Compliance profile of the human cornea as measured by atomic force microscopy

The inclusion of the epithelium in numerical models of the human cornea

We present a patient-specific finite element model of the human cornea that accounts for the presence of the epithelium. The thin anterior layer that protects the cornea from the external actions has a scant relevance from the mechanical point of view, and it has been neglected in most numerical models of the cornea, which assign to the entire cornea the mechanical properties of the stroma. Yet, modern corneal topographers capture the geometry of the epithelium, which can be naturally included into a patient-specific solid model of the cornea, treated as a multi-layer solid. For numerical applications, the presence of a thin layer on the anterior cornea requires a finer discretization and the definition of two constitutive models (including the corresponding properties) for stroma and epithelium. In this study, we want to assess the relevance of the inclusion of the epithelium in the model of the cornea, by analyzing the effects in terms of uncertainties of the mechanical properties, stress distribution across the thickness, and numerical discretization. We conclude that if the epithelium is modeled as stroma, the material properties should be reduced by 10%. While this choice represents a sufficiently good approximation for the simulation of in vivo mechanical tests, it might result into an under-estimation of the postoperative stress in the simulation of refractive surgery.

Basement Membranes, Brittlestar Tendons, and Their Mechanical Adaptability

Basement membranes (BMs) are thin layers of extracellular matrix that separate epithelia, endothelia, muscle cells, and nerve cells from adjacent interstitial connective tissue. BMs are ubiquitous in almost all multicellular animals, and their composition is highly conserved across the Metazoa. There is increasing interest in the mechanical functioning of BMs, including the involvement of altered BM stiffness in development and pathology, particularly cancer metastasis, which can be facilitated by BM destabilization. Such BM weakening has been assumed to occur primarily through enzymatic degradation by matrix metalloproteinases. However, emerging evidence indicates that non-enzymatic mechanisms may also contribute. In brittlestars (Echinodermata, Ophiuroidea), the tendons linking the musculature to the endoskeleton consist of extensions of muscle cell BMs. During the process of brittlestar autotomy, in which arms are detached for the purpose of self-defense, muscles break away from the endoskeleton as a consequence of the rapid destabilization and rupture of their BM-derived tendons. This contribution provides a broad overview of current knowledge of the structural organization and biomechanics of non-echinoderm BMs, compares this with the equivalent information on brittlestar tendons, and discusses the possible relationship between the weakening phenomena exhibited by BMs and brittlestar tendons, and the potential translational value of the latter as a model system of BM destabilization.

Balanced activation of Nrf-2/ARE mediates the protective effect of sulforaphane on keratoconus in the cell mechanical microenvironment

Keratoconus (KC) is a progressive degenerative disease that usually occurs bilaterally and is characterized by corneal thinning and apical protrusion of the cornea. Oxidative stress is an indicator of the accumulation of reactive oxygen species (ROS), and KC keratocytes exhibit increased ROS production compared with that of normal keratocytes. Therefore, oxidative stress in KC keratocytes may play a major role in the development and progression of KC. Here, we investigated the protective effect of sulforaphane (SF) antioxidants using a hydrogel-simulated model of the cell mechanical microenvironment of KC. The stiffness of the KC matrix microenvironment in vitro was 16.70 kPa and the stiffness of the normal matrix microenvironment was 34.88 kPa. Human keratocytes (HKs) were cultured for 24 h before observation or drug treatment with H2O2 in the presence or absence of SF. The levels of oxidative stress, nuclear factor E2-related factor 2 (Nrf-2) and antioxidant response element (ARE) were detected. The high-stress state of HKs in the mechanical microenvironment of KC cells compensates for the activation of the Nrf-2/ARE signaling pathway. H2O2 leads to increased oxidative stress and decreased levels of antioxidant proteins in KC. In summary, SF can reduce endogenous and exogenous oxidative stress and increase the antioxidant capacity of cells.

Use of discarded corneo-scleral rims to create cornea-like tissue

BACKGROUND

Corneal disease is a major cause of blindness. Transplantation of cadaver-derived corneas (keratoplasty) is still the current therapy of choice; however, the global shortage of donor corneas continues to drive a search for alternatives. To this end, biosynthetic corneal substitutes have recently begun to gain importance. Here, we present a novel method for the generation of a cornea-like tissue (CLT), using corneo-scleral rims discarded after keratoplasty.

METHODS AND RESULTS

Type I collagen was polymerized within the corneo-scleral rim, which functioned as a 'host' mould, directing the 'guest' collagen to polymerize into disc-shaped cornea-like material (CLM), displaying the shape, curvature, thickness, and transparency of normal cornea. This polymerization of collagen appears to derive from some morphogenetic influence exerted by the corneo-scleral rim. Once the CLM had formed naturally, we used collagen crosslinking to fortify it, and then introduced cells to generate a stratified epithelial layer to create cornea-like tissue (CLT) displaying characteristics of native cornea. Through the excision and reuse of rims, each rim turned out to be useful for the generation of multiple cornea-shaped CLTs.

CONCLUSIONS

The approach effectively helps to shorten the gap between demand and supply of CLMs/CLTs for transplantation. We are exploring the surgical transplantation of this CLT into animal eyes, as keratoprostheses, as a precursor to future applications involving human eyes. It is possible to use either the CLM or CLT, for patients with varying corneal blinding diseases.

Squishy matters - Corneal mechanobiology in health and disease

The cornea, as a dynamic and responsive tissue, constantly interacts with mechanical forces in order to maintain its structural integrity, barrier function, transparency and refractive power. Cells within the cornea sense and respond to various mechanical forces that fundamentally regulate their morphology and fate in development, homeostasis and pathophysiology. Corneal cells also dynamically regulate their extracellular matrix (ECM) with ensuing cell-ECM crosstalk as the matrix serves as a dynamic signaling reservoir providing biophysical and biochemical cues to corneal cells. Here we provide an overview of mechanotransduction signaling pathways then delve into the recent advances in corneal mechanobiology, focusing on the interplay between mechanical forces and responses of the corneal epithelial, stromal, and endothelial cells. We also identify species-specific differences in corneal biomechanics and mechanotransduction to facilitate identification of optimal animal models to study corneal wound healing, disease, and novel therapeutic interventions. Finally, we identify key knowledge gaps and therapeutic opportunities in corneal mechanobiology that are pressing for the research community to address especially pertinent within the domains of limbal stem cell deficiency, keratoconus and Fuchs' endothelial corneal dystrophy. By furthering our understanding corneal mechanobiology, we can contextualize discoveries regarding corneal diseases as well as innovative treatments for them.

Collagen type XII is undetectable in keratoconus Bowman's layer

PURPOSE

Corneal biomechanical failure is the hallmark of keratoconus (KC); however, the cause of this failure remains elusive. Collagen type XII (COL12A1), which localises to Bowman's layer (BL), is thought to function in stress-bearing areas, such as BL. Given the putative protective role of COL12A1 in biomechanical stability, this study aims to characterise COL12A1 expression in all corneal layers involved in KC.

METHODS

TaqMan quantitative PCR was performed on 31 corneal epithelium samples of progressive KC and myopic control eyes. Tissue microarrays were constructed using full-thickness corneas from 61 KC cases during keratoplasty and 18 non-KC autopsy eyes and stained with an antibody specific to COL12A1. Additionally, COL12A1 was knocked out in vitro in immortalised HEK293 cells.

RESULTS

COL12A1 expression was reduced at transcript levels in KC epithelium compared with controls (ratio: 0.58, p<0.03). Immunohistochemical studies demonstrated that COL12A1 protein expression in BL was undetectable, with reduced expression in KC epithelium, basement membrane and stroma.

CONCLUSIONS

The apparent absence of COL12A1 in KC BL, together with the functional importance that COL12A1 is thought to have in stress bearing areas, suggests that COL12A1 may play a role in the pathogenesis of KC. Further studies are necessary to investigate the mechanisms that lead to COL12A1 dysregulation in KC.

Corneal refractive error and astigmatism in patients aged 6 to 18 years with a history of retinopathy of prematurity and birth weight of <1500 g

PURPOSE

To investigate corneal refractive power (CR) and astigmatism (AS) in 6- to 18-year-old children with a history of retinopathy of prematurity (ROP) and birth weight of <1500 g who either did or did not undergo retinal photocoagulation (PC).

STUDY DESIGN

Retrospective study.

METHODS

We examined 143 eyes of 77 children in 2021. The children were divided into three groups for evaluation of CR and AS: those with a birth weight of ≥2500 g (normal birth weight [NBW] group, 13 eyes) as controls, those with spontaneously resolved ROP (sr-ROP group, 27 eyes), and those who underwent PC for treatment of ROP (PC-ROP group, 103 eyes). Swept-source anterior segment optical coherence tomography was used to analyze the cornea.

RESULTS

The median CR in the NBW, sr-ROP, and PC-ROP groups was 42.2 (41.3, 42.8) diopters (D), 44.5 (43.2, 45.5) D, and 45.2 (43.8, 46.6) D, respectively. The median AS in the NBW, sr-ROP, and PC-ROP groups was 1.2 (1.0, 1.5) D, 1.1 (0.8, 1.6) D, and 2.1 (1.4, 2.7) D. In the PC-ROP group, the with-the-rule astigmatic axis was 97%. In all three groups, a strong positive correlation was found between the mean anterior and posterior CR (NBW: r=0.795, sr-ROP: r=0.842, PC-ROP: r=0.890) and AS (NBW: r=0.883, sr-ROP: r=0.841, PC-ROP: r=0.860).

CONCLUSION

CR was significantly higher in the sr-ROP (p=0.013) and PC-ROP (p<0.001) groups than in the NBW group. The PC-ROP group had significantly more AS than the sr-ROP group. There was a strong correlation between the anterior and posterior CR and AS.

Collagen Mimetic Peptides Promote Repair of MMP-1-Damaged Collagen in the Rodent Sclera and Optic Nerve Head

The structural and biomechanical properties of collagen-rich ocular tissues, such as the sclera, are integral to ocular function. The degradation of collagen in such tissues is associated with debilitating ophthalmic diseases such as glaucoma and myopia, which often lead to visual impairment. Collagen mimetic peptides (CMPs) have emerged as an effective treatment to repair damaged collagen in tissues of the optic projection, such as the retina and optic nerve. In this study, we used atomic force microscopy (AFM) to assess the potential of CMPs in restoring tissue stiffness in the optic nerve head (ONH), including the peripapillary sclera (PPS) and the glial lamina. Using rat ONH tissue sections, we induced collagen damage with MMP-1, followed by treatment with CMP-3 or vehicle. MMP-1 significantly reduced the Young's modulus of both the PPS and the glial lamina, indicating tissue softening. Subsequent CMP-3 treatment partially restored tissue stiffness in both the PPS and the glial lamina. Immunohistochemical analyses revealed reduced collagen fragmentation after MMP-1 digestion in CMP-3-treated tissues compared to vehicle controls. In summary, these results demonstrate the potential of CMPs to restore collagen stiffness and structure in ONH tissues following enzymatic damage. CMPs may offer a promising therapeutic avenue for preserving vision in ocular disorders involving collagen remodeling and degradation.

Toward Corneal Limbus In Vitro Model: Regulation of hPSC-LSC Phenotype by Matrix Stiffness and Topography During Cell Differentiation Process

A functional limbal epithelial stem cells (LSC) niche is a vital element in the regular renewal of the corneal epithelium by LSCs and maintenance of good vision. However, little is known about its unique structure and mechanical properties on LSC regulation, creating a significant gap in development of LSC-based therapies. Herein, the effect of mechanical and architectural elements of the niche on human pluripotent derived LSCs (hPSC-LSC) phenotype and growth is investigated in vitro. Specifically, three formulations of polyacrylamide gels with different controlled stiffnesses are used for culture and characterization of hPSC-LSCs from different stages of differentiation. In addition, limbal mimicking topography in polydimethylsiloxane is utilized for culturing hPSC-LSCs at early time point of differentiation. For comparison, the expression of selected key proteins of the corneal cells is analyzed in their native environment through whole mount staining of human donor corneas. The results suggest that mechanical response and substrate preference of the cells is highly dependent on their developmental stage. In addition, data indicate that cells may carry possible mechanical memory from previous culture matrix, both highlighting the importance of mechanical design of a functional in vitro limbus model.

Use of Nanoindentation in Determination of Regional Biomechanical Properties of Rabbit Cornea After UVA Cross-Linking

Purpose

To evaluate the regional effects of different corneal cross-linking (CXL) protocols on corneal biomechanical properties.

Methods

The study involved both eyes of 50 rabbits, and the left eyes were randomized to the five intervention groups, which included the standard CXL group (SCXL), which was exposed to 3-mW/cm2 irradiation, and three accelerated CXL groups (ACXL1-3), which were exposed to ultraviolet-A at irradiations of 9 mW/cm2, 18 mW/cm2, and 30 mW/cm2, respectively, but with the same total dose (5.4 J/cm2). A control (CO) group was not exposed to ultraviolet-A. No surgery was done on the contralateral eyes. The corneas of each group were evaluated by the effective elastic modulus (Eeff) and the hydraulic conductivity (K) within a 7.5-mm radius using nanoindentation measurements.

Results

Compared with the CO group, Eeff (in regions with radii of 0-1.5 mm, 1.5-3.0 mm, and 3.0-4.5 mm) significantly increased by 309%, 276%, and 226%, respectively, with SCXL; by 222%, 209%, and 173%, respectively, with ACXL1; by 111%, 109%, and 94%, respectively, with ACXL2; and by 59%, 41%, and 37%, respectively, with ACXL3 (all P < 0.05). K was also significantly reduced by 84%, 81%, and 78%, respectively, with SCXL; by 75%, 74%, and 70%, respectively, with ACXL1; by 64%, 62%, and 61%, respectively, with ACXL2; and by 33%, 36%, and 32%, respectively, with ACXL3 (all P < 0.05). For the other regions(with radii between 4.5 and 7.5 mm), the SCXL and ACXL1 groups (but not the ACXL2 and ACXL3 groups) still showed significant changes in Eeff and K.

Conclusions

CXL had a significant effect on corneal biomechanics in both standard and accelerated procedures that may go beyond the irradiated area. The effect of CXL in stiffening the tissue and reducing permeability consistently decreased with reducing the irradiance duration.

Possible depth-resolved reconstruction of shear moduli in the cornea following collagen crosslinking (CXL) with optical coherence tomography and elastography

Corneal collagen crosslinking (CXL) is commonly used to prevent or treat keratoconus. Although changes in corneal stiffness induced by CXL surgery can be monitored with non-contact dynamic optical coherence elastography (OCE) by tracking mechanical wave propagation, depth dependent changes are still unclear if the cornea is not crosslinked through the whole depth. Here, phase-decorrelation measurements on optical coherence tomography (OCT) structural images are combined with acoustic micro-tapping (AµT) OCE to explore possible reconstruction of depth-dependent stiffness within crosslinked corneas in an ex vivo human cornea sample. Experimental OCT images are analyzed to define the penetration depth of CXL into the cornea. In a representative ex vivo human cornea sample, crosslinking depth varied from ∼100 µm in the periphery to ∼150 µm in the cornea center and exhibited a sharp in-depth transition between crosslinked and untreated areas. This information was used in an analytical two-layer guided wave propagation model to quantify the stiffness of the treated layer. We also discuss how the elastic moduli of partially CXL-treated cornea layers reflect the effective engineering stiffness of the entire cornea to properly quantify corneal deformation.

Characterization of the Immediate and Delayed Biomechanical Response to UV-A Crosslinking of Human Corneas

PURPOSE

Keratoconus leads to visual deterioration due to irregular astigmatism and corneal thinning. Riboflavin-based corneal UV-A crosslinking (CXL) induces novel intramolecular and intermolecular links resulting in corneal tissue stiffening, thereby halting disease progression. The purpose of this study was to analyze the immediate and delayed biomechanical responses of human donor corneas to CXL.

METHODS

CXL was performed according to the Dresden protocol to corneas not suitable for transplantation. Biomechanical properties were subsequently monitored by measuring the Young modulus using nanoindentation. The immediate tissue response was determined after 0, 1, 15, and 30 minutes of irradiation. Delayed biomechanical effects were investigated with follow-up measurements immediately and 1, 3, and 7 days after CXL.

RESULTS

Young's modulus indicated a linear trend in direct response to increasing irradiation times (mean values: total 61.31 kPa [SD 25.53], 0 minutes 48.82 kPa [SD 19.73], 1 minute 53.44 kPa [SD 25.95], 15 minutes 63.56 kPa [SD 20.99], and 30 minutes 76.76 kPa [SD 24.92]). The linear mixed model for the elastic response of corneal tissue was 49.82 kPa + (0.91 kPa/min × time [minutes]); P < 0.001. The follow-up measurements showed no significant delayed changes in the Young modulus (mean values: total 55,28 kPa [SD 15.95], immediately after CXL 56,83 kPa [SD 18.74], day 1 50.28 kPa [SD 14.15], day 3 57.08 kPa [SD 14.98], and day 7 56.83 kPa [SD 15.07]).

CONCLUSIONS

This study suggests a linear increase of corneal Young modulus as a function of CXL timing. No significant short-term delayed biomechanical changes posttreatment were observed.

Predictability of Astigmatism Correction by Arcuate Incisions with a Femtosecond Laser Using the Gaussian Approximation Calculation

Planning astigmatic correction is a complex task. Biomechanical simulation models are useful for predicting the effects of the physical procedure on the cornea. Algorithms based on these models allow preoperative planning and simulate the outcome of patient-specific treatment. The objective of this study was to develop a customised optimisation algorithm and determine the predictability of astigmatism correction by femtosecond laser arcuate incisions. In this study, biomechanical models and Gaussian approximation curve calculations were used for surgical planning. Thirty-four eyes with mild astigmatism were included, and corneal topographies were evaluated before and after femtosecond laser-assisted cataract surgery with arcuate incisions. The follow-up time was up to 6 weeks. Retrospective data showed a significant reduction in postoperative astigmatism. A total of 79.4% showed a postoperative astigmatic value less than 1 D. Clinical refraction was significantly reduced from -1.39 ± 0.79 D preoperatively to -0.86 ± 0.67 D postoperatively (p 0.02). A positive reduction in topographic astigmatism was also observed (p < 0.00). The best-corrected visual acuity increased postoperatively (p < 0.001). We can conclude that customised simulations based on corneal biomechanics are a valuable tool for correcting mild astigmatism with corneal incisions in cataract surgery to improve postoperative visual outcomes.

In vivo corneal elastography: A topical review of challenges and opportunities

Clinical measurement of corneal biomechanics can aid in the early diagnosis, progression tracking, and treatment evaluation of ocular diseases. Over the past two decades, interdisciplinary collaborations between investigators in optical engineering, analytical biomechanical modeling, and clinical research has expanded our knowledge of corneal biomechanics. These advances have led to innovations in testing methods (ex vivo, and recently, in vivo) across multiple spatial and strain scales. However, in vivo measurement of corneal biomechanics remains a long-standing challenge and is currently an active area of research. Here, we review the existing and emerging approaches for in vivo corneal biomechanics evaluation, which include corneal applanation methods, such as ocular response analyzer (ORA) and corneal visualization Scheimpflug technology (Corvis ST), Brillouin microscopy, and elastography methods, and the emerging field of optical coherence elastography (OCE). We describe the fundamental concepts, analytical methods, and current clinical status for each of these methods. Finally, we discuss open questions for the current state of in vivo biomechanics assessment techniques and requirements for wider use that will further broaden our understanding of corneal biomechanics for the detection and management of ocular diseases, and improve the safety and efficacy of future clinical practice.

[Corneal epithelial biomechanics: Resistance to stress and role in healing and remodeling]

The corneal epithelium is one of the first tissue barriers of the eye against the environment. In recent years, many studies provided better knowledge of its healing, its behavior and its essential role in the optical system of the eye. At the crossroads of basic science and clinical medicine, the study of the mechanical stresses applied to the cornea makes it possible to learn the behavior of epithelial cells and better understand ocular surface disease. We describe herein the current knowledge about the adhesion systems of the corneal epithelium and their resistance to mechanical stress. We will also describe the involvement of these mechanisms in corneal healing and their role in epithelial dynamics. Adhesion molecules of the epithelial cells, especially hemidesmosomes, allow the tissue cohesion required to maintain the integrity of the corneal epithelium against the shearing forces of the eyelids as well as external forces. Their regeneration after a corneal injury is mandatory for the restoration of a healthy epithelium. Mechanotransduction plays a significant role in regulating epithelial cell behavior, and the study of the epithelium's response to mechanical forces helps to better understand the evolution of epithelial profiles after refractive surgery. A better understanding of corneal epithelial biomechanics could also help improve future therapies, particularly in the field of tissue engineering.

Micro-mechanical fingerprints of the rat bladder change in actinic cystitis and tumor presence

Tissue mechanics determines tissue homeostasis, disease development and progression. Bladder strongly relies on its mechanical properties to perform its physiological function, but these are poorly unveiled under normal and pathological conditions. Here we characterize the mechanical fingerprints at the micro-scale level of the three tissue layers which compose the healthy bladder wall, and identify modifications associated with the onset and progression of pathological conditions (i.e., actinic cystitis and bladder cancer). We use two indentation-based instruments (an Atomic Force Microscope and a nanoindenter) and compare the micromechanical maps with a comprehensive histological analysis. We find that the healthy bladder wall is a mechanically inhomogeneous tissue, with a gradient of increasing stiffness from the urothelium to the lamina propria, which gradually decreases when reaching the muscle outer layer. Stiffening in fibrotic tissues correlate with increased deposition of dense extracellular matrix in the lamina propria. An increase in tissue compliance is observed before the onset and invasion of the tumor. By providing high resolution micromechanical investigation of each tissue layer of the bladder, we depict the intrinsic mechanical heterogeneity of the layers of a healthy bladder as compared with the mechanical properties alterations associated with either actinic cystitis or bladder tumor.

Therapeutic contact lenses for the treatment of corneal and ocular surface diseases: advances in extended and targeted drug delivery

The eye is one of the most important organs in the human body providing critical information on the environment. Many corneal diseases can lead to vision loss affecting the lives of people around the world. Ophthalmic drug delivery has always been a major challenge in the medical sciences. Since traditional methods are less efficient (∼ 5%) at delivering drugs to ocular tissues, contact lenses have generated growing interest in ocular drug delivery due to their potential to enhance drug bioavailability in ocular tissues. The main techniques used to achieve sustained release are discussed in this review, including soaking in drug solutions, incorporating drug into multilayered contact lenses, use of vitamin E barriers, molecular imprinting, nanoparticles, micelles and liposomes. The most clinically relevant results on different eye pathologies are presented. In addition, this review summarizes the benefits of contact lenses over eye drops, strategies for incorporating drugs into lenses to achieve sustained release, results of in vitro and in vivo studies, and the recent advances in the commercialization of therapeutic contact lenses for allergic conjunctivitis.

Hydroxycamptothecin and Substratum Stiffness Synergistically Regulate Fibrosis of Human Corneal Fibroblasts

Corneal fibrosis is a common outcome of inappropriate repair associated with trauma or ocular infection. Altered biomechanical properties with increased corneal stiffness is a feature of fibrosis that cause corneal opacities, resulting in severe visual impairment and even blindness. The present study aims to determine the effect of hydroxycamptothecin (HCPT) and matrix stiffness on transforming growth factor-β1 (TGF-β1)-induced fibrotic processes in human corneal fibroblasts (HTK cells). HTK cells were cultured on substrates with different stiffnesses ("soft", ∼261 kPa; "stiff", ∼2.5 × 10³ kPa) and on tissue culture plastic (TCP, ∼10⁶ kPa) and simultaneously treated with or without 1 μg/mL HCPT and 10 ng/mL TGF-β1. We found that HCPT induced decreased cell viability and antiproliferative effects on HTK cells. TGF-β1-induced expression of fibrosis-related genes (FN1, ACTA2) was reduced if the cells were simultaneously treated with HCPT. Substrate stiffness did not affect the expression of fibrosis-related genes. The TGF-β1 induced expression of FN1 on both soft and stiff substrates was reduced if cells were simultaneously treated with HCPT. However, this trend was not seen for ACTA2, i.e., the TGF-β1 induced expression of ACTA2 was not reduced by simultaneous treatment of HCPT in either soft or stiff substrate. Instead, HCPT treatment in the presence of TGF-β1 resulted in increased gene expression of keratocyte phenotype makers (LUM, KERA, AQP1, CHTS6) on both substrate stiffnesses. In addition, the protein expression of keratocyte phenotype makers LUM and ALDH3 was increased in HTK cells simultaneously treated with TGF-β1 and HCPT on stiff substrate as compared to control, i.e., without HCPT. In conclusion, we found that HCPT can reduce TGF-β1-induced fibrosis and promote the keratocyte phenotype in a substrate stiffness dependent manner. Thus, HCPT stimulation might be an approach to stimulate keratocytes in the appropriate healing stage to avoid or reverse fibrosis and achieve more optimal corneal wound healing.

Keratoconus: A Biomechanical Perspective

PURPOSE

The relevance of corneal biomechanics and the importance of including it in the clinical assessment of corneal ectasias are being increasingly recognized. The connection between corneal ultrastructure, biomechanical properties, and optical function is exemplified by a condition like keratoconus. Biomechanical instability is seen as the underlying basis for the secondary morphological changes in the cornea. Asymmetric biomechanical weakening is believed to drive progressive corneal steepening and thinning. Biomechanical strengthening is the principle of collagen crosslinking that has been shown to effectively arrest progression of the keratoconus. Corneal biomechanics has therefore ignited the interest of researchers and clinicians alike and has given us new insights into the cause and course of the disease. This article is an overview of the extensive work published, predominantly in the last two decades, on the biomechanical aspect of keratoconus.

METHODS

Published articles on corneal biomechanics in the specific context of keratoconus were reviewed, based on an electronic search using PubMed, Elsevier, and Science Direct. The search terms used included "Corneal Biomechanics," "Mechanical properties of the cornea," "Corneal ultrastructure," "Corneal Collagen," and "Keratoconus". Articles pertaining to refractive surgery, keratoplasty, collagen crosslinking, or intrastromal rings were excluded.

RESULTS

The electronic search revealed more than 500 articles, from which 80 were chosen for this article.

CONCLUSIONS

The structural and organizational pattern of the corneal stroma determines its mechanical properties and are responsible for the maintenance of the normal shape and function of the cornea. Changes in the ultrastructure are responsible for the biomechanical instability that leads to corneal ectasia. As non-invasive methods for evaluating corneal biomechanics in vivo evolve, our ability to diagnose subclinical keratoconus will improve, allowing identification of patients at risk to develop ectasia and to allow early treatment to arrest progression of the disease.

The TGM2 inhibitor cysteamine hydrochloride does not impact corneal epithelial and stromal wound healing in vitro and in vivo

Corneal wound healing is integral for resolution of corneal disease or for post-operative healing. However, corneal scarring that may occur secondary to this process can significantly impair vision. Tissue transglutaminase 2 (TGM2) inhibition has shown promising antifibrotic effects and thus holds promise to prevent or treat corneal scarring. The commercially available ocular solution for treatment of ocular manifestations of Cystinosis, Cystaran®, contains the TGM2 inhibitor cysteamine hydrochloride (CH). The purpose of this study is to assess the safety of CH on corneal epithelial and stromal wounds, its effects on corneal wound healing, and its efficacy against corneal scarring following wounding. Quantitative polymerase chain reaction (qPCR) and immunohistochemistry (IHC) were first used to quantify and localize TGM2 expression in the cornea. Subsequently, (i) the in vitro effects of CH at 0.163, 1.63, and 16.3 mM on corneal epithelial cell migration was assessed with an epithelial cell migration assay, and (ii) the in vivo effects of application of 1.63 mM CH on epithelial and stromal wounds was assessed in a rabbit model with ophthalmic examinations, inflammation scoring, color and fluorescein imaging, optical coherence tomography (OCT), and confocal biomicroscopy. Post-mortem assessment of corneal tissue post-stromal wounding included biomechanical characterization (atomic force microscopy (AFM)), histology (H&E staining), and determining incidence of myofibroblasts (immunostaining against α-SMA) in wounded corneal tissue. TGM2 expression was highest in corneal epithelial cells. Application of the TGM2 inhibitor CH did not affect in vitro epithelial cell migration at the two lower concentrations tested. At 16.3 mM, decreased cell migration was observed. In vivo application of CH at 57 mM was well tolerated and did not adversely affect wound healing. No difference in corneal scarring was found between CH treated and vehicle control eyes. This study shows that the TGM2 inhibitor CH, at the FDA-approved dose, is well tolerated in a rabbit model of corneal wound healing and does not adversely affect epithelial or stromal wound healing. This supports the safe use of this medication in Cystinosis patients with open corneal wounds. CH did not have an effect on corneal scarring in this study, suggesting that Cystaran® administration to patients with corneal wounds is unlikely to decrease corneal fibrosis.

In Vivo Evaluation of the Effects of SMILE with Different Amounts of Stromal Ablation on Corneal Biomechanics by Optical Coherence Elastography

This work aims to depth-resolved quantitatively analyze the effect of different stromal ablation amounts on the corneal biomechanical properties during small incision lenticule extraction (SMILE) using optical coherence elastography (OCE). A 4.5-MHz ultrasonic transducer was used to excite elastic waves in the corneal tissue. The OCE system combined with the antisymmetric Lamb wave model was employed to achieve a high-resolution, high-sensitivity, and depth-resolved quantitative detection of the corneal Young's modulus. Eighteen rabbits were randomly divided into three groups; each group had six rabbits. The first and second groups underwent -3D and -6D SMILE surgeries, and the third group was the control group, respectively. Young's modulus of the corneal cap and residual stromal bed (RSB) were both increased after SMILE, which shared the stress under intraocular pressure (IOP). Furthermore, the Young's modulus of both the corneal cap and RSB after 3D SMILE group were significantly lower than that in the -6D group, which indicated that the increases in the post-operative corneal Young's modulus were positively correlated with the amount of stromal ablation. The OCE system for quantitative spatial characterization of corneal biomechanical properties can provide useful information on the extent of safe ablation for SMILE procedures.

Hyaluronan Modulates the Biomechanical Properties of the Cornea

Purpose

Hyaluronan (HA) is a major constituent of the extracellular matrix (ECM) that has high viscosity and is essential for maintaining tissue hydration. In the cornea, HA is enriched in the limbal region and is a key component of the limbal epithelial stem cell niche. HA is upregulated after injury participating in the formation of the provisional matrix, and has a key role in regulating the wound healing process. This study investigated whether changes in the distribution of HA before and after injury affects the biomechanical properties of the cornea in vivo.

Methods

Corneas of wild-type (wt) mice and mice lacking enzymes involved in the biosynthesis of HA were analyzed before, immediately after, and 7 and 14 days after a corneal alkali burn (AB). The corneas were evaluated using both a ring light and fluorescein stain by in vivo confocal microscopy, optical coherence elastography (OCE), and immunostaining of corneal whole mounts.

Results

Our results show that wt mice and mice lacking HA synthase (Has)1 and 3 present an increase in corneal stiffness 7 and 14 days after AB without a significant increase in HA expression and absence of scarring at 14 days after AB. In contrast, mice lacking Has2 present a significant decrease in corneal stiffness, with a significant increase in HA expression and scarring at 14 days after AB.

Conclusions

Our findings show that the mechanical properties of the cornea are significantly modulated by changes in HA distribution following alkali burn.

Unraveling the mechanobiology of cornea: From bench side to the clinic

The cornea is a transparent, dome-shaped structure on the front part of the eye that serves as a major optic element and a protector from the external environment. Recent evidence shows aberrant alterations of the corneal mechano-environment in development and progression of various corneal diseases. It is, thus, critical to understand how corneal cells sense and respond to mechanical signals in physiological and pathological conditions. In this review, we summarize the corneal mechano-environment and discuss the impact of these mechanical cues on cellular functions from the bench side (in a laboratory research setting). From a clinical perspective, we comprehensively review the mechanical changes of corneal tissue in several cornea-related diseases, including keratoconus, myopia, and keratectasia, following refractive surgery. The findings from the bench side and clinic underscore the involvement of mechanical cues in corneal disorders, which may open a new avenue for development of novel therapeutic strategies by targeting corneal mechanics.

Advances in Regulatory Strategies of Differentiating Stem Cells towards Keratocytes

Corneal injury is a commonly encountered clinical problem which led to vision loss and impairment that affects millions of people worldwide. Currently, the available treatment in clinical practice is corneal transplantation, which is limited by the accessibility of donors. Corneal tissue engineering appears to be a promising alternative for corneal repair. However, current experimental strategies of corneal tissue engineering are insufficient due to inadequate differentiation of stem cell into keratocytes and thus cannot be applied in clinical practice. In this review, we aim to clarify the role and effectiveness of both biochemical factors, physical regulation, and the combination of both to induce stem cells to differentiate into keratocytes. We will also propose novel perspectives of differentiation strategy that may help to improve the efficiency of corneal tissue engineering.

[Experimental studies of the biomechanical properties of the cornea]

The review presents the results of experimental studies of the biomechanical properties of the cornea. Selective evaluation of the individual corneal structures (for example, limiting membranes) using classical mechanical tests is to a certain extent limited due to the rather small thickness of these structures and the related difficulties in sample fixation. In real practice, the use of a method better adapted for conducting such studies - atomic force microscopy (AFM) - remains promising, since on the one hand it eliminates the need for mechanical capture and retention of the sample, and on the other - provides the capability for studying its segments separately.

Recent Advances in Natural Materials for Corneal Tissue Engineering

Given the incidence of corneal dysfunctions and diseases worldwide and the limited availability of healthy, human donors, investigators are working to generate engineered cellular and acellular therapeutic approaches as alternatives to corneal transplants from human cadavers. These engineered strategies aim to address existing complications with human corneal transplants, including graft rejection, infection, and complications resulting from surgical methodologies. The main goals of these research endeavors are to (1) determine ideal mechanical properties, (2) devise methodologies to improve the efficacy of engineered corneal grafts and cell-based therapies, and (3) optimize transplantation of engineered tissue structures in the eye. Thus, recent innovations have sought to address these challenges through both in vitro and in vivo studies. This review covers recent work aimed at evaluating engineered materials, potential therapeutic cells, and the resulting cell-material interactions that lead to optimal corneal graft properties. Furthermore, we discuss promising strategies in corneal tissue engineering techniques and in vivo studies in animal models.

Microstructure characteristics of cornea of some birds: a comparative study

Background

Light is the critical factor that affects the eye's morphology and auxiliary plans. The ecomorphological engineering of the cornea aids the physiological activities of the cornea during connections between photoreceptor neurons and light photons. Cornea was dissected free from the orbit from three avian species as ibis (Eudocium albus), duck (Anas platyrhynchus domesticus) and hawk (Buteo Buteo) and prepared for light and scanning electron microscopy and special stain for structural comparison related to function.

Results

The three investigated avian species are composed of three identical layers; epithelium, stroma, and endothelium, and two basement membranes; bowman's and Descemet’s membrane, separating two cellular layers, except for B. buteo which only has a Descemet’s membrane. The corneal layers in the investigated species display different affinity to stain with Periodic Acid Schiff stain. The external corneal surface secured by different normal epithelial cells ran from hexagonal to regular polygonal cells. Those epithelial cells are punctured by different diameter microholes and microplicae and microvilli of various length. Blebs are scarcely distributed over their surface. The present investigation utilized histological, histochemical and SEM examination.

Conclusions

The study presents a brief image/account of certain structures of cornea for three of Avian’s species. Data distinguish the anatomic structures of the owl's eye. The discussion explains the role of some functional anatomical structures all through the vision.

Transcorneal delivery of topically applied silver nanoparticles does not delay epithelial wound healing

Silver nanoparticles (AgNPs) are a common antimicrobial additive for a variety of applications, including wound care. However, AgNPs often undergo dissolution resulting in release of silver ions, with subsequent toxicity to mammalian cells. The cornea is a primary exposure site to topically administered AgNPs in and around the eye but their impact on corneal wound healing is understudied. Thus, the purpose of this study was to determine in vitro toxicity of AgNPs on corneal epithelial cells and fibroblasts as well as their effects on corneal epithelial wound healing utilizing an in vivo rabbit model. Non-coated 20 nm sized AgNP (AgNP-20) as well as 1% and 10% silver silica NPs (AgSiO₂NPs) were tested at concentrations ranging from 0.05-250 μg/mL. Immortalized human corneal epithelial (hTCEpi) cells and primary rabbit corneal fibroblasts (RCFs) were incubated for 24 h with AgNPs and cell viability was tested. Additionally, a round wound healing assay was performed to determine hTCEpi cell migration. Quantitative real-time PCR and western blot analysis was performed to determine α-smooth muscle actin (α-SMA, a myofibroblast marker) mRNA and protein expression, respectively, in RCFs treated with 50 μg/mL of AgNPs. Corneal epithelial wound healing was evaluated with 1%-AgSiO₂NPs (10 and 250 μg/mL) using an in vivo rabbit model. Rabbits were subsequently euthanized, and histologic sections of the enucleated globes were used to determine corneal penetration of 1%-AgSiO₂NPs with autometallography and hyperspectral darkfield microscopy. Cell viability of both the hTCEpi cells and fibroblasts was significantly decreased by the three AgNPs in a dose dependent manner. Migration of hTCEpi cells was significantly inhibited by the three AgNPs. Alpha-SMA mRNA expression was significantly inhibited with three AgNPs, but only the 1%-AgSiO₂NPs inhibited protein expression of α-SMA. In vivo epithelial wound closure did not significantly differ between groups treated with 10 or 250 μg/mL of 1%-AgSiO₂NPs or vehicle control. The 1%-AgSiO₂NPs penetrated throughout all corneal layers and into the anterior chamber in all treated eyes with no histopathological changes observed. In conclusion, the 1%-AgSiO₂NPs are safe and have potential therapeutic applications through its efficacy of the corneal penetration and reduced scar formation during corneal wound healing.

Contribution of Bowman layer to corneal biomechanics

PURPOSE

To compare the elastic modulus of thin corneal lamellas using 2D stress-strain extensometry in healthy ex vivo human corneal lamellas with or without the presence of Bowman layer.

SETTING

Center for Applied Biotechnology and Molecular Medicine, University of Zurich, Switzerland; ELZA Institute, Dietikon, Switzerland; Department of Ophthalmology, Philipps University of Marburg, Germany.

DESIGN

Prospective experimental laboratory study.

METHODS

Healthy human corneas were stripped of Descemet membrane and the endothelium for Descemet membrane endothelial keratoplasty. After epithelium removal, corneas were divided into 2 groups. In Group 1, Bowman layer was ablated with an excimer laser (20 μm thick, 10 mm). In Group 2, Bowman layer was left intact. Then, a lamella was cut from the anterior cornea with an automated microkeratome. Elastic and viscoelastic material properties were analyzed by 2D stress-strain extensometry between 0.03 and 0.70 N.

RESULTS

Twenty-six human corneas were analyzed. The mean lamella thickness was 160 ± 37 μm in corneas with Bowman layer and 155 ± 22 μm in corneas without. No statistically significant differences between flaps with and without Bowman layer were observed in the tangential elastic modulus between 5% and 20% strain (11.5 ± 2.9 kPa vs 10.8 ± 3.7 kPa, P > .278).

CONCLUSIONS

The presence or absence of Bowman layer did not reveal a measurable difference in corneal stiffness. This may indicate that the removal of Bowman layer during photorefractive keratectomy does not represent a disadvantage to corneal biomechanics.

The impact of biomechanics on corneal endothelium tissue engineering

The integrity of innermost layer of the cornea, the corneal endothelium, is key to sustaining corneal transparency. Therefore, disease or injury causing loss or damage to the corneal endothelial cell population may threaten vision. Transplantation of corneal tissue is the standard treatment used to replace malfunctioning corneal endothelial cells. However, this surgery is dependent upon donor tissue, which is limited in supply. Hence, tissue engineers have attempted to construct alternative transplantable tissues or cell therapies to alleviate this problem. Nevertheless, the intrinsic non-dividing nature of corneal endothelial cells continues to foil scientists in their attempts to yield large numbers of cells in the laboratory for use in such novel therapies. Interestingly, the contribution of the biomechanical properties of the underlying extracellular matrix (ECM) on cell division, tissue development and maintenance has been extensively investigated in other many cell types. However, the impact of biomechanics on corneal endothelial cell behaviour is relatively unexplored. Here, we describe contemporary tissue engineering solutions aimed at circumventing donor tissue scarcity. We review the ECM structure and biomechanical features of corneal endothelial cells. We discuss the alterations of ECM in endothelial disease development and progression and point out the role of ECM in developing a tissue-engineered corneal endothelium. We highlight the main biomechanical cues, including topographical and mechanical features, that impact cellular behaviors. Finally, we discuss the influence of biomechanical cues on cell and tissue development, and how corneal endothelial cells response to individual biomechanical stimuli in tissue engineering, which have implications for designing an engineered endothelium and maintaining cell function.

Transcriptomic and Immunohistochemical Analysis of Progressive Keratoconus Reveal Altered WNT10A in Epithelium and Bowman's Layer

Purpose

To identify global gene expression changes in the corneal epithelium of keratoconus (KC) patients compared to non-KC myopic controls.

Methods

RNA-sequencing was performed on corneal epithelium samples of five progressive KC and five myopic control patients. Selected results were validated using TaqMan quantitative PCR (qPCR) on 31 additional independent samples, and protein level validation was conducted using western blot analysis on a subset. Immunohistochemistry was performed on tissue microarrays containing cores from over 100 KC and control cases. WNT10A transcript levels in corneal epithelium were correlated with tomographic indicators of KC disease severity in 15 eyes. Additionally, WNT10A was overexpressed in vitro in immortalized corneal epithelial cells.

Results

WNT10A was found to be underexpressed in KC epithelium at the transcript (ratio KC/control = 0.59, P = 0.02 per RNA-sequencing study; ratio = 0.66, P = 0.03 per qPCR) and protein (ratio = 0.07, P = 0.06) levels. Immunohistochemical analysis also indicated WNT10A protein was decreased in Bowman's layer of KC patients. In contrast, WNT10A transcript level positively correlated with increased keratometry (Kmax ρ = 0.57, P = 0.02). Finally, WNT10A positively regulated COL1A1 expression in corneal epithelial cells.

Conclusions

A specific Wnt ligand, WNT10A, is reduced at the mRNA and protein level in KC epithelium and Bowman's layer. This ligand positively regulates collagen type I expression in corneal epithelial cells. The results suggest that WNT10A expression in the corneal epithelium may play a role in progressive KC.

Micro-mechanical properties of corneal scaffolds from two different bio-models obtained by an efficient chemical decellularization

The present study elucidates the impact of detergent-based chemical decellularization on the micro-mechanical properties of porcine and rabbit corneas for the purpose of extracellular matrix (ECM) derived scaffolds. Aiming to optimize the decellularization process, different concentrations of Sodium Dodecyl Sulfate (SDS), Triton X-100 and CHAPS detergents were assessed on their ability to decellularize corneas from both bio-models at incubation periods of 12 and 24h. We evaluated the effect of decellularization on corneal ECM Young's Modulus and various area's roughness parameters (topography features) at a microscale by using Atomic Force Microscopy (AFM). Only SDS presented adequate decellularization properties at the selected concentrations (0.2, 0.5 and 1%) and incubation periods. All topography features displayed by native corneas were preserved after SDS treatments, while no statistically significant differences were identified for the average value of Young's Modulus between the control samples and those treated with 0.2% SDS (rabbit corneas) and 0.5% SDS (porcine corneas) after 12h. In this sense, cornea decellularization procedures can be improved by simultaneously reducing SDS concentration and incubation period. AFM is a useful tool to perform biomechanical analysis of the effect of decellularization on scaffold micro-mechanics. Evaluation of the scaffold mechanical behavior at a microscale could help in understanding cell-scaffold interactions in terms of mechanotransduction, complementing macroscale techniques (e.g. tensile tests) relevant for tissue engineering quality control and decision-making.

Silk Film Stiffness Modulates Corneal Epithelial Cell Mechanosignaling

Silk fibroin films are excellent candidate biomaterials for corneal tissue engineering due to their optical transparency, biocompatibility, and mechanical strength. Their tunable chemical and mechanical properties open the possibility of engineering cellular microenvironments that can both mimic native corneal tissue and provide stimuli to actively promote wound regeneration. While silk film mechanical properties, such as surface topography, have demonstrated the ability to control corneal epithelial cell wound regenerating behavior, few studies have explored the stiffness tunability of these films and its cellular effects. Cells are known actively sense the stiffness of their surroundings and processes such as cell adhesion, migration, proliferation, and expression of stem markers can be strongly influenced by matrix stiffness. This study develops technical solutions that allow for both the fabrication of films with stiffnesses similar to corneal tissue and also for their characterization in an aqueous, native-like environment at a scale relevant to cellular forces. Physiological evidence demonstrates that corneal epithelial cells are mechanosensitive to films of different stiffnesses and show that cell spreading, cytoskeletal tension, and molecular mechanotransducer localization are associated with film stiffness. These results indicate that silk film stiffness can be used to regulate cell behavior for the purposes of ocular surface repair.

Nano-Scale Stiffness and Collagen Fibril Deterioration: Probing the Cornea Following Enzymatic Degradation Using Peakforce-QNM AFM

Under physiological conditions, the cornea is exposed to various enzymes, some of them have digestive actions, such as amylase and collagenase that may change the ultrastructure (collagen morphology) and sequentially change the mechanical response of the cornea and distort vision, such as in keratoconus. This study investigates the ultrastructure and nanomechanical properties of porcine cornea following incubation with α-amylase and collagenase. Atomic force microscopy (AFM) was used to capture nanoscale topographical details of stromal collagen fibrils (diameter and D-periodicity) and calculate their elastic modulus. Samples were incubated with varying concentrations of α-amylase and collagenase (crude and purified). Dimethylmethylene blue (DMMB) assay was utilised to detect depleted glycosaminoglycans (GAGs) following incubation with amylase. Collagen fibril diameters were decreased following incubation with amylase, but not D-periodicity. Elastic modulus was gradually decreased with enzyme concentration in amylase-treated samples. Elastic modulus, diameter, and D-periodicity were greatly reduced in collagenase-treated samples. The effect of crude collagenase on corneal samples was more pronounced than purified collagenase. Amylase was found to deplete GAGs from the samples. This enzymatic treatment may help in answering some questions related to keratoconus, and possibly be used to build an empirical animal model of keratoconic corneas with different progression levels.

Quasi-Static Optical Coherence Elastography to Characterize Human Corneal Biomechanical Properties

Purpose

Quasi-static optical coherence elastography (OCE) is an emerging technology to investigate corneal biomechanical behavior in situations similar to physiological stress conditions. Herein OCE was applied to evaluate previously inaccessible biomechanical characteristics of human corneal tissue and to study the role of Bowman's layer in corneal biomechanics.

Methods

Human corneal donor buttons (n = 23) were obtained and Descemet's membrane and endothelium were removed. In 11 corneas, Bowman's layer was ablated by a 20 µm stromal excimer laser ablation. Buttons were mounted on an artificial anterior chamber and subjected to a pressure modulation from 10 to 30 mm Hg, and back to 10 mm Hg, in steps of 1 mm Hg. At each step, a spectral-domain optical coherence tomography scan was obtained. Displacements were analyzed by optical flow tracking, and strain over the entire stromal depth was retrieved from the phase gradient of the complex interference signal.

Results

During pressure increase, corneal tissue moved upward (486–585 nm/mm Hg) but did not fully recover (Δ= 2.63 to 8.64 µm) after pressure decrease. Vertical corneal strain distribution was negative in the anterior and positive in the posterior cornea, indicating simultaneous corneal compression and expansion, respectively. Bowman's layer caused minor localized differences in corneal strain distribution.

Conclusions

Corneal strain distribution is more complex than previously assumed, with a fundamental difference in mechanical response between the anterior and posterior stroma. Clinically, OCE technology might be used to monitor the progression of corneal ectatic diseases and to determine the success of corneal cross-linking.

Deciphering the mechanoresponsive role of β-catenin in keratoconus epithelium

Keratoconus (KC) is a corneal dystrophy characterized by progressive ectasia that leads to severe visual impairment and remains one of the leading indications for corneal transplantation. The etiology is believed to be multifactorial and alterations have been documented in the biomechanical, biochemical and ultrastructural characteristics of the cornea. While the exact site of disease origin is still debated, changes in the corneal epithelium are believed to occur even before the disease is clinically manifested. In this study we investigate the possible role of β-catenin as mechanotransducer in KC corneal epithelium. The sheets of corneal epithelium removed from keratoconic eyes when they underwent collagen crosslinking as a therapeutic procedure were used for this study. The healthy corneal epithelium of patients undergoing Laser Refractive Surgery for the correction of their refractive error, served as controls. Immunoblotting and tissue immunofluorescence studies were performed on KC epithelium to analyse the expression and localization of β-catenin, E-cadherin, ZO1, α-catenin, Cyclin D1, α-actinin, RhoA, and Rac123. Co-immunoprecipitation of β-catenin followed by mass spectrometry of KC epithelium was performed to identify its interacting partners. This was further validated by using epithelial tissues grown on scaffolds of different stiffness. Histology data reported breaks in the Bowman’s layer in KC patients. We hypothesize that these breaks expose the epithelium to the keratoconic corneal stroma, which, is known to have a decreased elastic modulus and that β-catenin acts as a mechanotransducer that induces structural changes such as loss of polarity (Syntaxin3) and barrier function (ZO1) through membrane delocalization. The results of our study strongly suggest that β-catenin could be a putative mechanotransducer in KC epithelium, thus supporting our hypothesis.

Ballistic delivery of compounds to inner layers of the cornea is limited by tough mechanical properties of stromal tissue

The barrier characteristics of the cornea are interrogated using the impact of micro-particles into ex vivo porcine cornea. Using a commercial gene gun (BioRad; PDS1000), microparticles were accelerated and made to embed in target materials: either ballistic gelatin as a reference or corneal tissue. Statistical analysis of penetration of polydisperse spherical microparticles (5-22 μm dia.) with density of 2.5 g/cc, 4.2 g/cc, and 7.8 g/cc (soda-lime glass, barium-titanate glass and stainless steel; more limited examination of 1.1 g/cc polyethylene and 19.2 g/cc tungsten) spanned almost two decades in kinetic energy. Penetration profiles in ballistic gelatin show that the particle embedding depth is sensitive to particle size and density. In the cornea, penetration is a weak function of size and density, and the corneal stroma is an effective stopping medium for high velocity microparticles. Despite the high water content of corneal tissue (76% w/w) compared to the stratum corneum of skin (40% w/w), the resistance to penetration of the cornea is comparable to what is seen in previous research of penetration in skin tissue. Using low density polymer particles with a therapeutic agent payload, it is demonstrated that bulk material can be ballistically delivered to the central 1 cm² of the corneal epithelium in an even layer with high bioavailability of therapeutic compound.

Biomechanical Modulation Therapy-A Stem Cell Therapy Without Stem Cells for the Treatment of Severe Ocular Burns

Ocular injuries caused by chemical and thermal burns are often unmanageable and frequently result in disfigurement, corneal haze/opacification, and vision loss. Currently, a considerable number of surgical and pharmacological approaches are available to treat such injuries at either an acute or a chronic stage. However, these existing interventions are mainly directed at (and limited to) suppressing corneal inflammation and neovascularization while promoting re-epithelialization. Reconstruction of the ocular surface represents a suitable but last-option recourse in cases where epithelial healing is severely impaired, such as due to limbal stem cell deficiency. In this concise review, we discuss how biomechanical modulation therapy (BMT) may represent a more effective approach to promoting the regeneration of ocular tissues affected by burn injuries via restoration of the limbal stem cell niche. Specifically, the scientific basis supporting this new therapeutic modality is described, along with our growing understanding of the role that tissue biomechanics plays in stem cell fate and function. The potential impact of BMT as a future treatment option for the management of injuries affecting tissue compliance is also further discussed.

Alterations in corneal biomechanics underlie early stages of autoimmune-mediated dry eye disease

Autoimmune-mediated dry eye disease is a pathological feature of multiple disorders including Sjögren's syndrome, lupus and rheumatoid arthritis that has a life-long, detrimental impact on vision and overall quality of life. Although late stage disease outcomes such as epithelial barrier dysfunction, reduced corneal innervation and chronic inflammation have been well characterized in both human patients and mouse models, there is little to no understanding of early pathological processes. Moreover, the mechanisms underlying the loss of cornea homeostasis and disease progression are unknown. Here, we utilize the autoimmune regulatory (Aire)-deficient mouse model of autoimmune-mediated dry eye disease in combination with genome wide transcriptomics, high-resolution imaging and atomic force microscopy to reveal a potential extracellular matrix (ECM)-biomechanical-based mechanism driving cellular and morphological changes at early disease onset. Early disease in the Aire-deficient mouse model is associated with a mild reduction in tear production and moderate immune cell infiltration, allowing for interrogation of cellular, molecular and biomechanical changes largely independent of chronic inflammation. Using these tools, we demonstrate for the first time that the emergence of autoimmune-mediated dry eye disease is associated with an alteration in the biomechanical properties of the cornea. We reveal a dramatic disruption of the synthesis and organization of the extracellular matrix as well as degradation of the epithelial basement membrane during early disease. Notably, we provide evidence that the nerve supply to the cornea is severely reduced at early disease stages and that this is independent of basement membrane destruction or significant immune cell infiltration. Furthermore, diseased corneas display spatial heterogeneity in mechanical, structural and compositional changes, with the limbal compartment often exhibiting the opposite response compared to the central cornea. Despite these differences, however, epithelial hyperplasia is apparent in both compartments, possibly driven by increased activation of IL-1R1 and YAP signaling pathways. Thus, we reveal novel perturbations in corneal biomechanics, matrix organization and cell behavior during the early phase of dry eye that may underlie disease development and progression, presenting new potential targets for therapeutic intervention.

Regulation of Keratocyte Phenotype and Cell Behavior by Substrate Stiffness

Corneal tissue engineering is an alternative way to solve the problem of lack of corneal donor tissue in corneal transplantation. Keratocytes with a normal phenotype and function in tissue-engineered cornea would be critical for corneal regeneration. Although the role of extracellular/substrate material stiffness is well-known for the regulation of the cell phenotype and cell behavior in many different cell types, its effects in keratocyte culture have not yet been thoroughly studied. This project studied the effect of substrate stiffness on the keratocyte phenotype marker expression and typical cell behavior (cell adhesion, proliferation, and migration), and the possible mechanisms involved. Human primary keratocytes were cultured on tissue culture plastic (TCP, ∼10⁶ kPa) or on plates with the stiffness equivalent of physiological human corneal stroma (25 kPa) or vitreous body (1 kPa). The expression of keratocyte phenotype markers, cell adhesion, proliferation, and migration were compared. The results showed that the stiffness of the substrate material regulates the phenotype marker expression and cell behavior of cultured keratocytes. Physiological corneal stiffness (25 kPa) superiorly preserved the cell phenotype when compared to the TCP and 1 kPa group. Keratocytes had a larger cell area when cultured on 25 kPa plates as compared to on TCP. Treatment of cells with NSC 23766 (Rac1 inhibitor) mimicked the response in the cell phenotype and behavior seen in the transition from soft materials to stiff materials, including the cytoskeletal structure, expression of keratocyte phenotype markers, and cell behavior. In conclusion, this study shows that substrate stiffness regulates the cell phenotype marker expression and cell behavior of keratocytes by Rac1-mediated cytoskeletal reorganization. This knowledge contributes to the development of corneal tissue engineering.

An eye on the dog as the scientist's best friend for translational research in ophthalmology: Focus on the ocular surface

Preclinical animal studies provide valuable opportunities to better understand human diseases and contribute to major advances in medicine. This review provides a comprehensive overview of ocular parameters in humans and selected animals, with a focus on the ocular surface, detailing species differences in ocular surface anatomy, physiology, tear film dynamics and tear film composition. We describe major pitfalls that tremendously limit the translational potential of traditional laboratory animals (i.e., rabbits, mice, and rats) in ophthalmic research, and highlight the benefits of integrating companion dogs with clinical analogues to human diseases into preclinical pharmacology studies. This One Health approach can help accelerate and improve the framework in which ophthalmic research is translated to the human clinic. Studies can be conducted in canine subjects with naturally occurring or noninvasively induced ocular surface disorders (e.g., dry eye disease, conjunctivitis), reviewed herein, and tear fluid can be easily retrieved from canine eyes for various bioanalytical purposes. In this review, we discuss common tear collection methods, including capillary tubes and Schirmer tear strips, and provide guidelines for tear sampling and extraction to improve the reliability of analyte quantification (drugs, proteins, others).

Ionically and Enzymatically Dual Cross-Linked Oxidized Alginate Gelatin Hydrogels with Tunable Stiffness and Degradation Behavior for Tissue Engineering

Hydrogels that allow for the successful long-term in vitro culture of cell-biomaterial systems to enable the maturation of tissue engineering constructs are highly relevant in regenerative medicine. Naturally derived polysaccharide-based hydrogels promise to be one material group with enough versatility and chemical functionalization capability to tackle the challenges associated with long-term cell culture. We report a marine derived oxidized alginate, alginate dialdehyde (ADA), and gelatin (GEL) system (ADA-GEL), which is cross-linked via ionic (Ca²⁺) and enzymatic (microbial transglutaminase, mTG) interaction to form dually cross-linked hydrogels. The cross-linking approach allowed us to tailor the stiffness of the hydrogels in a wide range (from <5 to 120 kPa), without altering the initial ADA and GEL hydrogel chemistry. It was possible to control the degradation behavior of the hydrogels to be stable for up to 30 days of incubation. Increasing concentrations of mTG cross-linker solutions allowed us to tune the degradation behavior of the ADA-GEL hydrogels from fast (<7 days) to moderate (14 days) and slow (>30 days) degradation kinetics. The cytocompatibility of mTG cross-linked ADA-GEL was assessed using NIH-3T3 fibroblasts and ATDC-5 mouse teratocarcinoma cells. Both cell types showed highly increased cellular attachment on mTG cross-linked ADA-GEL in comparison to Ca²⁺ cross-linked hydrogels. In addition, ATDC-5 cells showed a higher proliferation on mTG cross-linked ADA-GEL hydrogels in comparison to tissue culture polystyrene control substrates. Further, the attachment of human umbilical vein endothelial cells (HUVEC) on ADA-GEL (+) mTG was confirmed, proving the suitability of mTG+Ca²⁺ cross-linked ADA-GEL for several cell types. Summarizing, a promising platform to control the properties of ADA-GEL hydrogels is presented, with the potential to be applied in long-term cell culture investigations such as cartilage, bone, and blood-vessel engineering, as well as for biofabrication.

High-throughput 3D bioprinting of corneal stromal equivalents

In this study we designed high-throughput 3D bioprinting of corneal equivalents which may address the need for in vitro models. In our digital 3D cornea model, average dimensions of adult cornea were converted to 3D shapes, then to G-code files which were printed by BIOX printer (CELLINK). To maintain the curvature of cornea, a support scaffold was designed using stereolithographic printer. The support scaffold could facilitate the printing of 6-12 corneas at a time thus enabling high-throughput printing. Human corneal keratocytes (HCKs) were incorporated in the optimized bio-ink, and cell-laden corneal stromal equivalents were printed. Printed structures were cross-linked by calcium chloride 100 mM, washed with Hanks' Balanced Salt Solution and incubated at 37°C in fibroblast media. Printed corneas were analyzed for live dead assay, Alamar assay, and expression of fibronectin and actin green markers. Printed corneas were able to maintain their structure, integrity, and clarity. Live dead assay and Alamar assay demonstrated that HCKs maintained high viability (>95%) for 2 weeks. HCKs in the printed corneas showed expression for fibronectin and actin green. In conclusion, high-throughput fabrication of 3D printed corneal stromal equivalents using a combination of stereolithography printing, extrusion based printing, and micro-transfer molding techniques was achieved.

Bioinspired Precision Engineering of Three-Dimensional Epithelial Stem Cell Microniches

Maintenance of the epithelium relies on stem cells residing within specialized microenvironments, known as epithelial crypts. Two-photon polymerization (2PP) is a valuable tool for fabricating 3D micro/nanostructures for stem cell niche engineering applications. Herein, biomimetic gelatin methacrylate-based constructs, replicating the precise geometry of the limbal epithelial crypt structures (limbal stem cell "microniches") as an exemplar epithelial niche, are fabricated using 2PP. Human limbal epithelial stem cells (hLESCs) are seeded within the microniches in xeno-free conditions to investigate their ability to repopulate the crypts and the expression of various differentiation markers. Cell proliferation and a zonation in cell phenotype along the z-axis are observed without the use of exogenous signaling molecules. Significant differences in cell phenotype between cells located at the base of the microniche and those situated towards the rim are observed, demonstrating that stem cell fate is strongly influenced by its location within a niche and the geometrical details of where it resides. This study provides insight into the influence of the niche's spatial geometry on hLESCs and demonstrates a flexible approach for the fabrication of biomimetic crypt-like structures in epithelial tissues. This has significant implications for regenerative medicine applications and can ultimately lead to implantable synthetic "niche-based" treatments.

A clinical method for estimating the modulus of elasticity of the human cornea in vivo

BACKGROUND

To develop a method, using current clinical instrumentation, to estimate the Young's modulus of the human cornea in vivo.

METHODS

Central corneal curvature (CCC), central corneal thickness(CCT), intraocular pressure (IOP) was measured with the Goldmann tonometer (IOPG) and the Pascal Dynamic Corneal Tonometer(PDCT) in one eye of 100 normal young human subjects (21.07 ± 2.94 years) in vivo. The Orssengo and Pye algorithm was used to calculate the Young's modulus of the corneas of these subjects.

RESULTS

The Young's modulus(E) of the corneas of the subjects using the PDCT and IOPG results (Ecalc) was 0.25 ± 0.10MPa, and without the PDCT results (Eiopg) was 0.29 ± 0.06MPa. The difference in these results is due to the difference in tonometry results between the two instruments, as the mean PDCT result for the subjects was 16.89 ± 2.49mmHg and the IOPG result 15.06 ± 2.71mmHg. E was affected by the CCC of the subjects but more particularly by the CCT and IOP measurements. Corneal stiffness results are also presented.

CONCLUSION

Two methods have been developed to estimate the Young's modulus of the human cornea in vivo using current clinical instrumentation. One method (Ecalc) is applicable to the general corneal condition, and Eiopg to the normal cornea, and these results can be used to calculate corneal stiffness.

Stromal Collagen Arrangement Correlates with Stiffness of the Canine Cornea

The cornea is the most external layer of the eye and serves two important roles in (1) the refraction of light and (2) protection from the outside environment, both of which are highly dependent on the collagen assembly of the corneal stroma. This study sought to determine the collagen fiber arrangement of the canine corneal stroma and correlate the stromal organization with tissue stiffness in the anterior and posterior cornea. Collagen organization of the canine cornea was visualized through second-harmonic generation (SHG) imaging, and tissue stiffness of the anterior and posterior corneal stroma was determined by atomic force microscopy. Analysis of the canine anterior corneal stroma using SHG imaging documented intertwining of the collagen fibers with a high degree of fiber branching, with a more lamellar and non-branching posterior stroma. The anterior stroma had significantly higher tissue stiffness in both dogs and humans, when compared with the posterior corneal stroma (canine median: 1.3 kPa vs. 0.3 kPa; human median: 14.6 kPa vs. 2.1 kPa, respectively). There was a direct correlation between corneal collagen stromal organization and tissue stiffness in the dog, which was consistent with other mammalian species previously examined and likely reflects the need for maintenance of rigidity and corneal curvature.

Influence of polydimethylsiloxane substrate stiffness on corneal epithelial cells

Many cell types are known to modulate their behaviour in response to changes in material stiffness; however, little is known about how stiffness affects corneal epithelial cells. This study aims to investigate the response of a corneal epithelial cell line to polydimethylsiloxane (PDMS) substrates with a range of Young's moduli from 10 to 1500 kPa. Cellular morphology, proliferation, differentiation and mechanobiology were examined. Cells grown on PDMS adopted the typical cobblestone morphology exhibited by the corneal epithelium. Proliferative markers pERK and Ki67 were higher in cells cultured on stiffer substrates compared with those on softer substrates. Material stiffness was also found to influence the cell phenotype with cells on stiffer substrates having higher cytokeratin 3 gene expression, a mature epithelial marker, while cells on softer substrates expressed more cytokeratin 14, a basal epithelial marker. Cells grown on softer substrates also displayed higher levels of focal adhesions and intermediate filaments compared with cells on stiff substrates. This research will aid in designing novel biomaterials for the culture and transplantation of corneal epithelial cells.