Prelude to corneal tissue engineering - gaining control of collagen organization

Molecular and Cellular Mechanisms of the Therapeutic Effect of Mesenchymal Stem Cells and Extracellular Vesicles in Corneal Regeneration

The cornea is a vital component of the visual system, and its integrity is crucial for optimal vision. Damage to the cornea resulting from trauma, infection, or disease can lead to blindness. Corneal regeneration using mesenchymal stem cells (MSCs) and MSC-derived extracellular vesicles (MSC-EVs) offers a promising alternative to corneal transplantation. MSCs are multipotent stromal cells that can differentiate into various cell types, including corneal cells. They can also secrete a variety of anti-inflammatory cytokines and several growth factors, promoting wound healing and tissue reconstruction. This review summarizes the current understanding of the molecular and cellular mechanisms by which MSCs and MSC-EVs contribute to corneal regeneration. It discusses the potential of MSCs and MSC-EV for treating various corneal diseases, including corneal epithelial defects, dry eye disease, and keratoconus. The review also highlights finalized human clinical trials investigating the safety and efficacy of MSC-based therapy in corneal regeneration. The therapeutic potential of MSCs and MSC-EVs for corneal regeneration is promising; however, further research is needed to optimize their clinical application.

Crosstalk among miR-29, α-SMA, and TGFβ1/β3 in melatonin-induced exosome (Mel-prExo) treated human limbal mesenchymal stem cells (hLMSCs): An insight into scarless healing of the cornea

Inflammatory mediators that infiltrate the corneal stroma after corneal infections, trauma or refractive surgery can trigger the transformation of corneal keratocytes into myofibroblasts, resulting in highly irregular collagen deposition and subsequently corneal scarring. Mesenchymal stem cells (MSCs) can be used as therapeutic agents to regenerate corneal and conjunctival tissue damage, regulate inflammation, and reduce the development of limbal stem cell failure. The use of MSC-derived exosomes as a cell-free therapeutic vector is a novel therapeutic approach. This study aimed to assess the effect of exosomes obtained from melatonin (Mel)-treated human limbal mesenchymal stem cells (hLMSCs) on naïve hLMSCs and to determine their influence on the antifibrotic and pro-regenerative pathways involved in corneal scarring. hLMSCs were treated with varying concentrations of Mel, followed by isolation and characterization of the procured exosomes (Mel-prExos). These exosomes were added to the cell culture media of naïve hLMSCs to examine their antifibrotic and pro-regenerative effects. The expression of miR-155, miR-29, TGFβ1, TGFβ3, PPARγ, and α-SMA miRNAs and genes were compared between Mel-treated hLMSCs and Mel-prExo-treated hLMSCs by using real-time PCR. We found that at 1 μM Mel and in the presence of Mel-prExos, TGFβ1 was expressed 0.001-fold, while TGFβ3 was expressed 0.6-fold. miR-29 expression was increased 38-fold in the control-Exo group compared to that in the control group. Changes in TGFβ1/β3 and α-SMA expression are associated with miR-29 and miR-155. This approach could prove beneficial for ocular surface tissue engineering applications.

Regenerative treatment of ophthalmic diseases with stem cells: Principles, progress, and challenges

Background

Degenerate eye disorders, such as glaucoma, cataracts and age-related macular degeneration (AMD), are prevalent causes of blindness and visual impairment worldwide. Other eye disorders, including limbal stem cell deficiency (LSCD), dry eye diseases (DED), and retinitis pigmentosa (RP), result in symptoms such as ocular discomfort and impaired visual function, significantly impacting quality of life. Traditional therapies are limited, primarily focus on delaying disease progression, while emerging stem cell therapy directly targets ocular tissues, aiming to restore ocular function by reconstructing ocular tissue.

Main text

The utilization of stem cells for the treatment of diverse degenerative ocular diseases is becoming increasingly significant, owing to the regenerative and malleable properties of stem cells and their functional cells. Currently, stem cell therapy for ophthalmopathy involves various cell types, such as embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), mesenchymal stem cells (MSCs), and retinal progenitor cells (RPCs). In the current article, we will review the current progress regarding the utilization of stem cells for the regeneration of ocular tissue covering key eye tissues from the cornea to the retina. These therapies aim to address the loss of functional cells, restore damaged ocular tissue and or in a paracrine-mediated manner. We also provide an overview of the ocular disorders that stem cell therapy is targeting, as well as the difficulties and opportunities in this field.

Conclusions

Stem cells can not only promote tissue regeneration but also release exosomes to mitigate inflammation and provide neuroprotection, making stem cell therapy emerge as a promising approach for treating a wide range of eye disorders through multiple mechanisms.

3D bioprinted photo crosslinkable GelMA/methylcellulose hydrogel mimicking native corneal model with enhanced in vitro cytocompatibility and sustained keratocyte phenotype for stromal regeneration

Corneal transplantation serves as the standard clinical therapy for serious corneal disorders. However, rejection of grafts, significant expenditures, and most crucially, the global donor shortage, may affect the outcome. Recently, 3D bioprinting using biodegradable polymeric materials has become a suitable method for creating tissue replicas with identical architecture. One such most renowned material is GelMA, for its scaffold's three-dimensional structure, biocompatibility, robust mechanics, and favourable optical transmittance. However, GelMA's inadequate viscosity to print at body temperature with better form integrity remains an obstacle. The aim of this work is to create 3D printed GelMA/MC hydrogels for corneal stroma tissue engineering using MC's printability at room temperature and GelMA's irreversible photo cross-linking with UV irradiation. The print speed and pressure conditions for 3D GelMA/MC hydrogels were tuned. Thermal, morphological and physicochemical characteristics were studied for two distinct concentrations of GelMA/MC hydrogels. The hydrogels achieved a transparency of ~78 % (at 700 nm), which was on par with that of the normal cornea (80 %). The in vitro studies conducted using goat corneal stromal cells demonstrated the ability of both hydrogels to promote cell adhesion and proliferation. Expression of Vimentin and keratan sulphate validated the phenotype of keratocytes in the hydrogel. This 3D printed GelMA/MC hydrogel model mimics biophysical characteristics of the native corneal stroma, which may hold promise for clinical corneal stromal tissue engineering.

Mini review: human clinical studies of stem cell therapy in keratoconus

Treatment of keratoconus is one of the most interesting research fields for researchers in the world. Regenerative medicine based on human stem cells in the treatment of keratoconus has recently received attention. Despite extensive laboratory and animal studies in regenerative medicine of cornea, there are limited clinical studies in keratoconus. These studies showed promising results of stem cell therapy. In initial studies, the transplantation of these cells into stroma was associated with increased vision and improved corneal parameters without side effects. In this article, we tried to review different aspects of keratoconus stem cell therapy, including cell extraction and culture, surgical procedure, effectiveness and safety of this method in human clinical studies.

Direct measurements of collagen fiber recruitment in the posterior pole of the eye

Collagen is the main load-bearing component of the peripapillary sclera (PPS) and lamina cribrosa (LC) in the eye. Whilst it has been shown that uncrimping and recruitment of the PPS and LC collagen fibers underlies the macro-scale nonlinear stiffening of both tissues with increased intraocular pressure (IOP), the uncrimping and recruitment as a function of local stretch have not been directly measured. This knowledge is crucial for the development of constitutive models associating micro and macro scales. In this project we measured local stretch-induced collagen fiber bundle uncrimping and recruitment curves of the PPS and LC. Thin coronal samples of PPS and LC of sheep eyes were mounted and stretched biaxially quasi-statically using a custom system. At each step, we imaged the PPS and LC with instant polarized light microscopy and quantified pixel-level (1.5 μm/pixel) collagen fiber orientations. We used digital image correlation to measure the local stretch and quantified collagen crimp by the circular standard deviation of fiber orientations, or waviness. Local stretch-recruitment curves of PPS and LC approximated sigmoid functions. PPS recruited more fibers than the LC at the low levels of stretch. At 10% stretch the curves crossed with 75% bundles recruited. The PPS had higher uncrimping rate and waviness remaining after recruitment than the LC: 0.9° vs. 0.6° and 3.1° vs. 2.7°. Altogether our findings support describing fiber recruitment of both PPS and LC with sigmoid curves, with the PPS recruiting faster and at lower stretch than the LC, consistent with a stiffer tissue.

Effect of Mechanical Microenvironment on Collagen Self-Assembly In Vitro

Collagen, as a structural protein, is widely distributed in the human body. Many factors influence collagen self-assembly in vitro, including physical-chemical conditions and mechanical microenvironment, and play a key role in driving the structure and arrangement. However, the exact mechanism is unknown. The purpose of this paper is to investigate the changes in the structure and morphology of collagen self-assembly in vitro under mechanical microenvironment, as well as the critical role of hyaluronic acid in this process. Using bovine type I collagen as the research object, collagen solution is loaded into tensile and stress-strain gradient devices. The morphology and distribution of collagen is observed using an atomic force microscope while changing the concentration of collagen solution, mechanical loading strength, tensile speed, and ratio of collagen to hyaluronic acid. The results demonstrate that the mechanics field governs collagen fibers and changes their orientation. Stress magnifies the differences in results caused by different stress concentrations and sizes, and hyaluronic acid improves collagen fiber orientation. This research is critical for expanding the use of collagen-based biomaterials in tissue engineering.

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.

A "T.E.S.T." hydrogel bioadhesive assisted by corneal cross-linking for in situ sutureless corneal repair

Corneal transplantation is an effective clinical treatment for corneal diseases, which, however, is limited by donor corneas. It is of great clinical value to develop bioadhesive corneal patches with functions of "Transparency" and "Epithelium & Stroma generation", as well as "Suturelessness" and "Toughness". To simultaneously meet the "T.E.S.T." requirements, a light-curable hydrogel is designed based on methacryloylated gelatin (GelMA), Pluronic F127 diacrylate (F127DA) & Aldehyded Pluronic F127 (AF127) co-assembled bi-functional micelles and collagen type I (COL I), combined with clinically applied corneal cross-linking (CXL) technology for repairing damaged cornea. The patch formed after 5 min of ultraviolet irradiation possesses transparent, highly tough, and strongly bio-adhesive performance. Multiple cross-linking makes the patch withstand deformation near 600% and exhibit a burst pressure larger than 400 mmHg, significantly higher than normal intraocular pressure (10-21 mmHg). Besides, the slower degradation than GelMA-F127DA&AF127 hydrogel without COL I makes hydrogel patch stable on stromal beds in vivo, supporting the regrowth of corneal epithelium and stroma. The hydrogel patch can replace deep corneal stromal defects and well bio-integrate into the corneal tissue in rabbit models within 4 weeks, showing great potential in surgeries for keratoconus and other corneal diseases by combining with CXL.

Overview of Corneal Transplantation for the Nonophthalmologist

Corneal transplant is a procedure that aims to replace dysfunctional corneal tissue with a transparent graft and is one of the most widely performed transplant surgeries, but its public and professional awareness is low outside of ophthalmology. Corneal tissue consists of 5 major layers that serve to maintain its structural integrity and refractive shape: the epithelium, Bowman's layer, the stroma, Descemet's membrane, and the endothelium. Failure or irreversible damage to any layer of the cornea may be an indication for corneal transplant, and variants of this procedure may be full thickness or selectively lamellar. Complications related to corneal transplantation may occur anywhere from during surgery to years afterward, including rejection, dehiscence, cataract, and glaucoma. Complications should be managed by an ophthalmologist, but other physicians should be aware of prophylactic medications. Topical immunosuppressants and steroids are effective for preventing and treating rejection episodes, whereas there is little evidence to support the use of systemic immunosuppression. Eye protection is recommended for any corneal transplant recipient. Physicians should counsel patients on corneal donation, especially if outside the United States, where donor tissue is in short supply.

Prolactin-Induced Protein facilitates corneal wound healing

The purpose of the study was to investigate the role of Prolactin-Induced Protein (PIP) in corneal wound healing, in vivo and in vitro. In C57BL/6J mice, corneal epithelia was removed using an ocular burr. Phosphate buffered saline (PBS) or PIP (0.5 and 1.0 μg/mL) was applied topically or subconjunctivally injected. PIP accelerated wound closure as early as 24 h. PIP treatment promoted corneal wound healing and epithelial integrity and thickness. Integrin α6, integrin β4, Thrombospondin-1, and TGF-β1 expressions were all downregulated by PIP after wound closure. In vitro, scratch assays were performed using primary human epithelial cells (HCECs) and human corneal fibroblasts (HCFs), stimulated with PIP at various dosages. PIP treatment promoted both HCECs and HCFs migration. PIP upregulated expression of integrin α6, integrin β4, and Thrombospondin-1 in HCECs. Expression of TGF-β1 in HCECs and expression of smooth muscle actin (SMA) and Type III Collagen (Col III) in HCFs were significantly downregulated at 150 ng/mL PIP. PIP exhibits noteworthy anti-fibrotic potentiality. While the mechanism of how PIP is impactful on the corneal wound healing cascade is unknown, our findings are novel and further studies are warranted in order to unravel any therapeutic potential.

Highly Aligned Bacterial Nanocellulose Films Obtained During Static Biosynthesis in a Reproducible and Straightforward Approach

Bacterial nanocellulose (BNC) is usually produced as randomly-organized highly pure cellulose nanofibers films. Its high water-holding capacity, porosity, mechanical strength, and biocompatibility make it unique. Ordered structures are found in nature and the properties appearing upon aligning polymers fibers inspire everyone to achieve highly aligned BNC (A-BNC) films. This work takes advantage of natural bacteria biosynthesis in a reproducible and straightforward approach. Bacteria confined and statically incubated biosynthesized BNC nanofibers in a single direction without entanglement. The obtained film is highly oriented within the total volume confirmed by polarization-resolved second-harmonic generation signal and Small Angle X-ray Scattering. The biosynthesis approach is improved by reusing the bacterial substrates to obtain A-BNC reproducibly and repeatedly. The suitability of A-BNC as cell carriers is confirmed by adhering to and growing fibroblasts in the substrate. Finally, the thermal conductivity is evaluated by two independent approaches, i.e., using the well-known 3ω-method and a recently developed contactless thermoreflectance approach, confirming a thermal conductivity of 1.63 W mK-1 in the direction of the aligned fibers versus 0.3 W mK-1 perpendicularly. The fivefold increase in thermal conductivity of BNC in the alignment direction forecasts the potential of BNC-based devices outperforming some other natural polymer and synthetic materials.

Corneal Regeneration Using Adipose-Derived Mesenchymal Stem Cells

Adipose-derived stem cells are a subtype of mesenchymal stem cell that offers the important advantage of being easily obtained (in an autologous manner) from low invasive procedures, rendering a high number of multipotent stem cells with the potential to differentiate into several cellular lineages, to show immunomodulatory properties, and to promote tissue regeneration by a paracrine action through the secretion of extracellular vesicles containing trophic factors. This secretome is currently being investigated as a potential source for a cell-free based regenerative therapy for human tissues, which would significantly reduce the involved costs, risks and law regulations, allowing for a broader application in real clinical practice. In the current article, we will review the existing preclinical and human clinical evidence regarding the use of such adipose-derived mesenchymal stem cells for the regeneration of the three main layers of the human cornea: the epithelium (derived from the surface ectoderm), the stroma (derived from the neural crest mesenchyme), and the endothelium (derived from the neural crest cells).

3D Printed Hydrogels for Ocular Wound Healing

Corneal disease is one of the most significant causes of blindness around the world. Presently, corneal transplantation is the only way to treat cornea blindness. It should be noted that the amount of cornea that people donate is so much less than that required (1:70). Therefore, scientists have tried to resolve this problem with tissue engineering and regenerative medicine. Fabricating cornea with traditional methods is difficult due to their unique properties, such as transparency and geometry. Bioprinting is a technology based on additive manufacturing that can use different biomaterials as bioink for tissue engineering, and the emergence of 3D bioprinting presents a clear possibility to overcome this problem. This new technology requires special materials for printing scaffolds with acceptable biocompatibility. Hydrogels have received significant attention in the past 50 years, and they have been distinguished from other materials because of their unique and outstanding properties. Therefore, hydrogels could be a good bioink for the bioprinting of different scaffolds for corneal tissue engineering. In this review, we discuss the use of different types of hydrogel for bioink for corneal tissue engineering and various methods that have been used for bioprinting. Furthermore, the properties of hydrogels and different types of hydrogels are described.

Current Perspectives on Corneal Transplantation (Part 2)

Disease of the cornea is the third leading cause of blindness worldwide. Corneal graft surgery is one of the most successful forms of solid organ transplantations in humans, with ever increasing developments in surgical technique. To date, approximately 4504 corneal transplants are performed in the UK each year. While full thickness transplantation was the most commonly performed keratoplasty over the last few decades, selective lamellar transplantation of the diseased layers of the cornea has been universally adopted. This comprehensive review aims to provide an updated synthesis on different types of corneal transplantations, their treatment outcomes, and the associated complications of each procedure both in adult and pediatric populations. In addition, we also present an up-to-date summary of the emerging therapeutic approaches that have the potential to reduce the demand for donor-dependent keratoplasty.

Preparation and In Vitro Characterization of Gelatin Methacrylate for Corneal Tissue Engineering

BACKGROUND

Corneal disease is second only to cataract considered as the leading cause of blindness in the world, with high morbidity. Construction of corneal substitutes in vitro by tissue engineering technology to achieve corneal regeneration has become a research hotspot in recent years. We conducted in-depth research on the biocompatibility, physicochemical and mechanical properties of rat bone marrow mesenchymal stem cells (rBM-MSCs)-seeded gelatin methacrylate (GelMA) as a bioengineered cornea.

METHODS

Four kinds of GelMA with different concentrations (7, 10, 15 and 30%) were prepared, and their physic-chemical, optical properties, and biocompatibility with rBM-MSCs were characterized. MTT, live/dead staining, cell morphology, immunofluorescence staining and gene expression of keratocyte markers were performed.

RESULTS

7%GelMA hydrogel had higher equilibrium water content and porosity, better optical properties and hydrophilicity. In addition, it is more beneficial to the growth and proliferation of rBM-MSCs. However, the 30%GelMA hydrogel had the best mechanical properties, and could be more conducive to promote the differentiation of rBM-MSCs into keratocyte-like cells.

CONCLUSION

As a natural biological scaffold, GelMA hydrogel has good biocompatibility. And it has the ability to promote the differentiation of rBM-MSCs into keratocyte-like cells, which laid a theoretical and experimental foundation for further tissue-engineered corneal stromal transplantation, and provided a new idea for the source of seeded cells in corneal tissue engineering.

Hydrogels as Emerging Materials for Cornea Wound Healing

Hydrogel biomaterials have many favorable characteristics including tuneable mechanical behavior, cytocompatibility, optical properties suitable for regeneration and restoration of the damaged cornea tissue. The cornea is a tissue susceptible to various injuries and traumas with a complicated healing cascade, in which conserving its transparency and integrity is critical. Accordingly, the hydrogels' known properties along with the stimulation of nerve and cell regeneration make them ideal scaffold for corneal tissue engineering. Hydrogels have been used extensively in clinical applications for the repair and replacement of diseased organs. The development and optimizing of novel hydrogels to repair/replace corneal injuries have been the main focus of researches within the last decade. This research aims to critically review in vitro, preclinical, as well as clinical trial studies related to corneal wound healing using hydrogels in the past 10 years, as this is considered as an emerging technology for corneal treatment. Several unique modifications of hydrogels with smart behaviors have undergone early phase clinical trials and showed promising outcomes. Financially, this considers a multibillion dollars industry and with huge interest from medical devices as well as pharmaceutical industries with several products may emerge within the next five years.

Significance of Crosslinking Approaches in the Development of Next Generation Hydrogels for Corneal Tissue Engineering

Medical conditions such as trachoma, keratoconus and Fuchs endothelial dystrophy can damage the cornea, leading to visual deterioration and blindness and necessitating a cornea transplant. Due to the shortage of donor corneas, hydrogels have been investigated as potential corneal replacements. A key factor that influences the physical and biochemical properties of these hydrogels is how they are crosslinked. In this paper, an overview is provided of different crosslinking techniques and crosslinking chemical additives that have been applied to hydrogels for the purposes of corneal tissue engineering, drug delivery or corneal repair. Factors that influence the success of a crosslinker are considered that include material composition, dosage, fabrication method, immunogenicity and toxicity. Different crosslinking techniques that have been used to develop injectable hydrogels for corneal regeneration are summarized. The limitations and future prospects of crosslinking strategies for use in corneal tissue engineering are discussed. It is demonstrated that the choice of crosslinking technique has a significant influence on the biocompatibility, mechanical properties and chemical structure of hydrogels that may be suitable for corneal tissue engineering and regenerative applications.

Corneal Stromal Regeneration: A Review of Human Clinical Studies in Keratoconus Treatment

The use of advanced therapies with stem cells to reconstruct the complex tissue of corneal stroma has gained interest in recent years. Besides, collagen-based scaffolds bioengineering has been offered as another alternative over the last decade. The outcomes of the first clinical experience with stem cells therapy on corneal stroma regeneration in patients with advanced keratoconus were recently reported. Patients were distributed into three experimental groups: Group 1 (G-1) patients underwent implantation of autologous adipose-derived adult stem cells (ADASCs) alone, Group 2 (G-2) received a 120 μm decellularized donor corneal stromal laminas, and Group 3 (G-3) received a 120 μm recellularized donor laminas with ADASCs. A follow up of 36 months of clinical data, and 12 months of confocal microscopy study was performed, the authors found significant clinical improvement in almost all studied mean values of primary and secondary outcomes. Corneal confocal microscopy demonstrated an increase in cell density in the host stroma, as well as in the implanted tissue. Using different approaches, allogenic small incision lenticule extraction (SMILE) implantation was applied in cases with advanced keratoconus. Some authors reported the implantation of SMILE intrastromal lenticules combined with accelerated collagen cross-linking. Others performed intrastromal implantation of negative meniscus-shaped corneal stroma lenticules. Others have compared the outcomes of penetrating keratoplasty (PKP) vs. small-incision Intralase femtosecond (IFS) intracorneal concave lenticule implantation (SFII). Femtosecond laser-assisted small incision sutureless intrasotromal lamellar keratoplasty (SILK) has been also investigated. The published evidence shows that the implantation of autologous ADASCs, decellularized or recellularized human corneal stroma, allogenic SMILE lenticules corneal inlay, and recombinant cross-linked collagen have shown initially to be potentially effective for the treatment of advanced keratoconus. In light of the present evidence available, it can be said that the era of corneal stromal regeneration therapy has been already started.

Corneal Stroma Regeneration: New Approach for the Treatment of Cornea Disease

Corneal grafting is one of the most common forms of human tissue transplantation. The corneal stroma is responsible for many characteristics of the cornea. For these reasons, an important volume of research has been made to replicate the corneal stroma in the laboratory to find an alternative to classical corneal transplantation techniques.There is an increasing interest today in cell therapy of the corneal stroma using induced pluripotent stem cells or mesenchymal stem cells since these cells have shown to be capable of producing new collagen within the host stroma and even to improve its transparency.The first clinical experiment on corneal stroma regeneration in advanced keratoconus cases has been reported and included. Fourteen patients were randomized and enrolled into 3 experimental groups: (1) patients underwent implantation of autologous adipose-derived adult stem cells alone, (2) patients received decellularized donor corneal stroma laminas, and (3) patients received implantation of recellularized donor laminas with adipose-derived adult stem cells. Clinical improvement was detected with all cases in their visual, pachymetric, and topographic parameters of the operated corneas.Other recent studies have used allogenic SMILE implantation lenticule corneal inlays, showing also an improvement in different visual, topographic, and keratometric parameters.In the present report, we try to summarize the available preclinical and clinical evidence about the emerging topic of corneal stroma regeneration.

The progress in corneal translational medicine

Cornea tissue is in high demand by tissue donation centres globally, and thus tissue engineering cornea, which is the main topic of corneal translational medicine, can serve as a limitless alternative to a donated human cornea tissue. Tissue engineering aims to produce solutions to the challenges associated with conventional cornea tissue, including transplantation and use of human amniotic membrane (HAM), which have issues with storage and immune rejection in patients. Accordingly, by carefully selecting biomaterials and fabrication methods to produce these therapeutic tissues, the demand for cornea tissue can be met, with an improved healing outcome for recipients with less associated harmful risks. In this review paper, we aim to present the recent advancements in the research and clinical applications of cornea tissue, applications including biomaterial selection, fabrication methods, scaffold structure, cellular response to these scaffolds, and future advancements of these techniques.

Corneal stroma regeneration: Preclinical studies

Corneal grafting is one of the most common and successful forms of human tissue transplantation in the world, but the need for corneal grafting is growing and availability of human corneal donor tissue to fulfill this increasing demand is not assured worldwide. The stroma is responsible for many features of the cornea, including its strength, refractive power and transparency, so enormous efforts have been put into replicating the corneal stroma in the laboratory to find an alternative to classical corneal transplantation. Unfortunately this has not been yet accomplished due to the extreme difficulty in mimicking the highly complex ultrastructure of the corneal stroma, and none of the obtained substitutes that have been assayed has been able to replicate this complexity yet. In general, they can neither match the mechanical properties nor recreate the local nanoscale organization and thus the transparency and optical properties of a normal cornea. In this context, there is an increasing interest in cellular therapy of the corneal stroma using Induced Pluripotent Stem Cells (iPSCs) or mesenchymal stem cells (MSCs) from either ocular or extraocular sources, as they have proven to be capable of producing new collagen within the host stroma, modulate preexisting scars and enhance transparency by corneal stroma remodeling. Despite some early clinical data is already available, in the current article we will summary the available preclinical evidence about the topic corneal stroma regeneration. Both, in vitro and in vivo experiments in the animal model will be shown.

Bioengineering Approaches for Corneal Regenerative Medicine

BACKGROUND

Since the cornea is responsible for transmitting and focusing light into the eye, injury or pathology affecting any layer of the cornea can cause a detrimental effect on visual acuity. Aging is also a reason for corneal degeneration. Depending on the level of the injury, conservative therapies and donor tissue transplantation are the most common treatments for corneal diseases. Not only is there a lack of donor tissue and risk of infection/rejection, but the inherent ability of corneal cells and layers to regenerate has led to research in regenerative approaches and treatments.

METHODS

In this review, we first discussed the anatomy of the cornea and the required properties for reconstructing layers of the cornea. Regenerative approaches are divided into two main categories; using direct cell/growth factor delivery or using scaffold-based cell delivery. It is expected delivered cells migrate and integrate into the host tissue and restore its structure and function to restore vision. Growth factor delivery also has shown promising results for corneal surface regeneration. Scaffold-based approaches are categorized based on the type of scaffold, since it has a significant impact on the efficiency of regeneration, into the hydrogel and non-hydrogel based scaffolds. Various types of cells, biomaterials, and techniques are well covered.

RESULTS

The most important characteristics to be considered for biomaterials in corneal regeneration are suitable mechanical properties, biocompatibility, biodegradability, and transparency. Moreover, a curved shape structure and spatial arrangement of the fibrils have been shown to mimic the corneal extracellular matrix for cells and enhance cell differentiation.

CONCLUSION

Tissue engineering and regenerative medicine approaches showed to have promising outcomes for corneal regeneration. However, besides proper mechanical and optical properties, other factors such as appropriate sterilization method, storage, shelf life and etc. should be taken into account in order to develop an engineered cornea for clinical trials.

Using Acoustic Fields to Fabricate ECM-Based Biomaterials for Regenerative Medicine Applications

Ultrasound is emerging as a promising tool for both characterizing and fabricating engineered biomaterials. Ultrasound-based technologies offer a diverse toolbox with outstanding capacity for optimization and customization within a variety of therapeutic contexts, including improved extracellular matrix-based materials for regenerative medicine applications. Non-invasive ultrasound fabrication tools include the use of thermal and mechanical effects of acoustic waves to modify the structure and function of extracellular matrix scaffolds both directly, and indirectly via biochemical and cellular mediators. Materials derived from components of native extracellular matrix are an essential component of engineered biomaterials designed to stimulate cell and tissue functions and repair or replace injured tissues. Thus, continued investigations into biological and acoustic mechanisms by which ultrasound can be used to manipulate extracellular matrix components within three-dimensional hydrogels hold much potential to enable the production of improved biomaterials for clinical and research applications.

Influence of micropatterned substrates on keratocyte phenotype

Substrate topographic patterning is a powerful tool that can be used to manipulate cell shape and orientation. To gain a better understanding of the relationship between surface topography and keratocyte behavior, surface patterns consisting of linear aligned or orthogonally aligned microchannels were used. Photolithography and polymer molding techniques were used to fabricate micropatterns on the surface of polydimethylsiloxane (PDMS). Cells on linear aligned substrates were elongated and aligned in the channel direction, while cells on orthogonal substrates had a more spread morphology. Both linear and orthogonal topographies induced chromatin condensation and resulted in higher expressions of keratocyte specific genes and sulfated glycosaminoglycans (sGAG), compared with non-patterned substrates. However, despite differences in cell morphology and focal adhesions, many genes associated with a native keratocyte phenotype, such as keratocan and ALDH3A1, remain unchanged on the different patterned substrates. This information could be used to optimize substrates for keratocyte culture and to develop scaffolds for corneal regeneration.

Mechanical Roles in Formation of Oriented Collagen Fibers

Collagen is a structural protein that is widely present in vertebrates, being usually distributed in tissues in the form of fibers. In living organisms, fibers are organized in different orientations in various tissues. As the structural base in connective tissue and load-bearing tissue, the orientation of collagen fibers plays an extremely important role in the mechanical properties and physiological and biochemical functions. The study on mechanics role in formation of oriented collagen fibers enables us to understand how discrete cells use limited molecular materials to create tissues with different structures, thereby promoting our understanding of the mechanism of tissue formation from scratch, from invisible to tangible. However, the current understanding of the mechanism of fiber orientation is still insufficient. In addition, existing fabrication methods of oriented fibers are varied and involve interdisciplinary study, and the achievements of each experiment are favorable to the construction and improvement of the fiber orientation theory. To this end, this review focuses on the preparation methods of oriented fibers and proposes a model explaining the formation process of oriented fibers in tendons based on the existing fiber theory. Impact statement As the structural base in connective tissue and load-bearing tissue, the orientation of collagen fibers plays an extremely important role in the mechanical properties and physiological and biochemical functions. However, the current understanding of the mechanism of fiber orientation is still insufficient, which is greatly responsible for the challenge of functional tissue repair and regeneration. Understanding the mechanism of fiber orientation can promote the successful application of fiber orientation scaffolds in tissue repair and regeneration, as well as providing an insight for the mechanism of tissue histomorphology.

[Analysis of light scattering ability of the cornea before and after endothelial keratoplasty]

PURPOSE

To evaluate light scattering ability of the cornea before and after Descemet's stripping automated endothelial keratoplasty (DSAEK) and Descemet's membrane endothelial keratoplasty (DMEK) endothelial keratoplasty.

MATERIAL AND METHODS

The study included 70 patients (70 eyes) who had undergone DSAEK and DMEK (35 patients, 35 eyes in each group). In addition to standard ophthalmological examinations before and during the one-year follow-up after surgery, the patients had their central corneal thickness, stromal thickness, thickness of the interface area and the graft measured with optical coherence tomography (Optovue, U.S.A.). Intensity of light scattering by corneal structures was evaluated using Pentacam HR (Oculus, Germany) topographer with proprietary software and algorithms for calculation of corneal transparency.

RESULTS

Comparison of the outcomes revealed that indices of general integral and stromal light-scattering were higher in patients after DSAEK. Integral light scattering in the interface and graft areas in the DMEK group was significantly lower than in the DSAEK group during the whole follow-up period.

CONCLUSION

The reason for decrease of visual acuity after endothelial keratoplasty, regardless of the surgery type, may be higher light-scattering ability of patient's corneal stroma due to residual opacifications. Better visual acuity outcomes after DMEK in comparison to DSAEK are associated with higher transparency of the interface and graft areas. The suggested new method of assessing the intensity of light-scattering allows more precise evaluation of the influence of light-scattering ability of the stroma, interface and corneal graft after different modifications of endothelial keratoplasty, considering the respective layer thickness and excluding the influence of light scattering on the air-epithelium interface.

Cell loaded 3D bioprinted GelMA hydrogels for corneal stroma engineering

Tissue engineering aims to replace missing or damaged tissues and restore their functions.

Corneal Epithelial-Stromal Fibroblast Constructs to Study Cell-Cell Communication in Vitro

Cell–cell communication plays a fundamental role in mediating corneal wound healing following injury or infection. Depending on the severity of the wound, regeneration of the cornea and the propensity for scar development are influenced by the acute resolution of the pro-fibrotic response mediated by closure of the wound via cellular and tissue contraction. Damage of the corneal epithelium, basement membrane, and anterior stroma following a superficial keratectomy is known to lead to significant provisional matrix deposition, including secretion of fibronectin and thrombospondin-1, as well as development of a corneal scar. In addition, corneal wounding has previously been shown to promote release of extracellular vesicles from the corneal epithelium, which, in addition to soluble factors, may play a role in promoting tissue regeneration. In this study, we report the development and characterization of a co-culture system of human corneal epithelial cells and corneal stromal fibroblasts cultured for 4 weeks to allow extracellular matrix deposition and tissue maturation. The secretion of provisional matrix components, as well as small and large extracellular vesicles, was apparent within the constructs, suggesting cell–cell communication between epithelial and stromal cell populations. Laminin-1β was highly expressed by the corneal epithelial layer with the presence of notable patches of basement membrane identified by transmission electron microscopy. Interestingly, we identified expression of collagen type III, fibronectin, and thrombospondin-1 along the epithelial–stromal interface similar to observations seen in vivo following a keratectomy, as well as expression of the myofibroblast marker, α-smooth muscle actin, within the stroma. Our results suggest that this corneal epithelial–stromal model may be useful in the study of the biochemical phenomena that occur during corneal wound healing.

Regenerative Surgery of the Corneal Stroma for Advanced Keratoconus: 1-Year Outcomes

PURPOSE

This study evaluated 1-year safety and efficacy outcomes of corneal stroma cell therapy. Therapy consisted of implanting autologous adipose-derived adult stem cells (ADASc) with or without sheets of decellularized donor human corneal stroma within the stroma of patients with advanced keratoconus.

DESIGN

This was a prospective interventional non-randomized series of cases.

METHODS

Fourteen consecutive patients were selected and divided into 3 experimental groups. Group A patients underwent implantation of autologous ADASc alone (3 × 10⁶ cells/1 mL) (n = 5). Group B patients received decellularized donor 120-μm-thick corneal stroma lamina alone (n = 5). Group C patients had implantation of recellularized donor lamina with 1 × 10⁶ autologous ADASc plus another 1 × 10⁶ cells/1 mL at the time of the surgery (n = 4). Autologous ADASc were obtained by elective liposuction. Implantation was performed in the corneal stroma through a femtosecond-assisted 9.5-mm diameter lamellar dissection with the patient under topical anesthesia. Twelve months of follow-up data are presented.

RESULTS

No complications were observed during the 1-year follow-up, and full corneal transparency was recovered within 3 months in all patients. No patient lost lines of visual acuity. Corrected distance visual acuity improved 0.231, 0.264, and 0.094 Snellen lines in groups 1, 2, and 3, respectively. In group 1, refractive parameters showed an overall stability, whereas in groups 2 and 3, sphere improved 2.35 diopter (D) and 0.625 D, respectively. Anterior keratometry remained stable (group 1) and improved in groups 2 and 3 (mean improvement of 2D). Corneal aberrometry improved significantly. In optical coherence tomography scans, corneal thickness showed a mean improvement of 14.5 μm (group 1) and 116.4 μm (groups 2 and 3) in the central thickness, and new collagen production was observed at the surgical plane (group 1). Confocal biomicroscopy confirmed the host recellularization of the implanted laminas.

CONCLUSIONS

Intrastromal implantation of autologous ADASc and decellularized human corneal stroma did not show complications at 1 year of follow-up and were moderately effective for the treatment of advanced keratoconus. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

3D in vitro model for human corneal endothelial cell maturation

Corneal endothelium is a cellular monolayer positioned on the Descemet's membrane at the anterior cornea, and it plays a critical role in maintaining corneal clarity. Our present study examines the feasibility of utilizing our 3-dimensional (3D) corneal stromal construct, which consists of human corneal fibroblasts (HCF) and their self-assembled matrix, to observe the development and maturation of human corneal endothelial cells (HCEndoCs) in a co-culture model. Three-dimensional HCF constructs were created by growing the HCFs on Transwell membranes in Eagles' minimum essential medium (EMEM) + 10% FBS + 0.5 mM Vitamin C (VitC) for about 4 weeks. HCEndoCs, either primary (pHCEndoC) or cell line (HCEndoCL), were either seeded in chamber slides, directly on the Transwell membranes, or on the 3D HCF constructs and cultivated for 5 days or 2 weeks. The HCEndoCs that were seeded directly on the Transwell membranes were exposed indirectly to HCF by culturing the HCF on the plate beneath the membrane. Cultures were examined for morphology and ultrastructure using light and transmission electron microscopy (TEM). In addition, indirect-immunofluorescence microscopy (IF) was used to examine tight junction formation (ZO-1), maturation (ALDH1A1), basement membrane formation (Laminin), cell proliferation (Ki67), cell death (caspase-3), and fibrotic response (CTGF). As expected, both pHCEndoCs and HCEndoCLs formed monolayers on the constructs; however, the morphology of the HCEndoCLs appeared to be similar to that seen in vivo, uniform and closely packed, whereas the pHCEndoCs remained elongated. The IF data showed that laminin localization was present in the HCEndoCs' cytoplasm as cell-cell contact increased, and when they were grown in the 3D co-culture, the beginnings of what appears to be a continuous DM-like structure was observed. In addition, in co-cultures, ALDH1A1-positive HCEndoCs were present, ZO-1 expression localized within the tight junctions, minimal numbers of HCEndoCs were Ki67-or Caspase-3-positive, and CTGF was positive in both the HCEndoCs cytoplasm and the matrix of the co-culture. Also, laminin localization was stimulated in HCEndoCs upon indirect stimuli secreted by HCF. The present data suggests our 3D co-culture model is useful for studying corneal endothelium maturation in vitro since the co-culture promotes new DM-like formation, HCEndoCs develop in vivo-like characteristics, and the fibrotic response is activated. Our current findings are applicable to understanding the implications of corneal endothelial injection therapy, such as if the abnormal DM has to be removed from the patient, the newly injected endothelial cells will seed onto the wound area and deposit a new DM-like membrane. However, caution should be observed and as much of the normal DM should be left intact since removal of the DM can cause a posterior stromal fibrotic response.

Fibrosis: Shared Lessons From the Lens and Cornea

Regenerative repair in response to wounding involves cell proliferation and migration. This is followed by the reestablishment of cell structure and organization and a dynamic process of remodeling and restoration of the injured cells' extracellular matrix microenvironment and the integration of the newly synthesized matrix into the surrounding tissue. Fibrosis in the lungs, liver, and heart can lead to loss of life and in the eye to loss of vision. Learning to control fibrosis and restore normal tissue function after injury repair remains a goal of research in this area. Here we use knowledge gained using the lens and the cornea to provide insight into how fibrosis develops and clues to how it can be controlled. The lens and cornea are less complex than other tissues that develop life‐threatening fibrosis, but they are well characterized and research using them as model systems to study fibrosis is leading toward an improved understanding of fibrosis. Here we summarize the current state of the literature and how it is leading to promising new treatments. Anat Rec, 2019. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Safety and Feasibility of Intrastromal Injection of Cultivated Human Corneal Stromal Keratocytes as Cell-Based Therapy for Corneal Opacities

Purpose

To evaluate the safety and feasibility of intrastromal injection of human corneal stromal keratocytes (CSKs) and its therapeutic effect on a rodent early corneal opacity model.

Methods

Twelve research-grade donor corneas were used in primary culture to generate quiescent CSKs and activated stromal fibroblasts (SFs). Single and repeated intrastromal injections of 2 to 4 × 104 cells to rat normal corneas (n = 52) or corneas with early opacities induced by irregular phototherapeutic keratectomy (n = 16) were performed, followed by weekly examination of corneal response under slit-lamp biomicroscopy and in vivo confocal microscopy with evaluation of haze level and stromal reflectivity, and corneal thickness using anterior segment optical coherence tomography (AS-OCT). Time-lapse tracing of Molday ION-labelled cells was conducted using Spectralis OCT and label intensity was measured. Corneas were collected at time intervals for marker expression by immunofluorescence, cell viability, and apoptosis assays.

Results

Injected CSKs showed proper marker expression with negligible SF-related features and inflammation, hence maintaining corneal clarity and stability. The time-dependent loss of injected cells was recovered by repeated injection, achieving an extended expression of human proteoglycans inside rat stroma. In the early corneal opacity model, intrastromal CSK injection reduced stromal reflectivity and thickness, resulting in recovery of corneal clarity, whereas noninjected corneas were thicker and had haze progression.

Conclusions

We demonstrated the safety, feasibility, and therapeutic efficacy of intrastromal CSK injection. The cultivated CSKs can be a reliable cell source for potential cell-based therapy for corneal opacities.

Multiscale computational model of Achilles tendon wound healing: Untangling the effects of repair and loading

Mechanical stimulation of the healing tendon is thought to regulate scar anisotropy and strength and is relatively easy to modulate through physical therapy. However, in vivo studies of various loading protocols in animal models have produced mixed results. To integrate and better understand the available data, we developed a multiscale model of rat Achilles tendon healing that incorporates the effect of changes in the mechanical environment on fibroblast behavior, collagen deposition, and scar formation. We modified an OpenSim model of the rat right hindlimb to estimate physiologic strains in the lateral/medial gastrocnemius and soleus musculo-tendon units during loading and unloading conditions. We used the tendon strains as inputs to a thermodynamic model of stress fiber dynamics that predicts fibroblast alignment, and to determine local collagen synthesis rates according to a response curve derived from in vitro studies. We then used an agent-based model (ABM) of scar formation to integrate these cell-level responses and predict tissue-level collagen alignment and content. We compared our model predictions to experimental data from ten different studies. We found that a single set of cellular response curves can explain features of observed tendon healing across a wide array of reported experiments in rats–including the paradoxical finding that repairing transected tendon reverses the effect of loading on alignment–without fitting model parameters to any data from those experiments. The key to these successful predictions was simulating the specific loading and surgical protocols to predict tissue-level strains, which then guided cellular behaviors according to response curves based on in vitro experiments. Our model results provide a potential explanation for the highly variable responses to mechanical loading reported in the tendon healing literature and may be useful in guiding the design of future experiments and interventions.

Tendons and ligaments transmit force between muscles and bones throughout the body and are comprised of highly aligned collagen fibers that help bear high loads. The Achilles tendon is exposed to exceptionally high loads and is prone to rupture. When damaged Achilles tendons heal, they typically have reduced strength and stiffness, and while most believe that appropriate physical therapy can help improve these mechanical properties, both clinical and animal studies of mechanical loading following injury have produced highly variable and somewhat disappointing results. To help better understand the effects of mechanical loading on tendon healing and potentially guide future therapies, we developed a computational model of rat Achilles tendon healing and showed that we could predict the main effects of different mechanical loading and surgical repair conditions reported across a wide range of published studies. Our model offers potential explanations for some surprising findings of prior studies and for the high variability observed in those studies and may prove useful in designing future therapies or experiments to test new therapies.

Cellular therapy of the corneal stroma: a new type of corneal surgery for keratoconus and corneal dystrophies

Cellular therapy of the corneal stroma, with either ocular or extraocular stem cells, has been gaining a lot of interest over the last decade. Multiple publications from different research groups are showing its potential benefits in relation to its capacity to improve or alleviate corneal scars, improve corneal transparency in metabolic diseases by enhancing the catabolism of the accumulated molecules, generate new organized collagen within the host stroma, and its immunosuppressive and immunomodulatory properties. Autologous extraocular stem cells do not require a healthy contralateral eye and they do not involve any ophthalmic procedures for their isolation. Mesenchymal stem cells have been the most widely assayed and have the best potential to differentiate into functional adult keratocytes in vivo and in vitro. While embryonic stem cells have been partially abandoned due to ethical implications, the discovery of the induced pluripotent stem cells (iPSC) has opened a new and very promising field for future research as they are pluripotent cells with the capacity to theoretically differentiate into any cell type, with the special advantage that they are obtained from adult differentiated cells. Cellular delivery into the corneal stroma has been experimentally assayed in vivo in multiple ways: systemic versus local injections with or without a carrier. Encouraging preliminary human clinical data is already available although still very limited, and further research is necessary in order to consolidate the clinical applications of this novel therapeutic line.

Bio-Orthogonally Crosslinked, In Situ Forming Corneal Stromal Tissue Substitute

In this study, an in situ forming corneal stromal substitute based on collagen type I crosslinked by bio-orthogonal strain-promoted azide-alkyne cycloaddition (SPAAC) is presented. The crosslinked collagen gel has greater transparency compared to non-crosslinked collagen gels. The mechanical properties of the gels are controlled by changing functional group ratios and conjugated collagen concentrations. Higher concentrations of conjugated collagen yield enhances mechanical properties, where the storage modulus increases from 42.39 ± 8.95 to 112.03 ± 3.94 Pa after SPAAC crosslinking. Encapsulated corneal keratocytes grow within the SPAAC-crosslinked gels and corneal keratinocytes are supported on top of the gel surfaces. SPAAC-crosslinked gels support more favorable and stable keratinocyte morphology on their surface compared to non-crosslinked gels likely as a result of more optimal substrate stiffness, gel integrity, and resistance to degradation. SPAAC-crosslinked collagen gels with and without encapsulated keratocytes applied to rabbit corneas in an organ culture model after keratectomy exhibit surface epithelialization with multilayered morphology. The novel in situ forming gel is a promising candidate for lamellar and defect reconstruction of corneal stromal tissue.

Evaluation of an Improved Chitosan Scaffold Cross-Linked With Polyvinyl Alcohol and Amine Coupling Through 1-Ethyl-3-(3-Dimethyl Aminopropyl)-Carbodiimide (EDC) and 2 N-Hydroxysuccinimide (NHS) for Corneal Applications

BACKGROUND:

Corneal blindness resulting from various medical conditions affects millions worldwide. The rapid developing tissue engineering field offers design of a scaffold with mechanical properties and transparency similar to that of the natural cornea.

AIM:

The present study aimed at to prepare and investigate the properties of PVA/chitosan blended scaffold by further cross-linking with 1-Ethyl-3-(3-dimethyl aminopropyl)-carbodiimide (EDC) and 2 N-Hydroxysuccinimide (NHS) as potential in vitro carrier for human limbal stem cells delivery.

MATERIAL AND METHODS:

Acetic acid dissolved chitosan was added to PVA solution, uniformly mixed with a homogenizer until the mixture was in a colloidal state, followed by H₂SO₄ and formaldehyde added and the sample was allowed to cool, subsequently it was poured into a tube and heated in an oven at 60°C for 50 minutes. Finally, samples were soaked in a cross-linking bath with EDC, NHS and NaOH in H₂O/EtOH for 24 h consecutively stirred to cross-link the polymeric chains, reduce degradation. After soaking in the bath, the samples were carefully washed with 2% glycine aqueous solution several times to remove the remaining amount of cross-linkers, followed by washed with water to remove residual agents. Later the cross-linked scaffold subjected for various characterization and biological experiments.

RESULTS:

After viscosity measurement, the scaffold was observed by Fourier transform infrared (FT-IR). The water absorbency of PVA/Chitosan was increased 361% by swelling. Compression testing demonstrated that by increasing the amount of chitosan, the strength of the scaffold could be increased to 16×10−1 MPa. Our degradation results revealed by mass loss using equation shows that scaffold degraded gradually imply slow degradation. In vitro tests showed good cell proliferation and growth in the scaffold. Our assay results confirmed that the membrane could increase the cells adhesion and growth on the substrate.

CONCLUSION:

Hence, we strongly believe the use of this improved PVA/chitosan scaffold has potential to cut down the disadvantages of the human amniotic membrane (HAM) for corneal epithelium in ocular surface surgery and greater mechanical strength in future after successful experimentation with clinical trials.

From DMEK to Corneal Endothelial Cell Therapy: Technical and Biological Aspects

The main treatment available for restoration of the corneal endothelium is keratoplasty and DMEK provides faster visual recovery and better postoperative visual acuity when compared to DSAEK. However, the technical challenges related to this technique and the steep technical learning curve seem to prevent the overcoming of DSAEK in favor of DMEK. Furthermore, the outcome of lamellar keratoplasty techniques is influenced by problems related to corneal grafting tissue availability, management, and quality. On the other hand, improvements in the field of cell engineering have opened the way for the use of stem cells-derived corneal endothelial cells with regenerative intent. In this overview, latest findings in endothelial cell engineering are reported, and perspectives of clinical application of mesenchymal stem cells for corneal endothelial replacement and regeneration are evaluated.

3D bioprinting of a corneal stroma equivalent

Corneal transplantation constitutes one of the leading treatments for severe cases of loss of corneal function. Due to its limitations, a concerted effort has been made by tissue engineers to produce functional, synthetic corneal prostheses as an alternative recourse. However, successful translation of these therapies into the clinic has not yet been accomplished. 3D bioprinting is an emerging technology that can be harnessed for the fabrication of biological tissue for clinical applications. We applied this to the area of corneal tissue engineering in order to fabricate corneal structures that resembled the structure of the native human corneal stroma using an existing 3D digital human corneal model and a suitable support structure. These were 3D bioprinted from an in-house collagen-based bio-ink containing encapsulated corneal keratocytes. Keratocytes exhibited high cell viability both at day 1 post-printing (>90%) and at day 7 (83%). We established 3D bio-printing to be a feasible method by which artificial corneal structures can be engineered.

Cell-laden and orthogonal-multilayer tissue-engineered corneal stroma induced by a mechanical collagen microenvironment and transplantation in a rabbit model

The development of functional therapies for corneal repair and regeneration is a pressing issue. Corneal stroma provides the principal functions of the cornea. However, because of the highly organized nature of the stromal matrix, the attempts to reproduce corneal stroma might follow a scar model. Here, we have developed a protocol for the efficient generation of a cell-laden and orthogonal-multilayer tissue-engineered (TE) corneal stroma, which is induced by the mechanical effects of compressed collagen (CC) or stretched compressed collagen (SCC). Within SCC, with applied compression and force extension, collagen microfibres and corneal stromal cells (CSCs) are arranged orderly, while collagen fibres and CSCs in CC are randomly arranged. Dehydrated SCC has higher tensile strength than dehydrated CC. Hydrated SCC has similar transparency with hydrated native corneal stroma. Compared with those cultured on tissue culture plates (TCP), down-regulation of the genes and proteins of cytoskeleton, activation, proliferation, collagen and TRPV4, up-regulation of proteoglycans, gap junction proteins and TRPA1 are in CSCs of CC and SCC. Moreover, SCC and CC grafts displayed biocompatibility and integration with host corneal tissue after rabbit intra-corneal stromal transplantation by wk 6 under slit lamp microscopy, in vivo confocal microscopy and histological examination. The SCC model facilitates the construction of physiological feature TE corneal stroma, which serves as a foundation for physiological TE construction of other tissues.

STATEMENT OF SIGNIFICANCE

The development of functional therapies for corneal repair and regeneration is a pressing issue. Corneal stroma provides the principal functions of the cornea. Here, we have developed a protocol for the efficient generation of a cell-laden and orthogonal-multilayer tissue-engineered (TE) corneal stroma, which is induced by the mechanical effects of compressed collagen (CC) or stretched compressed collagen (SCC). These models facilitate the construction of physiological feature TE corneal stroma, which serves as a foundation for physiological TE construction of other tissues and helps to reverse fibrosis pathologies in general.

Cellular Therapy With Human Autologous Adipose-Derived Adult Stem Cells for Advanced Keratoconus

PURPOSE

The aim of this phase 1 study was to preliminarily evaluate the safety and efficacy of autologous adipose-derived adult stem cell (ADASC) implantation within the corneal stroma of patients with advanced keratoconus.

METHODS

Five consecutive patients were selected. Autologous ADASCs were obtained by elective liposuction. ADASCs (3 × 10) contained in 1 mL saline were injected into the corneal stroma through a femtosecond-assisted 9.5-mm diameter lamellar pocket under topical anesthesia. Patients were reviewed at 1 day, 1 week, 1, 3, and 6 months postoperatively. Visual function, manifest refraction, slit-lamp biomicroscopy, intraocular pressure, endothelial cell density, corneal topography, corneal optical coherence tomography, and corneal confocal biomicroscopy were recorded.

RESULTS

No intraoperative or postoperative complications were recorded, with full corneal transparency recovery within 24 hours. Four patients completed the full follow-up. All patients improved their visual function (mean: 1 line of unaided and spectacle-corrected distance vision and 2 lines of rigid contact lens distance vision). Manifest refraction and topographic keratometry remained stable. Corneal optical coherence tomography showed a mean improvement of 16.5 μm in the central corneal thickness, and new collagen production was observed as patchy hyperreflective areas at the level of the stromal pocket. Confocal biomicroscopy confirmed the survival of the implanted stem cells at the surgical plane. Intraocular pressure and endothelial cell density remained stable.

CONCLUSIONS

Cellular therapy of the human corneal stroma in vivo with autologous ADASCs appears to be safe. Stem cells survive in vivo with intrastromal new collagen production. Future studies with larger samples are required to confirm these preliminary results.

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.