donor cornea shortage for keratoplasties. In this study, human corneas were cryopreserved using two standard cryopreservation protocols that are employed in the Tissue Bank of the Teresa Herrera Hospital (Spain) to store corneas for tectonic keratoplasties (TK protocol) and aortic valves (AV protocol), and two vitrification protocols, VS55 and DP6. Endothelial viability and general corneal state were evaluated to determine the protocol that provides the best results. The potential corneal cryopreservation protocol was studied in detail taking into consideration some cryopreservation-related variables and the endothelial integrity and stroma arrangement of the resulting cryopreserved corneas. TK corneas showed mostly viable endothelial cells, while the others showed few (AV) or none (DP6 and VS55). The corneal structure was well maintained in TK and AV corneas. TK corneas showed endothelial acellular areas surrounded by injured cells and a normal-like stromal fiber arrangement. Cryoprotectant solutions of the TK protocol presented an increasing osmolality and a physiological pH value. Cooling temperature rate of TK protocol was of 1 °C/min to −40 °C and 3 °C/min to −120 °C, and almost all of dimethyl sulfoxide left the tissue after washing. Future studies should be done changing cryopreservation-related variables of the TK protocol to store corneas of optical grade.

Management of Corneal Clouding in Patients with Mucopolysaccharidosis

Mucopolysaccharidoses (MPS) are a rare group of lysosomal storage disorders characterized by the accumulation of incompletely degraded glycosaminoglycans (GAGs) in multiple organ systems including the eye. Visual loss occurs in MPS predominantly due to corneal clouding and retinopathy, but the sclera, trabecular meshwork and optic nerve may all be affected. Despite the success of therapies such as enzyme replacement therapy (ERT) and hematopoietic stem-cell transplantation (HSCT) in improving many of the systemic manifestations of MPS, their effect on corneal clouding is minimal. The only current definitive treatment for corneal clouding is corneal transplantation, usually in the form of a penetrating keratoplasty or a deep anterior lamellar keratoplasty. This article aims to provide an overview of corneal clouding, its current clinical and surgical management, and significant research progress.

CLEAR - Effect of contact lens materials and designs on the anatomy and physiology of the eye

This paper outlines changes to the ocular surface caused by contact lenses and their degree of clinical significance. Substantial research and development to improve oxygen permeability of rigid and soft contact lenses has meant that in many countries the issues caused by hypoxia to the ocular surface have largely been negated. The ability of contact lenses to change the axial growth characteristics of the globe is being utilised to help reduce the myopia pandemic and several studies and meta-analyses have shown that wearing orthokeratology lenses or soft multifocal contact lenses can reduce axial length growth (and hence myopia). However, effects on blinking, ptosis, the function of Meibomian glands, fluorescein and lissamine green staining of the conjunctiva and cornea, production of lid-parallel conjunctival folds and lid wiper epitheliopathy have received less research attention. Contact lens wear produces a subclinical inflammatory response manifested by increases in the number of dendritiform cells in the conjunctiva, cornea and limbus. Papillary conjunctivitis is also a complication of all types of contact lenses. Changes to wear schedule (daily disposable from overnight wear) or lens materials (hydrogel from SiHy) can reduce papillary conjunctivitis, but the effect of such changes on dendritic cell migration needs further study. These changes may be associated with decreased comfort but confirmatory studies are needed. Contact lenses can affect the sensitivity of the ocular surface to mechanical stimulation, but whether these changes affect comfort requires further investigation. In conclusion, there have been changes to lens materials, design and wear schedules over the past 20+ years that have improved their safety and seen the development of lenses that can reduce the myopia development. However, several changes to the ocular surface still occur and warrant further research effort in order to optimise the lens wearing experience.

Peripheral ulcerative keratitis

Peripheral ulcerative keratitis (PUK) is an inflammatory condition of the peripheral cornea with hallmark features of epithelial defects and stromal destruction as a result of a complex interplay of factors including host autoimmunity and the peculiar anatomic and physiologic features of the peripheral cornea and environmental factors. PUK may be the result of local or systemic causes and infectious or noninfectious causes. Arriving at a specific etiological diagnosis requires a meticulous clinical workup that may include a battery of laboratory and radiological investigations. Management by a team of internists or rheumatologists and ophthalmologists and judicious use of immunosuppressive agents may yield favorable results minimizing adverse effects. We review current clinical knowledge on the diagnosis and management of PUK.

Cell sheet technology: Influence of culture conditions on in vitro-cultivated corneal stromal tissue for regenerative therapies of the ocular surface

The in vitro reconstruction of stromal tissue by long-term cultivation of corneal fibroblasts is a smart approach for regenerative therapies of ocular surface diseases. However, systematic investigations evaluating optimized cultivation protocols for the realization of a biomaterial are lacking. This study investigated the influence of supplements to the culture media of human corneal fibroblasts on the formation of a cell sheet consisting of cells and extracellular matrix. Among the supplements studied are vitamin C, fetal bovine serum, L-glutamine, components of collagen such as L-proline, L-4-hydroxyproline and glycine, and TGF-β1, bFGF, IGF-2, PDGF-BB and insulin. After long-term cultivation, the proliferation, collagen and glycosaminoglycan content and light transmission of the cell sheets were examined. Biomechanical properties were investigated by tensile tests and the ultrastructure was characterized by electron microscopy, small-angle X-ray scattering, antibody staining and ELISA. The synthesis of extracellular matrix was significantly increased by cultivation with insulin or TGF-β1, each with vitamin C. The sheets exhibited a high transparency and suitable material properties. The production of a transparent, scaffold-free, potentially autologous, in vitro-generated construct by culturing fibroblasts with extracellular matrix synthesis-stimulating supplements represents a promising approach for a biomaterial that can be used for ocular surface reconstruction in slowly progressing diseases.

Direct Evidence of Symmetry between Bilateral Human Corneas in Biomechanical Properties: A Comparison Study with Fresh Corneal Tissue

Purpose. To investigate the difference between the eyes from the same human with respect to the biomechanical properties of fresh corneal tissues and investigate the assumption of similarity of the corneal biomechanical properties between the eyes. Methods. Strip specimens extracted through a small incision lenticule extraction (SMILE) surgery were tested using a uniaxial tensile test. The specimens were extracted vertically. Low-strain tangent modulus (LSTM), high-strain tangent modulus (HSTM), and tensile strength (${\sigma }_b$) were the biomechanical parameters used in the comparison of the eyes from the same human. Results. Ninety corneal specimens from 45 persons were included in this study. The LSTM of the left and right eyes were 1.34 ± 0.52 and 1.37 ± 0.46 MPa, while the HSTM were 50.53 ± 7.51 and 49.41 ± 7.01 MPa, respectively. There was no significant difference between the eyes in terms of LSTM, HSTM, and${\sigma }_b\left(P=0.813,0.335,\text{and\hspace{0.17em}}0.605,\text{resp}.\right)$. The LSTM and HSTM were significantly correlated with the spherical equivalent (SE) ($P\le 0.01,P=0.001$, resp.). Conclusions. The assumption that the corneal biomechanical properties of the eyes from the same human are similar has been confirmed for the first time using fresh human corneal tissue. This finding may be useful in further biomechanical studies.

Natural cross-linker-stabilized acellular porcine corneal stroma for lamellar keratoplasty

Acellular porcine corneal stroma (APCS) is a promising alternative to human donor cornea for lamellar keratoplasty (LKP). However, the detergents, enzymes and physical forces used during decellularization unavoidably alter the cornea's extracellular matrix composition and disrupt its ultrastructure, making it less mechanically stable and liable to degradation both in vitro and in vivo. Herein, we systematically analyzed the low biomechanics and easy degradability of APCS in terms of structure and protein composition. Then, we introduced natural cross-linkers, namely proanthocyanidin (PA), epigallocatechin-3-gallate and genipin, to stabilize the APCS that exhibited color variations during crosslinking. Then, we developed a protective crosslinking system by combining cross-linkers with bovine serum albumin (BSA) to reduce color change, maintain transparency and improve the mechanical property of APCS. PA/BSA-crosslinked APCS (PA/BSA-APCS) shows favorable corneal transparency and swelling property; the improved overall and surface corneal biomechanics were comparable to those of human cornea, revealing strong resistance to enzymatic degradation and good biocompatibility. Results from LKP in the rabbit model showed complete re-epithelialization without graft melting, the stitches were scarcely loosened after the operation and more host keratocytes had migrated in PA/BSA-APCS at six months post-operation. Therefore, PA/BSA-APCS could be useful as a corneal substitute for tissue regeneration and the protective crosslinking system could be applicable in other bioengineering fields.

On the Connection Between Geometry and Statically Determined Membrane Stresses in the Human Cornea

Under the action of the intraocular pressure (IOP), the human cornea is stressed and deforms acquiring a quasi-spherical configuration. If the stressed configuration is known, and the cornea is regarded as a membrane, disregarding flexural behaviors with an equilibrium analysis only is possible to estimate the distribution of the average stress across the thickness. In the cornea, the action of the intraocular pressure is supported by collagen fibrils, immersed into an elastin-proteoglycan matrix, and organized in a very precise architecture to provide the necessary confinement and transparency to the light. With the goal of understanding the static consequences of shape modifications due to pathological dilatation (ectasia), we present a simplified stress analysis of the human cornea modeled as a membrane. A numerical investigation over 40 patient-specific corneas (20 normal and 20 ectatic) is carried out to establish a relationship between the physiological geometry and the distribution of the membrane stresses, and to assess the possibility to obtain information on the stress state based on topographic images only. Comparative analyses reveal that, with respect to normal corneas, in ectatic corneas the pattern of the principal stress lines is modified markedly showing a deviation from the hypothetical dominant orientation of the collagen fibrils. The rotation of the principal stress with respect to the fibril orientation can be thought as responsible of the transmission of a large amount of shear stresses onto the elastin-proteoglycan matrix. The anomalous loading of the matrix could be correlated to the evolution of time-dependent shape modifications leading to ectasia.

Engineering topography: Effects on corneal cell behavior and integration into corneal tissue engineering

Cell-material interactions are important to tissue engineering. Inspired by the natural topographic structures on the extracellular matrix, a growing number of studies have integrated engineering topography into investigations of cell behavior on biomaterials. Engineering topography has a significant influence on cell behaviors. These cell-topography interactions play an important role in regenerative medicine and tissue engineering. Similarly, cell-topography interactions are important to corneal reconstruction and regeneration. In this review, we primarily summarized the effects of topographic cues on the behaviors of corneal cells, including cell morphology, adhesion, migration, and proliferation. Furthermore, the integration of engineering surface topography into corneal tissue engineering was also discussed.

Microgrooved collagen-based corneal scaffold for promoting collective cell migration and antifibrosis

Microgrooved collagen membrane can effectively promote the epithelialization of corneal epithelial cells and inhibit the fibrosis of corneal stromal cells.

Preliminary Investigation of the Mechanical Anisotropy of the Normal Human Corneal Stroma

Purpose

To investigate the anisotropic characteristics of the normal human corneal stroma using fresh corneal tissue.

Methods

Sixty-four corneal specimens extracted from stromal lenticules were included in this study. The specimens were cut in the temporal-nasal (horizontal) or superior-inferior (vertical) direction. Strip specimens were subjected to uniaxial tensile testing. The tensile properties of the specimens were measured and compared in the two directions.

Results

The low-strain tangent modulus was statistically significantly greater in the vertical direction than in the horizontal direction (1.32 ± 0.50 MPa vs 1.17 ± 0.43 MPa; P=0.035), as was the high-strain tangent modulus (51.26 ± 8.23 MPa vs 43.59 ± 7.96 MPa; P ≤ 0.001). The elastic modulus in the vertical direction was also higher than that in horizontal direction at stresses of 0.01, 0.02, and 0.03 MPa, but not statistically significant; so, P=0.338, 0.373, and 0.417, respectively.

Conclusions

The biomechanical behavior in normal human corneal stroma tissue is slightly stiffer in the vertical direction than in the horizontal direction. This information may aid our understanding of the biomechanical properties of the cornea and related diseases.

The influence of hydration on different mechanical moduli of the cornea

PURPOSE

To determine the interrelation of different elastic moduli of the cornea and to investigate their dependency on corneal hydration.

METHODS

Rabbit eyes were divided into four groups. Corneas were excised and mounted into a Barron artificial anterior chamber. Various corneal hydration steady states were achieved with different dextran T-500 concentrations in the anterior chamber, as well as on the corneal anterior surface. The treatment-solutions of each group contained either 5, 10, 15, or 20% w/w dextran. Ultrasound pachymetry was used to measure central corneal thickness. Brillouin microscopy of the central cornea determined the longitudinal bulk modulus by means of Brillouin frequency shift. Subsequently, a 5-mm-wide central strip was taken for extensiometry to measure the tangential elastic modulus.

RESULTS

The longitudinal bulk modulus was 1.2-times higher in corneas dehydrated with 20% dextran compared to those hydrated with 5% dextran. In contrast, the tangential elastic modulus increased by 4.4 times. The obtained longitudinal bulk moduli were two orders of magnitude bigger than the tangential elastic moduli. Regression analysis of longitudinal bulk modulus and tangential elastic modulus revealed a quadratic relation. The bulk modulus seemed to be independent of tension, whereas the elastic modulus was tension-dependent. Greater corneal hydration led to significantly thicker pachymetry.

CONCLUSION

Corneal biomechanics are highly dependent on the level of corneal hydration. Surprisingly, tangential elastic moduli were more sensitive to hydration changes than longitudinal bulk moduli. A quadratic relation was found between both moduli.

Transient viscous response of the human cornea probed with the Surface Force Apparatus

Knowledge of the biomechanical properties of the human cornea is crucial for understanding the development of corneal diseases and impact of surgical treatments (e.g., corneal laser surgery, corneal cross-linking). Using a Surface Force Apparatus we investigated the transient viscous response of the anterior cornea from donor human eyes compressed between macroscopic crossed cylinders. Corneal biomechanics was analyzed using linear viscoelastic theory and interpreted in the framework of a biphasic model of soft hydrated porous tissues, including a significant contribution from the pressurization and viscous flow of fluid within the corneal tissue. Time-resolved measurements of tissue deformation and careful determination of the relaxation time provided an elastic modulus in the range between 0.17 and 1.43 MPa, and fluid permeability of the order of 10⁻¹³ m⁴/(N∙s). The permeability decreased as the deformation was increased above a strain level of about 10%, indicating that the interstitial space between fibrils of the corneal stromal matrix was reduced under the effect of strong compression. This effect may play a major role in determining the observed rate-dependent non-linear stress-strain response of the anterior cornea, which underlies the shape and optical properties of the tissue.

Glycosaminoglycans from bovine eye vitreous humour and interaction with collagen type II

Glycosaminoglycans (GAGs) play an important role in stabilizing the gel state of eye vitreous humour. In this study, the composition of GAGs present in bovine eye vitreous was characterized through disaccharide analysis by liquid chromatography-mass spectrometry. The interaction of GAGs with collagen type II was assessed using surface plasmon resonance (SPR). The percentage of hyaluronic acid (HA), chondroitin sulfate (CS) and heparan sulfate (HS), of total GAG, were 96.2%, 3.5% and 0.3%, respectively. The disaccharide composition of CS consisted of 4S (49%), 0S (38%) 6S (12%), 2S6S (1.5%) and 2S4S (0.3%). The disaccharide composition of HS consisted of 0S (80%), NS2S (7%), NS (7%), 6S (4%), NS6S (2%), and TriS, 2S and 4S6S (each at 0.1%). The average molecular weights of CS and HS were 148 kDa and 204 kDa, respectively. SPR reveals that collagen type II binds to heparin (primarily composed of TriS) with a binding affinity (K _D) of 755 nM and interacts with other GAGs, including CSB and CSE. Both bovine vitreous CS and HS interact with collagen type II, with vitreous HS showing a higher binding affinity.

Highly concentrated collagen solutions leading to transparent scaffolds of controlled three-dimensional organizations for corneal epithelial cell colonization

Controlling both organizations and transparency of dense collagen scaffolds.

Bioinspiring Chondrosia reniformis (Nardo, 1847) Collagen-Based Hydrogel: A New Extraction Method to Obtain a Sticky and Self-Healing Collagenous Material

Collagen is a natural and abundant polymer that serves multiple functions in both invertebrates and vertebrates. As collagen is the natural scaffolding for cells, collagen-based hydrogels are regarded as ideal materials for tissue engineering applications since they can mimic the natural cellular microenvironment. Chondrosia reniformis is a marine demosponge particularly rich in collagen, characterized by the presence of labile interfibrillar crosslinks similarly to those described in the mutable collagenous tissues (MCTs) of echinoderms. As a result single fibrils can be isolated using calcium-chelating and disulphide-reducing chemicals. In the present work we firstly describe a new extraction method that directly produces a highly hydrated hydrogel with interesting self-healing properties. The materials obtained were then biochemically and rheologically characterized. Our investigation has shown that the developed extraction procedure is able to extract collagen as well as other proteins and Glycosaminoglycans (GAG)-like molecules that give the collagenous hydrogel interesting and new rheological properties when compared to other described collagenous materials. The present work motivates further in-depth investigations towards the development of a new class of injectable collagenous hydrogels with tailored specifications.

Establishment of an in vitro monolayer model of macular corneal dystrophy

Macular corneal dystrophy (MCD) is characterized by multiple punctate gray-white opacities in the corneal stromal region, due to the accumulation of abnormally sulfated keratan sulfates. We attempted to develop an in vitro model of MCD by simulating the sulfation inhibition using sodium chlorate, a chemical inhibitor of 3'-phosphoadenosine-5'-phosphosulfate (PAPs). The SEM and micro-Raman spectroscopy results showed the hallmark feature of MCD. Further the gene expression studies elucidated the direct effect of sulfation inhibition on the WNT pathway, that in turn downregulated production of matrix metalloproteinases (MMPs), which causes abnormal matrix deposits leading to loss of transparency in vivo. It also resulted in downregulation of integrin and cadherin complexation that leads to disruption of the epithelial layer in the MCD affected corneas. This study offers a promising initial step toward establishing a relevant in vitro MCD disease model, to assess signaling transduction pathways and devise potential treatment strategies based on MMP administration to the MCD affected corneas.

Characterizing depth-dependent refractive index of articular cartilage subjected to mechanical wear or enzymic degeneration

Utilizing a laser scanning confocal microscope system, the refractive indices of articular cartilage (AC) with mechanical or biochemical degenerations were characterized to investigate whether potential correlations exist between refractive index (RI) and cartilage degeneration. The cartilage samples collected from the medial femoral condyles of kangaroo knees were mechanically degenerated under different loading patterns or digested in trypsin solution with different concentrations. The sequences of RI were then measured from cartilage surface to deep region and the fluctuations of RI were quantified considering combined effects of fluctuating frequency and amplitude. The compositional and microstructural alterations of cartilage samples were assessed with histological methods. Along with the loss of proteoglycans, the average RI of cartilage increased and the local fluctuation of RI became stronger. Short-term high-speed test induced little influence to both the depth fluctuation and overall level of RI. Long-term low-speed test increased the fluctuation of RI but the average RI was barely changed. The results substantially demonstrate that RI of AC varies with both compositional and structural alterations and is potentially an indicator for the degeneration of AC.

Comparison of human corneal cell density by age and corneal location: an in vivo confocal microscopy study

Background

Peripheral and central regions of the cornea are optically different and have different repair capacity and pathology. For this reason, we characterized the cellular morphology and quantified the cell density of the central and peripheral regions of the cornea with age.

Methods

Eighty healthy subjects were enrolled in the study and divided into four groups according to age: A (0–19 years), B (20–39 years), C (40–59 years), and D (>60 years). In vivo confocal microscopy was used to measure the following parameters for the central and peripheral regions of the cornea: average cellular density and area of the superficial and basal epithelium; average density of the anterior and posterior keratocytes; average endothelial cell density and cellular area; percentage of hexagonal endothelial cells.

Results

Statistically significant differences between the central and peripheral cornea were observed for the cellular density of basal epithelial cells in group A. The density of keratocytes in the anterior stroma was significantly greater in the central region compared with the peripheral region in group B and group C. The percentage of hexagonal cells in the endothelial layer was significantly greater in the central region compared with the peripheral region. Age-related changes were found in peripheral basal epithelial cell density, central and peripheral endothelial cell density, and the percentage of hexagonal endothelial cells.

Conclusion

Both similarities and differences in morphology of the central and peripheral regions of the transparent cornea were observed. These observations would provide a histological basis for further studies to define its regional pathological mechanisms.

A structural model for the in vivo human cornea including collagen-swelling interaction

A structural model of the in vivo cornea, which accounts for tissue swelling behaviour, for the three-dimensional organization of stromal fibres and for collagen-swelling interaction, is proposed. Modelled as a binary electrolyte gel in thermodynamic equilibrium, the stromal electrostatic free energy is based on the mean-field approximation. To account for active endothelial ionic transport in the in vivo cornea, which modulates osmotic pressure and hydration, stromal mobile ions are shown to satisfy a modified Boltzmann distribution. The elasticity of the stromal collagen network is modelled based on three-dimensional collagen orientation probability distributions for every point in the stroma obtained by synthesizing X-ray diffraction data for azimuthal angle distributions and second harmonic-generated image processing for inclination angle distributions. The model is implemented in a finite-element framework and employed to predict free and confined swelling of stroma in an ionic bath. For the in vivo cornea, the model is used to predict corneal swelling due to increasing intraocular pressure (IOP) and is adapted to model swelling in Fuchs' corneal dystrophy. The biomechanical response of the in vivo cornea to a typical LASIK surgery for myopia is analysed, including tissue fluid pressure and swelling responses. The model provides a new interpretation of the corneal active hydration control (pump-leak) mechanism based on osmotic pressure modulation. The results also illustrate the structural necessity of fibre inclination in stabilizing the corneal refractive surface with respect to changes in tissue hydration and IOP.

Effect of corneal light scatter on vision: a review of the literature

The cornea is the transparent connective tissue window at the front of the eye. The physiological role of the cornea is to conduct external light into the eye, focus it, together with the lens, onto the retina, and to provide rigidity to the entire eyeball. Therefore, good vision requires maintenance of the transparency and proper refractive shape of the cornea. The surface structures irregularities can be associated with wavefront aberrations and scattering errors. Light scattering in the human cornea causes a reduction of visual quality. In fact, the cornea must be transparent and maintain a smooth and stable curvature since it contributes to the major part of the focusing power of the eye. In most cases, a simple examination of visual acuity cannot demonstrate the reduction of visual quality secondary light scattering. In fact, clinical techniques for examining the human cornea in vivo have greatly expanded over the last few decades. The measurement of corneal back scattering qualifies the degree of corneal transparency. The measurement of corneal forward-scattering quantifies the amount of visual impairment that is produced by the alteration of transparency. The aim of this study was to review scattering in the human cornea and methods of measuring it.

Characterization of a corneal endothelium engineered on a self-assembled stromal substitute

Endothelial dysfunctions are the first indication for allogeneic corneal transplantation. Development of a tissue-engineered posterior cornea could be an alternative to the use of native allogeneic tissues. In this paper, we used the self-assembly approach to form a cellularized stromal substitute that served as a carrier for the engineering of an endothelium. This endothelialized stromal substitute was then characterized using alizarin red staining, histology, scanning and transmission electron microscopy, as well as mass spectrometry and immunodetection of collagens and function-related proteins. We report the engineering of a monolayer of flattened endothelial cells with a cell density of 966 ± 242 cells/mm(2) (mean ± SD). Endothelial interdigitations were present between cells. The stromal fibroblasts deposited a dense and cohesive collagenous matrix. Collagen fibrils had a diameter of 39.1 ± 11.3 nm, and a mean center to center interfibrillar space of 50.9 ± 10.9 nm. The stromal substitute was composed of collagen types I, V, VI and XII, as well as lumican and decorin. Type IV collagen was also present underneath the endothelium. The endothelium expressed both the sodium/potassium (Na(+)/K(-)) ATPase and sodium/bicarbonate (Na(+)/ [Formula: see text] ) cotransporter pumps. These results indicate that the self-assembled stromal substitute is able to support the expression of endothelial cell functionality markers and therefore, is a suitable carrier for the engineering of an endothelium that could be used for the treatment of endothelial dysfunctions.

Multiscale Investigation of the Depth-Dependent Mechanical Anisotropy of the Human Corneal Stroma

PURPOSE

To investigate the depth-dependent mechanical anisotropy of the human corneal stroma at the tissue (stroma) and molecular (collagen) level by using atomic force microscopy (AFM).

METHODS

Eleven human donor corneas were dissected at different stromal depths by using a microkeratome. Mechanical measurements were performed in 15% dextran on the surface of the exposed stroma of each sample by using a custom-built AFM in force spectroscopy mode using both microspherical (38-μm diameter) and nanoconical (10-nm radius of curvature) indenters at 2-μm/s and 15-μm/s indentation rates. Young's modulus was determined by fitting force curve data using the Hertz and Hertz-Sneddon models for a spherical and a conical indenter, respectively. The depth-dependent anisotropy of stromal elasticity was correlated with images of the corneal stroma acquired by two-photon microscopy.

RESULTS

The force curves were obtained at stromal depths ranging from 59 to 218 μm. At the tissue level, Young's modulus (ES) showed a steep decrease at approximately 140-μm stromal depth (from 0.8 MPa to 0.3 MPa; P = 0.03) and then was stable in the posterior stroma. At the molecular level, Young's modulus (EC) was significantly greater than at the tissue level; EC decreased nonlinearly with increasing stromal depth from 3.9 to 2.6 MPa (P = 0.04). The variation of microstructure through the thickness correlated highly with a nonconstant profile of the mechanical properties in the stroma.

CONCLUSIONS

The corneal stroma exhibits unique anisotropic elastic behavior at the tissue and molecular levels. This knowledge may benefit modeling of corneal behavior and help in the development of biomimetic materials.

Peptide Amphiphiles in Corneal Tissue Engineering

The increasing interest in effort towards creating alternative therapies have led to exciting breakthroughs in the attempt to bio-fabricate and engineer live tissues. This has been particularly evident in the development of new approaches applied to reconstruct corneal tissue. The need for tissue-engineered corneas is largely a response to the shortage of donor tissue and the lack of suitable alternative biological scaffolds preventing the treatment of millions of blind people worldwide. This review is focused on recent developments in corneal tissue engineering, specifically on the use of self-assembling peptide amphiphiles for this purpose. Recently, peptide amphiphiles have generated great interest as therapeutic molecules, both in vitro and in vivo. Here we introduce this rapidly developing field, and examine innovative applications of peptide amphiphiles to create natural bio-prosthetic corneal tissue in vitro. The advantages of peptide amphiphiles over other biomaterials, namely their wide range of functions and applications, versatility, and transferability are also discussed to better understand how these fascinating molecules can help solve current challenges in corneal regeneration.

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.

Stiffening effects of riboflavin/UVA corneal collagen cross-linking is hydration dependent

The collagen cross-linking is a relatively new treatment option for strengthening the cornea, delaying, and in some cases stopping the progression of keratoconus. The uniaxial tensile experiments are among the most commonly used techniques to assess the effectiveness of this therapeutic option in enhancing tensile properties. In the present study, we investigated the possible effects of hydration on stiffening effects of corneal collagen cross-linking procedure, as measured by the uniaxial tensile testing method. For this purpose, after cross-linking bovine corneas, we let the strips to dehydrate in air or swell in a solution until their thickness reached an average thickness of 0.5, 0.7, 0.9, 1.1, and 1.5 mm. Using thickness as a representative of hydration, we divided corneal strips into five different groups and measured their stress-strain behavior by conducting uniaxial tensile experiments in mineral oil. It was observed that the collagen cross-linking treatment and hydration together affect the tensile behavior of the bovine cornea. While corneal collagen cross-linking resulted in a significant increase in the tensile stress-strain response of each thickness group (P<0.01), less hydrated collagen cross-linked samples showed a significantly stiffer response (P<0.01). A master curve was found for representing the tensile behavior of the collagen cross-linked bovine cornea at different levels of hydration. The results of the present research confirmed that the amount of mechanical stiffening of the corneal collagen cross-linking, as measured by uniaxial tensile testing, strongly depends on the hydration. Therefore, it is concluded that uniaxial tensile experiments could only be used to assess stiffening effects of the collagen cross-linking treatment if the hydration of specimens is fully controlled.

Analysis of the viscoelastic properties of the human cornea using Scheimpflug imaging in inflation experiment of eye globes

PURPOSE

To demonstrate a Scheimpflug-based imaging procedure for investigating the depth- and time-dependent strain response of the human cornea to inflation testing of whole eye globes.

METHODS

Six specimens, three of which with intact corneal epithelium, were mounted in a customized apparatus within a humidity and temperature-monitored wet chamber. Each specimen was subjected to two mechanical tests in order to measure corneal strain resulting from application of cyclic (cyclic regimen) and constant (creep regimen) stress by changing the intra-ocular pressure (IOP) within physiological ranges (18-42 mmHg). Corneal shape changes were analyzed as a function of IOP and both corneal stress-strain curves and creep curves were generated.

RESULTS

The procedure was highly accurate and repeatable. Upon cyclic stress application, a biomechanical corneal elasticity gradient was found in the front-back direction. The average Young's modulus of the anterior cornea ranged between 2.28±0.87 MPa and 3.30±0.90 MPa in specimens with and without intact epithelium (P = 0.05) respectively. The Young's modulus of the posterior cornea was on average 0.21±0.09 MPa and 0.17±0.06 MPa (P>0.05) respectively. The time-dependent strain response of the cornea to creep testing was quantified by fitting data to a modified Zener model for extracting both the relaxation time and compliance function.

CONCLUSION

Cyclic and creep mechanical tests are valuable for investigating the strain response of the intact human cornea within physiological IOP ranges, providing meaningful results that can be translated to clinic. The presence of epithelium influences the results of anterior corneal shape changes when monitoring deformation via Scheimpflug imaging in inflation experiments of whole eye globes.

Mechanisms of self-organization for the collagen fibril lattice in the human cornea

The transparency of the human cornea depends on the regular lattice arrangement of the collagen fibrils and on the maintenance of an optimal hydration--the achievement of both depends on the presence of stromal proteoglycans (PGs) and their linear sidechains of negatively charged glycosaminoglycans (GAGs). Although the GAGs produce osmotic pressure by the Donnan effect, the means by which they exert positional control of the lattice is less clear. In this study, a theoretical model based on equilibrium thermodynamics is used to describe restoring force mechanisms that may control and maintain the fibril lattice and underlie corneal transparency. Electrostatic-based restoring forces that result from local charge density changes induced by fibril motion, and entropic elastic restoring forces that arise from duplexed GAG structures that bridge neighbouring fibrils, are described. The model allows for the possibility that fibrils have a GAG-dense coating that adds an additional fibril force mechanism preventing fibril aggregation. Swelling pressure predictions are used to validate the model with results showing excellent agreement with experimental data over a range of hydration from 30 to 200% of normal. The model suggests that the electrostatic restoring force is dominant, with the entropic forces from GAG duplexes being an order or more smaller. The effect of a random GAG organization, as observed in recent imaging, is considered in a dynamic model of the lattice that incorporates randomness in both the spatial distribution of GAG charge and the topology of the GAG duplexes. A striking result is that the electrostatic restoring forces alone are able to reproduce the image-based lattice distribution function for the human cornea, and thus dynamically maintain the short-range order of the lattice.

Swelling pressure and hydration behavior of porcine corneal stroma

PURPOSE

The aim of this study was to characterize swelling pressure-thickness, swelling pressure-hydration and hydration-thickness relations of porcine cornea.

METHODS

Mechanical compression tests and free swelling experiments were performed on porcine cornea. A rheometer (DHR-2, TA Instruments) with a thermally controlled fluid chamber filled with 0.9% NaCl solution was used to measure the equilibrium swelling pressure of (n = 17) corneal stromal specimens. The samples were compressed incrementally and their swelling pressure-thickness relations were obtained. In parallel to this investigation, a transient digital imaging microscope (H800-CL, American Scope Inc.), a USB autofocus camera (UM05, ViTiny), and a precision weighing scale (AGZN100, Torbal) were simultaneously used to measure the weight-thickness relation of (n = 8) corneal specimens. This experimental study gave the thickness-hydration relationship required for expressing swelling pressure measurements as a function of hydration.

RESULTS

At the in vivo 666 ± 68 µm central corneal thickness, an average swelling pressure of 52 ± 13 mmHg and hydration of 3.36 ± 0.25 mg H2O/mg dry tissue were found. The swelling pressure was reported as functions of both tissue thickness and hydration. The average fixed charge density of ρF/F ~ 42.8 mM and dry density of 1.47±0.15 g/cm3 were found. The thickness-hydration relationship was only linear when the tissue thickness was within the range of physiological thickness.

CONCLUSION

Overall, the physiological hydration and swelling pressure of the porcine cornea were within the same range of those reported previously for other mammalian corneas such as steers, rabbits and humans. Nevertheless, the thickness-hydration behavior of the porcine cornea was only similar to that of the human cornea.

The insoluble TGFBIp fraction of the cornea is covalently linked via a disulfide bond to type XII collagen

TGFBIp, also known as keratoepithelin and βig-h3, is among the most abundant proteins in the human cornea, and approximately 60% is associated with the insoluble fraction following extraction in sodium dodecyl sulfate (SDS) sample buffer. TGFBIp is of particular interest because a wide range of mutations causes amyloid or fuchsinophilic crystalloid deposits in the cornea leading to visual impairment. We show that the SDS-insoluble fraction of TGFBIp from porcine and human corneas is covalently linked via a reducible bond to the NC3 domain of type XII collagen in a TGFBIp:type XII collagen stoichiometric ratio of 2:1. Because type XII collagen is anchored to striated collagen fibers of the extracellular matrix, its interaction with TGFBIp is likely to provide anchoring for cells to the extracellular matrix through the integrin binding capability of TGFBIp. Furthermore, the TGFBIp-type XII collagen molecule will affect our understanding of the molecular pathogenesis of the TGFBI-linked corneal dystrophies.

Depth-dependent Anisotropy of Proteoglycan in Articular Cartilage by Fourier Transform Infrared Imaging

Fourier transform infrared microscopic imaging (FTIRI) was used to quantitatively examine the anisotropies of proteoglycan (PG) and collagen in articular cartilage. Dried 6 μm thick sections of canine humeral cartilage were imaged at 6.25 μm pixel-size in FTIRI with an infrared analyzer set at 26 different angles between 0° and 180° polarization. Like the amide II and amide III peaks, the 1338 cm(-1) band confirms the anisotropy of collagen fibrils in cartilage. The absorption profile of the sugar band shows an anisotropic flipping at the deeper part in the radial zone, just above the tidemark. Together with the reduction in the PG concentration and subsequent increase in tissue calcification in this region, this anisotropy flipping of sugar might be caused by the orientational change in the collagen-attaching PG from orthogonal to parallel when the fibrils are entering the calcified zone.

Self-assembled matrix by umbilical cord stem cells

Corneal integrity is critical for vision. Corneal wounds frequently heal with scarring that impairs vision. Recently, human umbilical cord mesenchymal stem cells (cord stem cells) have been investigated for tissue engineering and therapy due to their availability and differentiation potential. In this study, we used cord stem cells in a 3-dimensional (3D) stroma-like model to observe extracellular matrix organization, with human corneal fibroblasts acting as a control. For 4 weeks, the cells were stimulated with a stable Vitamin C (VitC) derivative ±TGF-β1. After 4 weeks, the mean thickness of the constructs was ∼30 μm; however, cord stem cell constructs had 50% less cells per unit volume, indicating the formation of a dense matrix. We found minimal change in decorin and lumican mRNA, and a significant increase in perlecan mRNA in the presence of TGF-β1. Keratocan on the other hand decreased with TGF-β1 in both cell lineages. With both cell types, the constructs possessed aligned collagen fibrils and associated glycosaminoglycans. Fibril diameters did not change with TGF-β1 stimulation or cell lineage; however, highly sulfated glycosaminoglycans associated with the collagen fibrils significantly increased with TGF-β1. Overall, we have shown that cord stem cells can secrete their own extracellular matrix and promote the deposition and sulfation of various proteoglycans. Furthermore, these cells are at least comparable to commonly used corneal fibroblasts and present an alternative for the 3D in vitro tissue engineered model.

Biological glass: structural determinants of eye lens transparency

The purpose of the lens is to project a sharply focused, undistorted image of the visual surround onto the neural retina. The first pre-requisite, therefore, is that the tissue should be transparent. Despite the presence of remarkably high levels of protein, the lens cytosol remains transparent as a result of short-range-order interactions between the proteins. At a cellular level, the programmed elimination of nuclei and other light-scattering organelles from cells located within the pupillary space contributes directly to tissue transparency. Scattering at the cell borders is minimized by the close apposition of lens fibre cells facilitated by a plethora of adhesive proteins, some expressed only in the lens. Similarly, refractive index matching between lens membranes and cytosol is believed to minimize scatter. Refractive index matching between the cytoplasm of adjacent cells is achieved through the formation of cellular fusions that allow the intermingling of proteins. Together, these structural adaptations serve to minimize light scatter and enable this living, cellular structure to function as 'biological glass'.

Effect of substrate composition and alignment on corneal cell phenotype

Corneal blindness is a significant problem treated primarily by corneal transplants. Donor tissue supply is low, creating a growing need for an alternative. A tissue-engineered cornea made from patient-derived cells and biopolymer scaffold materials would be widely accessible to all patients and would alleviate the need for donor sources. Previous work in this lab led to a method for electrospinning type I collagen scaffolds for culturing corneal fibroblasts ex vivo that mimics the microenvironment in the native cornea. This electrospun scaffold is composed of small-diameter, aligned collagen fibers. In this study, we investigate the effect of scaffold nanostructure and composition on the phenotype of corneal stromal cells. Rabbit-derived corneal fibroblasts were cultured on aligned and unaligned collagen type I fibers ranging from 50 to 300 nm in diameter and assessed for expression of α-smooth muscle actin, a protein marker upregulated in hazy corneas. In addition, the optical properties of the cell-matrix constructs were assessed using optical coherence microscopy. Cells grown on collagen scaffolds had reduced myofibroblast phenotype expression compared to cells grown on tissue culture plates. Cells grown on aligned collagen type I fibers downregulated α-smooth muscle actin protein expression significantly more than unaligned collagen scaffolds, and also exhibited reduced overall light scattering by the tissue construct. These results suggest that aligned collagen type I fibrous scaffolds are viable platforms for engineering corneal replacement tissue.

Depth-dependent anisotropies of amides and sugar in perpendicular and parallel sections of articular cartilage by Fourier transform infrared imaging

Full thickness blocks of canine humeral cartilage were microtomed into both perpendicular sections and a series of 100 parallel sections, each 6 μm thick. Fourier transform infrared (IR) imaging was used to image each tissue section eleven times under different IR polarizations (from 0° to 180° polarization states in 20° increments and with an additional 90° polarization), at a spatial resolution of 6.25 μm and a wavenumber step of 8 cm⁻¹. With increasing depth from the articular surface, amide anisotropies increased in the perpendicular sections and decreased in the parallel sections. Both types of tissue sectioning identified a 90° difference between amide I and amide II in the superficial zone (SZ) of cartilage. The fibrillar distribution in the parallel sections from the SZ was shown to not be random. Sugar had a weak but recognizable anisotropy in the upper part of the radial zone (RZ) in the perpendicular sections. The depth-dependent anisotropic data were fitted with a theoretical equation that contained three signature parameters, which illustrate the arcade structure of collagens with the aid of a fibril model. Fourier-transform IR imaging of both perpendicular and parallel sections provides the possibility of determining the three-dimensional macromolecular structures in articular cartilage. Being sensitive to the orientation of the macromolecular structure in healthy articular cartilage aids the prospect of detecting the early onset of the tissue degradation that may lead to pathological conditions such as osteoarthritis.

On Mechanics of Connective Tissue: Assessing the Electrostatic Contribution to Corneal Stroma Elasticity

The extracellular matrix plays a crucial role in defining the mechanical properties of connective tissues like cornea, heart, tendon, bone and cartilage among many others. The unique properties of these collagenous tissues arise because of both the hierarchal structure of collagens and the presence of negatively charged proteoglycans (PGs) which hold collagen fibers together. Here, in an effort to understand the mechanics of these structures, using the nonlinear Poison-Boltzmann (PB) equation, we study the electrostatic contribution to the elasticity of corneal stroma due to the presence of negatively charged PG glycosminoglycans (GAGs). Since collagens and GAGs have a regular hexagonal arrangement inside the corneal stroma, a triangular unit cell is chosen. The finite element method is used to solve the PB equation inside this domain and to obtain the electric potential and ionic distributions. Having the ion and potential distributions throughout the unit cell, the electrostatic free energy is computed and the tissue elasticity is calculated using the energy method. It is shown that as the ionic bath concentration increases; the electrostatic contribution to tissue elasticity is reduced.