Prelude to corneal tissue engineering - gaining control of collagen organization

Corneal regeneration: A review of stromal replacements

Corneal blindness is traditionally treated by transplantation of a donor cornea, or in severe cases by implantation of an artificial cornea or keratoprosthesis. Due to severe donor shortages and the risks of complications that come with artificial corneas, tissue engineering in ophthalmology has become more focused on regenerative strategies using biocompatible materials either with or without cells. The stroma makes up the bulk of the corneal thickness and mainly consists of a tightly interwoven network of collagen type I, making it notoriously difficult to recreate in a laboratory setting. Despite the challenges that come with corneal stromal tissue engineering, there has recently been enormous progress in this field. A large number of research groups are working towards developing the ideal biomimetic, cytocompatible and transplantable stromal replacement. Here we provide an overview of the approaches directed towards tissue engineering the corneal stroma, from classical collagen gels, films and sponges to less traditional components such as silk, fish scales, gelatin and polymers. The perfect stromal replacement has yet to be identified and future research should be directed at combined approaches, in order to not only host native stromal cells but also restore functionality.

STATEMENT OF SIGNIFICANCE

In the field of tissue engineering and regenerative medicine in ophthalmology the focus has shifted towards a common goal: to restore the corneal stroma and thereby provide a new treatment option for patients who are currently blind due to corneal opacification. Currently the waiting lists for corneal transplantation include more than 10 million patients, due to severe donor shortages. Alternatives to the transplantation of a donor cornea include the use of artificial cornea, but these are by no means biomimetic and therefore do not provide good outcomes. In recent years a lot of work has gone into the development of tissue engineered scaffolds and other biomaterials suitable to replace the native stromal tissue. Looking at all the different approaches separately is a daunting task and up until now there was no review article in which every approach is discussed. This review does include all approaches, from classical tissue engineering with collagen to the use of various alternative biomaterials and even fish scales. Therefore, this review can serve as a reference work for those starting in the field and but also to stimulate collaborative efforts in the future.

Postnatal periodontal ligament as a novel adult stem cell source for regenerative corneal cell therapy

Corneal opacities are a leading cause of global blindness. They are conventionally treated by the transplantation of donor corneal tissue, which is, restricted by a worldwide donor material shortage and allograft rejection. Autologous adult stem cells with a potential to differentiate into corneal stromal keratocytes (CSKs) could offer a suitable choice of cells for regenerative cell therapy. Postnatal periodontal ligament (PDL) contains a population of adult stem cells, which has a similar embryological origin as CSK, that is cranial neural crest. We harvested PDL cells from young adult teeth extracted because of non-functional or orthodontic reason and differentiated them towards CSK phenotype using a two-step protocol with spheroid formation followed by growth factor and cytokine induction in a stromal environment (human amnion stroma and porcine corneal stroma). Our results showed that the PDL-differentiated CSK-like cells expressed CSK markers (CD34, ALDH3A1, keratocan, lumican, CHST6, B3GNT7 and Col8A2) and had minimal expression of genes related to fibrosis and other lineages (vasculogenesis, adipogenesis, myogenesis, epitheliogenesis, neurogenesis and hematogenesis). Introduction of PDL spheroids into the stroma of porcine corneas resulted in extensive migration of cells inside the host stroma after 14-day organ culture. Their quiescent nature and uniform cell distribution resembled to that of mature CSKs inside the native stroma. Our results demonstrated the potential translation of PDL cells for regenerative corneal cell therapy for corneal opacities.

Biomaterials for corneal bioengineering

Corneal transplantation is an important surgical treatment for many common corneal diseases. However, a worldwide shortage of tissue from suitable corneal donors has meant that many people are not able to receive sight-restoring operations. In addition, rejection is a major cause of corneal transplant failure. Bioengineering corneal tissue has recently gained widespread attention. In order to facilitate corneal regeneration, a range of materials is currently being investigated. The ideal substrate requires sufficient tectonic durability, biocompatibility with cultured cellular elements, transparency, and perhaps biodegradability and clinical compliance. This review considers the anatomy and function of the native cornea as a precursor to evaluating a variety of biomaterials for corneal regeneration including key characteristics for optimal material form and function. The integration of appropriate cells with the most appropriate biomaterials is also discussed. Taken together, the information provided offers insight into the requirements for fabricating synthetic and semisynthetic corneas for in vitro modeling of tissue development and disease, pharmaceutical screening, and in vivo application for regenerative medicine.

Corneal Stroma Enhancement With Decellularized Stromal Laminas With or Without Stem Cell Recellularization for Advanced Keratoconus

PURPOSE

This phase 1 study seeks to preliminarily evaluate the safety and efficacy of decellularized human corneal stromal lamina transplantation with or without autologous adipose-derived adult stem cell recellularization within the corneal stroma of patients with advanced keratoconus.

DESIGN

Phase 1 clinical trial.

METHODS

Femtosecond-assisted 120-μm thickness and 9-mm diameter laminas were obtained from the anterior stroma of human donor corneas and decellularized with a sodium dodecyl sulfate solution. Autologous adipose-derived adult stem cells were obtained by elective liposuction and cultured onto both sides of the lamina. Five patients received the decellularized lamina alone and 4 patients the recellularized lamina into a femtosecond-assisted 9.5-mm diameter lamellar pocket under topical anesthesia. The total duration of follow-up was 6 months.

RESULTS

No case showed clinical haze or scarring by month 3. Six months after surgery, patients showed a general improvement of all visual parameters, with a mean unaided visual acuity from 0.109 to 0.232 (P = .05) and corrected distance visual acuity from 0.22 to 0.356 (P = .068). Refractive sphere improved in all patients (from -4.55 diopters [D] to -2.69 D; P = .017), but refractive cylinder remained stable (from -2.83 to -2.61; P = .34). An improvement tendency of all anterior keratometric values was observed. A mean improvement of 120 μm in all thickness parameters was confirmed (P = .008), as well as an improvement in the spherical aberration (P = .018), coma (P = .23) and total higher order aberrations (P = .31). No significant differences among groups were detected.

CONCLUSIONS

Decellularized human corneal stromal laminas transplantation seems safe and moderately effective for advanced keratoconus. Potential benefits of its recellularization with autologous adipose-derived adult stem cells remains unclear.

Corneal Tissue Engineering: An In Vitro Model of the Stromal-nerve Interactions of the Human Cornea

Tissue engineering has gained substantial recognition due to the high demand for human cornea replacements with an estimated 10 million people worldwide suffering from corneal vision loss¹. To address the demand for viable human corneas, significant progress in three-dimensional (3D) tissue engineering has been made^2,3,4. These cornea models range from simple monolayer systems to multilayered models, leading to 3D full-thickness corneal equivalents². However, the use of a 3D tissue-engineered cornea in the context of in vitro disease models studied to date lacks resemblance to the multilayered 3D corneal tissue structure, function, and the networking of different cell types (i.e., nerve, epithelium, stroma, and endothelium)^2,3. In addition, the demand for in vitro cornea tissue models has increased in an attempt to reduce animal testing for pharmaceutical products. Thus, more sophisticated models are required to better match systems to human physiological requirements, and the development of a model that is more relevant to the patient population is absolutely necessary. Given that multiple cell types in the cornea are affected by diseases and dystrophies, such as Keratoconus, Diabetic Keratopathy, and Fuchs, this model includes a 3D co-culture model of primary human corneal fibroblasts (HCFs) from healthy donors and neurons from the SH-SY5Y cell line. This allows us for the first time to investigate the interactions between the two cell types within the human corneal tissue. We believe that this model could potentially dissect the underlying mechanisms associated with the stromal-nerve interactions of corneal diseases that exhibit nerve damages. This 3D model mirrors the basic anatomical and physiological nature of the corneal tissue in vivo and can be used in the future as a tool for investigating corneal defects as well as screening the efficacy of various agents before animal testing.

Substance P and patterned silk biomaterial stimulate periodontal ligament stem cells to form corneal stroma in a bioengineered three-dimensional model

Background

We aimed to generate a bioengineered multi-lamellar human corneal stroma tissue in vitro by differentiating periodontal ligament stem cells (PDLSCs) towards keratocytes on an aligned silk membrane.

Methods

Human PDLSCs were isolated and identified. The neuropeptide substance P (SP) was added in keratocyte differentiation medium (KDM) to evaluate its effect on keratocyte differentiation of PDLSCs. PDLSCs were then seeded on patterned silk membrane and cultured with KDM and SP. Cell alignment was evaluated and the expression of extracellular matrix (ECM) components of corneal stroma was detected. Finally, multi-lamellar tissue was constructed in vitro by PDLSCs seeded on patterned silk membranes, which were stacked orthogonally and stimulated by KDM supplemented with SP for 18 days. Sections were prepared and subsequently stained with hematoxylin and eosin or antibodies for immunofluorescence observation of human corneal stroma-related proteins.

Results

SP promoted the expression of corneal stroma-related collagens (collagen types I, III, V, and VI) during the differentiation induced by KDM. Patterned silk membrane guided cell alignment of PDLSCs, and important ECM components of the corneal stroma were shown to be deposited by the cells. The constructed multi-lamellar tissue was found to support cells growing between every two layers and expressing the main type of collagens (collagen types I and V) and proteoglycans (lumican and keratocan) of normal human corneal stroma.

Conclusions

Multi-lamellar human corneal stroma-like tissue can be constructed successfully in vitro by PDLSCs seeded on orthogonally aligned, multi-layered silk membranes with SP supplementation, which shows potential for future corneal tissue engineering.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0715-y) contains supplementary material, which is available to authorized users.

Scaffold-free tissue engineering of functional corneal stromal tissue

Blinding corneal scarring is predominately treated with allogeneic graft tissue; however, there is a worldwide shortage of donor tissue leaving millions in need of therapy. Human corneal stromal stem cells (CSSC) have been shown produce corneal tissue when cultured on nanofibre scaffolding, but this tissue cannot be readily separated from the scaffold. In this study, scaffold-free tissue engineering methods were used to generate biomimetic corneal stromal tissue constructs that can be transplanted in vivo without introducing the additional variables associated with exogenous scaffolding. CSSC were cultured on substrates with aligned microgrooves, which directed parallel cell alignment and matrix organization, similar to the organization of native corneal stromal lamella. CSSC produced sufficient matrix to allow manual separation of a tissue sheet from the grooved substrate. These constructs were cellular and collagenous tissue sheets, approximately 4 μm thick and contained extracellular matrix molecules typical of corneal tissue including collagen types I and V and keratocan. Similar to the native corneal stroma, the engineered corneal tissues contained long parallel collagen fibrils with uniform diameter. After being transplanted into mouse corneal stromal pockets, the engineered corneal stromal tissues became transparent, and the human CSSCs continued to express human corneal stromal matrix molecules. Both in vitro and in vivo, these scaffold-free engineered constructs emulated stromal lamellae of native corneal stromal tissues. Scaffold-free engineered corneal stromal constructs represent a novel, potentially autologous, cell-generated, biomaterial with the potential for treating corneal blindness. Copyright © 2016 John Wiley & Sons, Ltd.

Controlling the 3D architecture of Self-Lifting Auto-generated Tissue Equivalents (SLATEs) for optimized corneal graft composition and stability

Ideally, biomaterials designed to play specific physical and physiological roles in vivo should comprise components and microarchitectures analogous to those of the native tissues they intend to replace. For that, implantable biomaterials need to be carefully designed to have the correct structural and compositional properties, which consequently impart their bio-function. In this study, we showed that the control of such properties can be defined from the bottom-up, using smart surface templates to modulate the structure, composition, and bio-mechanics of human transplantable tissues. Using multi-functional peptide amphiphile-coated surfaces with different anisotropies, we were able to control the phenotype of corneal stromal cells and instruct them to fabricate self-lifting tissues that closely emulated the native stromal lamellae of the human cornea. The type and arrangement of the extracellular matrix comprising these corneal stromal Self-Lifting Analogous Tissue Equivalents (SLATEs) were then evaluated in detail, and was shown to correlate with tissue function. Specifically, SLATEs comprising aligned collagen fibrils were shown to be significantly thicker, denser, and more resistant to proteolytic degradation compared to SLATEs formed with randomly-oriented constituents. In addition, SLATEs were highly transparent while providing increased absorption to near-UV radiation. Importantly, corneal stromal SLATEs were capable of constituting tissues with a higher-order complexity, either by creating thicker tissues through stacking or by serving as substrate to support a fully-differentiated, stratified corneal epithelium. SLATEs were also deemed safe as implants in a rabbit corneal model, being capable of integrating with the surrounding host tissue without provoking inflammation, neo-vascularization, or any other signs of rejection after a 9-months follow-up. This work thus paves the way for the de novo bio-fabrication of easy-retrievable, scaffold-free human tissues with controlled structural, compositional, and functional properties to replace corneal, as well as other, tissues.

Surface Topography and Mechanical Strain Promote Keratocyte Phenotype and Extracellular Matrix Formation in a Biomimetic 3D Corneal Model

The optimal functionality of the native corneal stroma is mainly dependent on the well-ordered arrangement of extracellular matrix (ECM) and the pressurized structure. In order to develop an in vitro corneal model, it is crucial to mimic the in vivo microenvironment of the cornea. In this study, the influence of surface topography and mechanical strain on keratocyte phenotype and ECM formation within a biomimetic 3D corneal model is studied. By modifying the surface topography of materials, it is found that patterned silk fibroin film with 600 grooves mm^-1 optimally supports cell alignment and ECM arrangement. Furthermore, treatment with 3% dome-shaped mechanical strain, which resembles the shape and mechanics of native cornea, significantly enhances the expression of keratocyte markers as compared to flat-shaped strain. Accordingly, a biomimetic 3D corneal model, in the form of a collagen-modified, silk fibroin-patterned construct subjected to 3% dome-shaped strain, is created. Compared to traditional 2D cultures, it supports a significantly higher expression of keratocyte and ECM markers, and in conclusion better maintains keratocyte phenotype, alignment, and fusiform cell shape. Therefore, the novel biomimetic 3D corneal model developed in this study serves as a useful in vitro 3D culture model to improve current 2D cultures for corneal studies.

3D Stacked Construct: A Novel Substitute for Corneal Tissue Engineering

Corneal trauma/injury often results in serious complications including permanent vision loss or loss of visual acuity which demands corneal transplantations or treatment with allogenic graft tissues. There is currently a huge shortage of donor tissue worldwide and the need for human corneal equivalents increases annually. In order to meet such demand the current clinical approach of treating corneal injuries is limited and involves synthetic and allogenic materials which have various shortcomings when it comes to actual transplantations. In this study we introduce the newly developed, next generation of our previously established 3D self-assembled constructs, where multiple constructs are grown and stacked on top of each other without any other artificial product. This new technology brings our 3D in vitro model closer to what is seen in vivo and provides a solid foundation for future studies on corneal biology.Lipids are known for playing a vital role during metabolism and diseased state of various tissues and Sphingolipids are one such class of lipids which are involved in various cellular mechanisms and signaling processes. The impacts of Sphingolipids that have been documented in several human diseases often involve inflammation, neovascularization, tumorigenesis, and diabetes, but these conditions are not yet thoroughly studied. There is very little information about the exact role of Sphingolipids in the human cornea and future studies aiming at dissecting the mechanisms and pathways involved in order to develop novel therapies. We believe that our novel 3D stacked model can be used to delineate the role of Sphingolipids in the human cornea and provide new insights for understanding and treating various human corneal diseases.

Current advances in electrospun gelatin-based scaffolds for tissue engineering applications

The development of biomimetic highly-porous scaffolds is essential for successful tissue engineering. Electrospun nanofibers are highly versatile platforms for a broad range of applications in different research areas. In the biomedical field, micro/nanoscale fibrous structures have gained great interest for wound dressings, drug delivery systems, soft and hard-tissue engineering scaffolds, enzyme immobilization, among other healthcare applications. In this mini-review, electrospun gelatin-based scaffolds for a variety of tissue engineering applications, such as bone, cartilage, skin, nerve, and ocular and vascular tissue engineering, are reviewed and discussed. Gelatin blends with natural or synthetic polymers exhibit physicochemical, biomechanical, and biocompatibility properties very attractive for scaffolding. Current advances and challenges on this research field are presented.

Electrospun Scaffolds for Corneal Tissue Engineering: A Review

Corneal diseases constitute the second leading cause of vision loss and affect more than 10 million people globally. As there is a severe shortage of fresh donated corneas and an unknown risk of immune rejection with traditional heterografts, it is very important and urgent to construct a corneal equivalent to replace pathologic corneal tissue. Corneal tissue engineering has emerged as a practical strategy to develop corneal tissue substitutes, and the design of a scaffold with mechanical properties and transparency similar to that of natural cornea is paramount for the regeneration of corneal tissues. Nanofibrous scaffolds produced by electrospinning have high surface area-to-volume ratios and porosity that simulate the structure of protein fibers in native extra cellular matrix (ECM). The versatilities of electrospinning of polymer components, fiber structures, and functionalization have made the fabrication of nanofibrous scaffolds with suitable mechanical strength, transparency and biological properties for corneal tissue engineering feasible. In this paper, we review the recent developments of electrospun scaffolds for engineering corneal tissues, mainly including electrospun materials (single and blended polymers), fiber structures (isotropic or anisotropic), functionalization (improved mechanical properties and transparency), applications (corneal cell survival, maintenance of phenotype and formation of corneal tissue) and future development perspectives.

Progress in Corneal Stromal Repair: From Tissue Grafts and Biomaterials to Modular Supramolecular Tissue-Like Assemblies

Corneal injuries and degenerative conditions have major socioeconomic consequences, given that in most cases, they result in blindness. In the quest of the ideal therapy, tissue grafts, biomaterials, and modular engineering approaches are under intense investigation. Herein, advancements and shortfalls are reviewed and future perspectives for these therapeutic strategies discussed.

Corneal surgery in keratoconus: which type, which technique, which outcomes?

Keratoconus is a disease characterized by progressive thinning, bulging, and distortion of the cornea. Advanced cases usually present with loss of vision due to high irregular astigmatism. A majority of these cases require surgical intervention. This review provides an update on the current treatment modalities of corneal surgery available for the management of advanced corneal ectasias.

Polarization microscopy for characterizing fiber orientation of ocular tissues

Characterizing the collagen fiber orientation and organization in the eye is necessary for a complete understanding of ocular biomechanics. In this study, we assess the performance of polarized light microscopy to determine collagen fiber orientation of ocular tissues. Our results demonstrate that the method provides objective, accurate, repeatable and robust data on fiber orientation with µm-scale resolution over a broad, cm-scale, field of view, unaffected by formalin fixation, without requiring tissue dehydration, labeling or staining. Together, this shows that polarized light microscopy is a powerful method for studying collagen architecture in the eye, with applications ranging from normal physiology and aging, to pathology and transplantation.

Osteogenic differentiation of adipose derived stem cells on high and low aspect ratio micropatterns

Adipose derived stem cells (ADSCs) were cultured on collagen-silk fibroin films with microchannel and micropillar patterns to investigate the effects of cell morphology changes on osteogenic differentiation. Channel and pillar micropatterned films were prepared from collagen type I and silk fibroin. While higher ADSC proliferation profiles were obtained on micropillar blend film, microchannel blend films, however, caused twice higher aspect ratio and effective orientation of cells. Alkaline phosphatase activity of ADSCs was several times higher on microchannel surface when the measured activities were normalized to cell number. Effective deposition of collagen type I and mineral by the cells were observed for patterned and unpatterned films, and these extracellular matrix components were oriented along the axis of the microchannels. In conclusion, the use of collagen-fibroin blend film with microchannel topography increased the aspect ratio and alignment of cells significantly, and was also effective in the differentiation of ADSCs into osteogenic lineage.

[Justification of the use of particular biopolymers as matrix materials for artificial corneas]

This article reviews distinctive features, advantages, and drawbacks of different biopolymers used to construct the 3D matrices of artificial corneas. Modern requirements for matrices are provided.

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.

The materials science of collagen

Collagen is the principal biopolymer in the extracellular matrix of both vertebrates and invertebrates. It is produced in specialized cells (fibroblasts) and extracted into the body by a series of intra and extracellular steps. It is prevalent in connective tissues, and the arrangement of collagen determines the mechanical response. In biomineralized materials, its fraction and spatial distribution provide the necessary toughness and anisotropy. We review the structure of collagen, with emphasis on its hierarchical arrangement, and present constitutive equations that describe its mechanical response, classified into three groups: hyperelastic macroscopic models based on strain energy in which strain energy functions are developed; macroscopic mathematical fits with a nonlinear constitutive response; structurally and physically based models where a constitutive equation of a linear elastic material is modified by geometric characteristics. Viscoelasticity is incorporated into the existing constitutive models and the effect of hydration is discussed. We illustrate the importance of collagen with descriptions of its organization and properties in skin, fish scales, and bone, focusing on the findings of our group.

Application of retinoic acid improves form and function of tissue engineered corneal construct

Retinoic acid has recently been shown to control the phenotype and extracellular matrix composition of corneal stromal cells cultured in vitro as monolayers. This study set out to investigate the effects of retinoic acid on human corneal keratocytes within a 3D environment. Human corneal keratocytes were encapsulated in collagen gels, which were subsequently compressed under load, and cultured in serum-free media supplemented with 10 µM retinoic acid or DMSO vehicle for 30 days. Cell proliferation was quantified on selected days, while the expression of several important keratocytes markers was evaluated at day 30 using RT-PCR and immunoblotting. The weight and size of the collagen constructs were measured before and after hydration and contraction analyses. Retinoic acid enhanced keratocyte proliferation until day 30, whereas cells in control culture conditions showed reduced numbers after day 21. Both gene and protein expressions of keratocyte-characteristic proteoglycans (keratocan, lumican and decorin), corneal crystallins and collagen type I and V were significantly increased following retinoic acid supplementation. Retinoic acid also significantly reduced the expression of matrix metalloproteases 1, 3 and 9 while not increasing α-smooth muscle actin and fibronectin expression. Furthermore, these effects were also correlated with the ability of retinoic acid to significantly inhibit the contractility of keratocytes while allowing the build-up of corneal stromal extracellular matrix within the 3D constructs. Thus, retinoic acid supplementation represents a promising strategy to improve the phenotype of 3D-cultured keratocytes, and their usefulness as a model of corneal stroma for corneal biology and regenerative medicine applications.

Promoting convergence: the Phi spiral in abduction of mouse corneal behaviors

Why do mouse corneal epithelial cells display spiraling patterns? We want to provide an explanation for this phenomenon by applying an idealized problem solving process. Specifically, we applied complementary line-fitting methods to measure transgenic epithelial reporter expression arrangements displayed on three mature, live enucleated globes to clarify the problem. Two prominent logarithmic curves were discovered, one of which displayed the ϕ ratio, an indicator of the optimal configuration in phyllotactic systems. We then utilized two different computational approaches to expose our current understanding of the behavior. In one procedure, which involved an isotropic mechanics-based finite element method, we successfully produced logarithmic spiral curves of maximum shear strain based pathlines but computed dimensions displayed pitch angles of 35° (ϕ spiral is ~17°), which was altered when we fitted the model with published measurements of coarse collagen orientations. We then used model-based reasoning in context of Peircean abduction to select a working hypothesis. Our work serves as a concise example of applying a scientific habit of mind and illustrates nuances of executing a common method to doing integrative science.

A microfabricated, optically accessible device to study the effects of mechanical cues on collagen fiber organization

As the primary structural protein of our bodies, fibrillar collagen and its organizational patterns determine the biomechanics and shape of tissues. While the molecular assembly of individual fibrils is well understood, the mechanisms determining the arrangement of fibers and thus the shape and form of tissues remain largely unknown. We have developed a cell culture model that successfully recapitulates early tissue development and the de novo deposition of collagen fibers to investigate the role of mechanical cues on collagen fiber alignment. The devices used a thin, collagen-coated deformable PDMS membrane inside a tissue culture well built on microscope-grade coverslips. Deformations and strains in the PDMS membrane were quantified by tracking fluorescent bead displacement and through the use of a COMSOL model. Cyclical strains were applied to serum-cultured rabbit corneal cells at 0.5 Hz for 24-48 h and showed a preferred alignment after 36 h of cyclical loading. Cells cultured with ascorbic acid under methylcellulose serum-free conditions deposited a collagenous matrix that was visible under live second harmonic generation microscopy at week 4. Our microfabricated tissue culture system allows for the controllable application of a wide range of stress profiles to cells, and for the observation and quantification of cells and de novo collagen formation in vitro. Future studies will involve the fabrication of models to study the formation and organization of collagen in ocular diseases.

[Decellularized collagen matrix from tilapia fish scales for corneal reconstruction (BioCornea)]

BACKGROUND

The worldwide need for donor corneal tissue clearly exceeds the availability of transplantable human tissue; therefore, recent efforts aim to identify and characterize alternative tissues, such as decellularized collagen scaffolds.

OBJECTIVES

The transparent fish scales of tilapia (Oreochromis mossambicus) were analyzed as a potential alternative for corneal reconstruction ("BioCornea").

MATERIAL AND METHODS

The article gives a review of the literature and own preliminary results. After decellularization the tissue characteristics of the fish scales, the repopulation with corneal epithelium and stromal cells, immunogenicity, the feasibility of corneal transplantation and the angiogenic properties were analyzed in vitro and in various animal models.

RESULTS

The fish scales mainly consist of collagen type I and show an architecture that is similar to the human cornea. Corneal epithelium and stromal cells are able to grow over and into the scaffold. It is possible to transplant fish scales in various animal models without severe inflammatory responses. Furthermore, in mice, less blood and lymphatic vessels grow into the xenograft when compared to conventional allogenic transplants.

CONCLUSION

Preliminary results with decellularized tilapia fish scales as an alternative for corneal reconstruction ("BioCornea") are promising.

Gelatin nanofiber-reinforced alginate gel scaffolds for corneal tissue engineering

A severe shortage of donor cornea is now an international crisis in public health. Substitutes for donor tissue need to be developed to meet the increasing demand for corneal transplantation. Current attempts in designing scaffolds for corneal tissue regeneration involve utilization of expensive materials. Yet, these corneal scaffolds still lack the highly-organized fibrous structure that functions as a load-bearing component in the native tissue. This work shows that transparent nanofiber-reinforced hydrogels could be developed from cheap, non-immunogenic and readily available natural polymers to mimic the cornea's microstructure. Electrospinning was employed to produce gelatin nanofibers, which were then infiltrated with alginate hydrogels. Introducing electrospun nanofibers into hydrogels improved their mechanical properties by nearly one order of magnitude, yielding mechanically robust composites. Such nanofiber-reinforced hydrogels could serve as alternatives to donor tissue for corneal transplantation.

Construction of a collagen-based, split-thickness cornea substitute

Tissue-engineered corneas may become a promising alternative to allografts in the treatment of serious cornea defects because of the tunable characteristics of the biomaterials, biomimetic designs, and incorporation of patient's own cells. In this study, collagen foam was coated with a fibrous mat to mimic the stromal layer and the Bowman's layer. The stromal layer substitute was made of N-ethyl-N-(3-dimethyl aminopropyl)carbodiimide/N-hydroxysuccinimide-cross-linked collagen-chondroitin sulfate foam and seeded with primary human corneal keratocytes (HK). Retinal pigment epithelium (RPE) cells served as the epithelial layer after seeding on a dehydrothermally cross-linked collagen type I fibrous mat deposited directly on top of the foams by electrospinning. The physical characterization and the in vitro studies showed that the designed cornea replacement was suitable for cell attachment and growth, and co-culture of the two cell types induced more extracellular matrix (ECM) deposition than the single cell-seeded constructs. The fiber layer was shown to be successful in separating the HK and RPE cells, and still allowed them to maintain cell-cell communication as the increase in ECM deposition and the maintenance of the high transparency (~80%) suggested. This split-thickness corneal substitute was also shown to be readily suturable without any major tears at the end of a short co-culture of 30 days.

Novel collagen membranes for the reconstruction of the corneal surface

No standardized biomaterial exists for the surgical treatment of persistent corneal erosions and ulcerations. We analyzed the suitability and biocompatibility of defined noncross-linked and UV/riboflavin cross-linked equine type I collagen membranes for the reconstruction of the corneal surface. Isolated human oral mucosa epithelial cells, a cell type in clinical use for the treatment of ocular surface diseases, were subcultivated on both types of membranes and examined concerning cell adhesion, proliferation, and differentiation. Biocompatibility was evaluated following superficial and intrastromal corneal transplantation in New Zealand white rabbits. In cell cultures all collagen membranes supported adhesion of oral mucosa epithelial cells leading to the formation of multilayered epithelial cell sheets. After intrastromal corneal implantation clinical signs of degradation were seen in all variants of collagen membranes, which was fastest in noncross-linked variants. The histological and ultrastructural level invasion of keratocytes and production of new collagen fibers inside the collagen membranes could be detected in noncross-linked variants. After superficial corneal implantation covering of the membranes by corneal epithelium over time was visible. Ultrastructural analysis showed a slower rate of degradation and less invading keratocytes in cross-linked variants compared with noncross-linked collagen membranes. Cross-linked and noncross-linked variants of the collagen membrane proofed to be suitable to serve as a carrier for epithelial stem cells in vitro and showed a high biocompatibility in vivo. These results indicate that the tested collagen membranes might be suitable for the reconstruction of the corneal surface in patients with nonhealing ulcerations. Whether membranes with faster or slower degradation properties are preferable for the treatment of persistent corneal ulcerations might depend on the underlying corneal pathology and the degree of concomitant inflammation.

Wounding the cornea to learn how it heals

Corneal wound healing studies have a long history and rich literature that describes the data obtained over the past 70 years using many different species of animals and methods of injury. These studies have lead to reduced suffering and provided clues to treatments that are now helping patients live more productive lives. In spite of the progress made, further research is required since blindness and reduced quality of life due to corneal scarring still happens. The purpose of this review is to summarize what is known about different types of wound and animal models used to study corneal wound healing. The subject of corneal wound healing is broad and includes chemical and mechanical wound models. This review focuses on mechanical injury models involving debridement and keratectomy wounds to reflect the authors' expertise.

Corneal stromal stem cells versus corneal fibroblasts in generating structurally appropriate corneal stromal tissue

Recapitulation of human corneal stromal tissue is believed to be among the most challenging steps in engineering human corneal tissue because of the difficulty in reproducing its highly-ordered hierarchical ultrastructure, which imparts its robust biomechanical properties and optical transparency. In this study, we compared the feasibility of utilizing human corneal stromal stem cells (hCSSCs) and human corneal fibroblasts (hCFs) in the generation of human corneal stromal tissue on a highly-aligned fibrous substrate made from poly(ester urethane) urea. In the serum-free keratocyte differentiation medium supplemented with FGF-2 (10 ng/mL) and TGF-β3 (0.1 ng/mL), hCSSCs successfully differentiated into keratocytes and secreted multilayered lamellae with orthogonally-oriented collagen fibrils, in a pattern mimicking human corneal stromal tissue. The constructs were 60-70 μm thick and abundant in cornea-specific extracellular matrix (ECM) components, including keratan sulfate, lumican, and keratocan. Under the identical conditions, hCFs tended to differentiate into myofibroblasts and deposited a less-organized collagen-fibrillar construct in a pattern with similarities to corneal scar tissue due to a lack of cornea-specific ECM components. These observations demonstrated that hCSSCs showed a much greater potential, under proper substrate and growth factor guidance, to facilitate the generation of a biological human cornea equivalent. Unlike hCSSCs, hCFs were less responsive to these environmental cues and under identical culture conditions generated an ECM that poorly mimicked the native, functional tissue structure and composition.

Corneal stromal bioequivalents secreted on patterned silk substrates

Emulating corneal stromal tissue is believed to be the most challenging step in bioengineering an artificial human cornea because of the difficulty in reproducing its highly ordered microstructure, the key to the robust biomechanical properties and optical transparency of this tissue. We conducted a comparative study to assess the feasibility of human corneal stromal stem cells (hCSSCs) and human corneal fibroblasts (hCFs) in the generation of human corneal stromal tissue on groove-patterned silk substrates. In serum-free keratocyte differentiation medium, hCSSCs successfully differentiated into keratocytes secreting multilayered lamellae with orthogonally-oriented collagen fibrils, in a pattern mimicking human corneal stromal tissue. The constructs were 90-100 μm thick, containing abundant cornea-specific extracellular matrix (ECM) components, including keratan sulfate, lumican, and keratocan. In contrast, hCFs tended to differentiate into myofibroblasts that deposited less organized collagen in a pattern resembling that of corneal scar tissue. RGD surface coupling coupling was an essential factor in enhancing cell attachment, orientation, proliferation, differentiation and ECM deposition on the silk substratum. These results demonstrated that an approach of combining hCSSCs with an RGD surface-coupled patterned silk film offers a powerful tool to develop highly ordered collagen fibril-based constructs for corneal regeneration and corneal stromal tissue repair.

Future perspectives for regenerative medicine in ophthalmology

Repair and reconstruction of the cornea has historically relied on synthetic materials or tissue transplantation. However, the future holds promise for treatments using smart biomaterials and stem cells that direct tissue repair and regeneration to ultimately create new ocular structures that are indistinguishable from the original native tissue. The cornea is a remarkable engineering structure. By understanding the physical structure of the tissue and the resulting impact of the structure on biological function, we can design novel materials for a number of ophthalmic clinical applications. Furthermore, by extending this structure-function approach to characterizing corneal disease processes, new therapies can be engineered.

Characterization of structural, mechanical and nano-mechanical properties of electrospun PGS/PCL fibers

Structural and mechanical properties of aligned PGS/PCL nanofibers for cornea tissue engineering are studied and compared to natural corneal stroma.

Lateral growth limitation of corneal fibrils and their lamellar stacking depend on covalent collagen cross-linking by transglutaminase-2 and lysyl oxidases, respectively

Corneal stroma contains an extracellular matrix of orthogonal lamellae formed by parallel and equidistant fibrils with a homogeneous diameter of ~35 nm. This is indispensable for corneal transparency and mechanical functions. However, the mechanisms controlling corneal fibrillogenesis are incompletely understood and the conditions required for lamellar stacking are essentially unknown. Under appropriate conditions, chick embryo corneal fibroblasts can produce an extracellular matrix in vitro resembling primary corneal stroma during embryonic development. Among other requirements, cross-links between fibrillar collagens, introduced by tissue transglutaminase-2, are necessary for the self-assembly of uniform, small diameter fibrils but not their lamellar stacking. By contrast, the subsequent lamellar organization into plywood-like stacks depends on lysyl aldehyde-derived cross-links introduced by lysyl oxidase activity, which, in turn, only weakly influences fibril diameters. These cross-links are introduced at early stages of fibrillogenesis. The enzymes are likely to be important for a correct matrix deposition also during repair of the cornea.

Modulation of keratocyte phenotype by collagen fibril nanoarchitecture in membranes for corneal repair

Type I collagen membranes with tailored fibril nanoarchitectures were fabricated through a vitrification processing, which mimicked, to a degree, the collagen maturation process of corneal stromal extracellular matrix in vivo. Vitrification was performed at a controlled temperature of either 5 °C or 39 °C at a constant relative humidity of 40% for various time periods from 0.5 wk up to 8 wk. During vitrification, the vitrified collagen membranes (collagen vitrigels, CVs) exhibited a rapid growth in fibrillar density through the evaporation of water and an increase in fibrillar stiffness due to the formation of new and/or more-stable interactions. On the other hand, the collagen fibrils in CVs maintained their D-periodicity and showed no significant difference in fibrillar diameter, indicating preservation of the native states of the collagen fibrils during vitrification. Keratocyte phenotype was maintained on CVs to varying degrees that were strongly influenced by the collagen fibril nanoarchitectures. Specifically, the vitrification time of CVs mainly governed the keratocyte morphology, showing significant increases in the cell protrusion number, protrusion length, and cell size along with CV vitrification time. The CV vitrification temperature affected the regulation of keratocyte fibroblasts' gene expressions, including keratocan and aldehyde dehydrogenase (ALDH), demonstrating a unique way to control the expression of specific genes in vitro.

A brief history of corneal transplantation: From ancient to modern

This review highlights many of the fundamental concepts and events in the development of corneal transplantation - from ancient times to modern. Tales of eye, limb, and even heart transplantation appear in ancient and medieval texts; however, in the scientific sense, the original concepts of corneal surgery date back to the Greek physician Galen (130-200 AD). Although proposals to provide improved corneal clarity by surgical interventions, including keratoprostheses, were better developed by the 17(th) and 18(th) centuries, true scientific and surgical experimentation in this field did not begin until the 19(th) century. Indeed, the success of contemporary corneal transplantation is largely the result of a culmination of pivotal ideas, experimentation, and perseverance by inspired individuals over the last 200 years. Franz Reisinger initiated experimental animal corneal transplantation in 1818, coining the term "keratoplasty". Subsequently, Wilhelmus Thorne created the term corneal transplant and 3 years later Samuel Bigger, 1837, reported successful corneal transplantation in a gazelle. The first recorded therapeutic corneal xenograft on a human was reported shortly thereafter in 1838-unsurprisingly this was unsuccessful. Further progress in corneal transplantation was significantly hindered by limited understanding of antiseptic principles, anesthesiology, surgical technique, and immunology. There ensued an extremely prolonged period of debate and experimentation upon the utility of animal compared to human tissue, and lamellar versus penetrating keratoplasty. Indeed, the first successful human corneal transplant was not performed by Eduard Zirm until 1905. Since that first successful corneal transplant, innumerable ophthalmologists have contributed to the development and refinement of corneal transplantation aided by the development of surgical microscopes, refined suture materials, the development of eye banks, and the introduction of corticosteroids. Recent developments, including the replacement of selected corneal layers rather than full-thickness keratoplasty, have the potential to improve or transform corneal transplant surgery in the future.

Optimization of human corneal endothelial cell culture: density dependency of successful cultures in vitro

Background

Global shortage of donor corneas greatly restricts the numbers of corneal transplantations performed yearly. Limited ex vivo expansion of primary human corneal endothelial cells is possible, and a considerable clinical interest exists for development of tissue-engineered constructs using cultivated corneal endothelial cells. The objective of this study was to investigate the density-dependent growth of human corneal endothelial cells isolated from paired donor corneas and to elucidate an optimal seeding density for their extended expansion in vitro whilst maintaining their unique cellular morphology.

Results

Established primary human corneal endothelial cells were propagated to the second passage (P2) before they were utilized for this study. Confluent P2 cells were dissociated and seeded at four seeding densities: 2,500 cells per cm² (‘LOW’); 5,000 cells per cm² (‘MID’); 10,000 cells per cm² (‘HIGH’); and 20,000 cells per cm² (‘HIGH^×2’), and subsequently analyzed for their propensity to proliferate. They were also subjected to morphometric analyses comparing cell sizes, coefficient of variance, as well as cell circularity when each culture became confluent. At the two lower densities, proliferation rates were higher than cells seeded at higher densities, though not statistically significant. However, corneal endothelial cells seeded at lower densities were significantly larger in size, heterogeneous in shape and less circular (fibroblastic-like), and remained hypertrophic after one month in culture. Comparatively, cells seeded at higher densities were significantly homogeneous, compact and circular at confluence. Potentially, at an optimal seeding density of 10,000 cells per cm², it is possible to obtain between 10 million to 25 million cells at the third passage. More importantly, these expanded human corneal endothelial cells retained their unique cellular morphology.

Conclusions

Our results demonstrated a density dependency in the culture of primary human corneal endothelial cells. Sub-optimal seeding density results in a decrease in cell saturation density, as well as a loss in their proliferative potential. As such, we propose a seeding density of not less than 10,000 cells per cm² for regular passage of primary human corneal endothelial cells.

Composite electrospun gelatin fiber-alginate gel scaffolds for mechanically robust tissue engineered cornea

A severe shortage of good quality donor cornea is now an international crisis in public health. Alternatives for donor tissue need to be urgently developed to meet the increasing demand for corneal transplantation. Hydrogels have been widely used as scaffolds for corneal tissue regeneration due to their large water content, similar to that of native tissue. However, these hydrogel scaffolds lack the fibrous structure that functions as a load-bearing component in the native tissue, resulting in poor mechanical performance. This work shows that mechanical properties of compliant hydrogels can be substantially enhanced with electrospun nanofiber reinforcement. Electrospun gelatin nanofibers were infiltrated with alginate hydrogels, yielding transparent fiber-reinforced hydrogels. Without prior crosslinking, electrospun gelatin nanofibers improved the tensile elastic modulus of the hydrogels from 78±19 kPa to 450±100 kPa. Stiffer hydrogels, with elastic modulus of 820±210 kPa, were obtained by crosslinking the gelatin fibers with carbodiimide hydrochloride in ethanol before the infiltration process, but at the expense of transparency. The developed fiber-reinforced hydrogels show great promise as mechanically robust scaffolds for corneal tissue engineering applications.