Novel collagen membranes for the reconstruction of the corneal surface

Corneal regeneration strategies: From stem cell therapy to tissue engineered stem cell scaffolds

Corneal epithelial defects and excessive wound healing might lead to severe complications. As stem cells can self-renew infinitely, they are a promising solution for regenerating the corneal epithelium and treating severe corneal epithelial injury. The chemical and biophysical properties of biological scaffolds, such as the amniotic membrane, fibrin, and hydrogels, can provide the necessary signals for stem cell proliferation and differentiation. Multiple researchers have conducted investigations on these scaffolds and evaluated them as potential therapeutic interventions for corneal disorders. These studies have identified various inherent benefits and drawbacks associated with these scaffolds. In this study, we provided a comprehensive overview of the history and use of various stem cells in corneal repair. We mainly discussed biological scaffolds that are used in stem cell transplantation and innovative materials that are under investigation.

Cultivated autologous limbal epithelial cell (CALEC) transplantation: Development of manufacturing process and clinical evaluation of feasibility and safety

To treat unilateral limbal stem cell (LSC) deficiency, we developed cultivated autologous limbal epithelial cells (CALEC) using an innovative xenobiotic-free, serum-free, antibiotic-free, two-step manufacturing process for LSC isolation and expansion onto human amniotic membrane with rigorous quality control in a good manufacturing practices facility. Limbal biopsies were used to generate CALEC constructs, and final grafts were evaluated by noninvasive scanning microscopy and tested for viability and sterility. Cultivated cells maintained epithelial cell phenotype with colony-forming and proliferative capacities. Analysis of LSC biomarkers showed preservation of "stemness." After preclinical development, a phase 1 clinical trial enrolled five patients with unilateral LSC deficiency. Four of these patients received CALEC transplants, establishing preliminary feasibility. Clinical case histories are reported, with no primary safety events. On the basis of these results, a second recruitment phase of the trial was opened to provide longer term safety and efficacy data on more patients.

Keratin films for ocular surface reconstruction: Wound healing in an in-vivo model

The most commonly used tissue substitute for ocular surface reconstruction is human amniotic membrane (AM). Because of its low biomechanical strength and intransparency there is a need to search for alternatives of consistent quality. This study, further explored the biocompatibility of Keratin Film (KF) and its ability to sustain corneal epithelial wound healing. In three equal groups of 5 New Zeeland white rabbits a 4 mm superficial keratectomy was created in the right eye. Five eyes received a KF, five a human AM graft and the remaining five no implant. All eyes were treated with ofloxacin and dexamethasone eye drops and followed up for 10 days. Corneal fluorescein staining, vascularization, and transparency were assessed using slit lamp biomicroscopy according to a standardized grading score during and at the end of follow-up. The corneal-scleral-button was excised and processed for histology. After 10 days all eyes which had received a KF showed complete epithelial healing and no signs of neovascularization. In the AM group 1 eye showed a persistent epithelial defect at day 10 and 2 eyes showed neovascularization at day 7 resolving at day 10. Transparency improved progressively both in the KF group as well as in the AM group towards the end of the follow. Histology showed a multilayer epithelium firmly adherent to the KF with no evidence of keratocyte migration or inflammatory reaction in the corneal stroma. In this study on rabbit eyes KF better supported corneal epithelial wound healing than amniotic membrane.

Corneal Stroma Regeneration with Collagen-Based Hydrogel as an Artificial Stroma Equivalent: A Comprehensive In Vivo Study

Restoring the anatomical and functional characteristics of the cornea using various biomaterials is especially relevant in the context of a global shortage of donor tissue. Such biomaterials must be biocompatible, strong, and transparent. Here, we report a Viscoll collagen membrane with mechanical and optical properties suitable for replacing damaged stromal tissue. After removing a portion of the stroma, a Viscoll collagen membrane was implanted into the corneas of rabbits. After 6 months, the active migration of host cells into Viscoll collagen membranes was noted, with the preservation of corneal transparency in all experimental animals. Effective integration of the Viscoll collagen membrane with corneal tissue promoted nerve regeneration in vivo, as confirmed by in vivo confocal microscopy. We also demonstrated the safety and efficacy of the Viscoll collagen membrane for corneal stroma regeneration. Thus, in combination with the proposed packaging format that provides long-term storage of up to 10 months, this material has great potential for replacing and regenerating damaged stromal tissues.

[Regenerative medicine for the corneal epithelium : Cell therapy from bench to bedside]

In the case of thermal or caustic burns of the ocular surface, loss of limbal epithelial stem cells leads to compromised self-renewal of the corneal epithelium. This results in permanent loss of vision. In these situations, transplantation of cultured limbal epithelial cells on an amniotic membrane or fibrin gel as substrate (Holoclar®) can help to regenerate the corneal surface. The required cells are obtained from the healthy partner eye, if available. Adult stem cells from other parts of the body potentially serve as alternative cell sources: hair follicles, oral mucosa, mesenchymal stromal cells, or induced pluripotent stem cells (originally, e.g., skin fibroblasts). The reprogramming of such cells can be achieved with the help of transcription factors. In addition, work is being done on biosynthetic or synthetic matrices, which not only serve as substrate material for the transplantation but also support the functional properties of these cells (self-renewal, corneal epithelial-typical phenotype).

A new collagen scaffold for the improvement of corneal biomechanical properties in a rabbit model

Although collagen based materials are widely used in corneal tissue engineering with promising results. The usage of such materials for the improvement of corneal biomechanical properties is still unclear. In this study, we aimed to investigate a new Viscoll collagen-based membrane for the improvement of corneal biomechanical characteristics.The right eyes of 15 Chinchilla rabbits were implanted with the membrane via an intrastromal pocket, with the contralateral intact eyes as controls. At 7, 30, 90, and 180 days post-surgery, the rabbits underwent anterior segment optical coherence topography, clinical examination, and slit-lamp microscopy. Additionally, the corneal samples also underwent histological examination followed by the assessment of the biomechanical characteristics of four treated and non-treated corneas at 30, 90, and 180 days, including keratometry at 180 days, post operation. Data are presented as means ± confidence intervals with a 95% confidence level. All the operated corneas retained their transparency throughout the study. Implantation approximately doubled the central corneal thickness. Corneas became stronger by approximately 87% between 1 and 6 months after surgery (maximum fracture load, 13.3 ± 0.8 and 24.9 ± 1.4 N, respectively), and their elasticity increased by approximately 27% over the same time frame (maximum slope of the elastic region of the stress-strain curve, 11.5 ± 0.2 and 14.6 ± 1.4 N/mm respectively). We have thus proposed a new method to increase corneal thickness and strengthen the corneal tissues while preserving their transparency and demonstrated its safety and efficacy in a rabbit model over 6 months. This may be a suitable alternative to the existing corneal collagen crosslinking procedure.

Ocular surface repair using decellularized porcine conjunctiva

The primary functions of the conjunctiva embody ocular surface protection and the maintenance of the tear film equilibrium. Severe conjunctival defects such as symblepharon may impair the integrity of ocular surface and cause loss of visual functions. Here we report the use of a decellularized porcine conjunctiva (DPC) for conjunctival reconstruction in rabbit models and in clinic. Our results show that the major xenoantigens are efficiently removed, while abundant matrix components and integrated microstructures are well preserved in the DPC. These characteristics provide mechanical support and favorable histocompatibility for repairing damaged conjunctiva. The DPC application has demonstrated enhanced transplant stability and improved epithelial regeneration in severe ocular surface damage comparing to those of amniotic membrane (AM), the most frequently applied matrix for ocular surface reconstruction nowadays. In order to test the DPC performance in clinic, three patients with pterygium and one patient with symblepharon underwent transplant with DPC. The grafts in all cases were completely re-epithelized and no graft melt or fibroplasia were observed. These results suggest that the strategy we developed is feasible and effective for conjunctival reconstruction and ocular surface repair. STATEMENT OF SIGNIFICANCE: In this study, we adopted an innovative approach to prepare decellularized porcine conjunctiva (DPC). The intricate conjunctiva-specific structures and abundant matrix components were preserved in DPC, which offers favorable mechanical properties for graft. DPC has shown positive effects in ocular surface repair, which has been proven particularly in a rabbit model with severe symblepharon. Reconstructed conjunctiva by DPC exhibited epithelial heterogeneity, extremely resembling that of native conjunctiva. In addition, results from clinical studies were encouraging for pterygium and symblepharon and clinical application of DPC is promising.

Recovering vision in corneal epithelial stem cell deficient eyes

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Corneal limbal epithelial stem cells deficiencies cause severe ocular surface instability and visual impairment.

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These conditions, caused by injury or disease, are very difficult to treat.

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Laboratory-grown epithelial cell sheets expanded from healthy limbal tissue can be used to reconstruct the ocular surface.

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Other epithelia, such as the oral mucosa, can be used to generate the therapeutic cell sheets.

A healthy corneal epithelium, which is essential for proper vision and protection from external pathogens, is continuously replenished throughout life by stem cells located at the limbus. In diseased or injured eyes, however, in which stem cells are deficient, severe ocular problems manifest themselves. These are notoriously difficult to manage and as a result the last 20 or so years has seen a number of therapeutic strategies emerge that aim to recover the ocular surface and restore vision in limbal stem cell deficient eyes. The dominant concept involves the generation of laboratory cultivated epithelial cell sheets expanded from small biopsies of the epithelial limbus (for patient or donors) or another non-corneal epithelial tissue such as the oral mucosa. Typically, cells are grown on sterilised human amniotic membrane as a substrate, which then forms part of the graft, or specially formulated plastic culture dishes from which cells sheets can be released by lowering the temperature, and thus the adherence of the plastic to the cells. Overall, clinical results are promising, as is discussed, with new cultivation methodologies and different cell lineages currently being investigated to augment the treatment options for visual disturbance caused by a corneal epithelial limbal stem cell deficiency.

Biomaterials for Regenerative Medicine Approaches for the Anterior Segment of the Eye

The role of biomaterials in tissue engineering and regenerative medicine strategies to treat vision loss associated with damage to tissues in the anterior segment of the eye has been studied for several years. This has mostly involved replacement and support for the cornea and conjunctiva. These are complex tissues with specific functional requirements for different parts of the tissue. Amniotic membrane (AM) is used in clinical practice to transplant autologous or allogenic cells to the corneal surface. Fibrin gels have also progressed to clinical use under specific conditions. Alternatives to AM such as collagen gels, other natural materials, for example keratin and silks, and synthetic polymers have received considerable attention in laboratory and animal studies. This experience is building a body of evidence to demonstrate the potential of tissue engineering and regenerative medicine in corneal and conjunctival reconstruction and can also lead to other applications in the anterior segment of the eye, for example, the trabecular meshwork. There is a real clinical need for new procedures to overcome vision loss but there are also opportunities for developments in ocular applications to lead to biomaterials innovations for use in other clinical areas.

Role of corneal collagen fibrils in corneal disorders and related pathological conditions

The cornea is a soft tissue located at the front of the eye with the principal function of transmitting and refracting light rays to precisely sense visual information. Corneal shape, refraction, and stromal stiffness are to a large part determined by corneal fibrils, the arrangements of which define the corneal cells and their functional behaviour. However, the modality and alignment of native corneal collagen lamellae are altered in various corneal pathological states such as infection, injury, keratoconus, corneal scar formation, and keratoprosthesis. Furthermore, corneal recuperation after corneal pathological change is dependent on the balance of corneal collagen degradation and contraction. A thorough understanding of the characteristics of corneal collagen is thus necessary to develop viable therapies using the outcome of strategies using engineered corneas. In this review, we discuss the composition and distribution of corneal collagens as well as their degradation and contraction, and address the current status of corneal tissue engineering and the progress of corneal cross-linking.

Optimization of Corneal Epithelial Progenitor Cell Growth on Bombyx mori Silk Fibroin Membranes

Scaffolds prepared from silk fibroin derived from cocoons of the domesticated silkworm moth Bombyx mori have demonstrated potential to support the attachment and growth of human limbal epithelial (HLE) cells in vitro. In this study, we attempted to further optimize protocols to promote the expansion of HLE cells on B. mori silk fibroin- (BMSF-) based scaffolds. BMSF films were initially coated with different extracellular matrix proteins and then analysed for their impact on corneal epithelial cell adhesion, cell morphology, and culture confluency. Results showed that collagen I, collagen III, and collagen IV consistently improved HCE-T cell adherence, promoted an elongated cell morphology, and increased culture confluency. By contrast, ECM coating had no significant effect on the performance of primary HLE cells cultured on BMSF films. In the second part of this study, primary HLE cells were grown on BMSF films in the presence of medium (SHEM) supplemented with keratinocyte growth factor (KGF) and the Rho kinase inhibitor, Y-27632. The results demonstrated that SHEM medium supplemented with KGF and Y-27632 dramatically increased expression of corneal differentiation markers, keratin 3 and keratin 12, whereas expression of the progenitor marker, p63, did not appear to be significantly influenced by the choice of culture medium.

A novel suture retention test for scaffold strength characterization in ophthalmology

Sutures are a common way to attach scaffolds in patients. For tubular cardiac scaffolds, the 'suture retention strength' is commonly used to evaluate the resistance of a scaffold against the pull-out of a suture. In order to make this quantity accessible for ophthalmological scaffolds the test procedure has been modified in a novel way. Polycaprolactone (PCL) films of different thicknesses and an amniotic membrane (AM) were used for the experiments. Circular samples with a radius of 7mm were taken and a suture was passed through each sample and tied to a loop. The sample was clamped in a tensile tester and a bolt was passed through the loop. The suture was then pulled with a constant deformation rate until pull-out occurred. The suture retention strength, the deformation at the suture retention strength, and the deformation at rupture were determined for each sample. The presented modified suture retention test allows to measure the relevant parameters of samples on the scale of ophthalmological scaffolds in a reproducible way. A comparison between the first data on PCL and AM has been made.

Human pluripotent stem cell-derived limbal epithelial stem cells on bioengineered matrices for corneal reconstruction

Corneal epithelium is renewed by limbal epithelial stem cells (LESCs), a type of tissue-specific stem cells located in the limbal palisades of Vogt at the corneo-scleral junction. Acute trauma or inflammatory disorders of the ocular surface can destroy these stem cells, leading to limbal stem cell deficiency (LSCD) - a painful and vision-threatening condition. Treating these disorders is often challenging and complex, especially in bilateral cases with extensive damage. Human pluripotent stem cells (hPSCs) provide new opportunities for corneal reconstruction using cell-based therapy. Here, we investigated the use of hPSC-derived LESC-like cells on bioengineered collagen matrices in serum-free conditions, aiming for clinical applications to reconstruct the corneal epithelium and partially replace the damaged stroma. Differentiation of hPSCs towards LESC-like cells was directed using small-molecule induction followed by maturation in corneal epithelium culture medium. After four to five weeks of culture, differentiated cells were seeded onto bioengineered matrices fabricated as transparent membranes of uniform thickness, using medical-grade porcine collagen type I and a hybrid cross-linking technology. The bioengineered matrices were fully transparent, with high water content and swelling capacity, and parallel lamellar microstructure. Cell proliferation of hPSC-LESCs was significantly higher on bioengineered matrices than on collagen-coated control wells after two weeks of culture, and LESC markers p63 and cytokeratin 15, along with proliferation marker Ki67 were expressed even after 30 days in culture. Overall, hPSC-LESCs retained their capacity to self-renew and proliferate, but were also able to terminally differentiate upon stimulation, as suggested by protein expression of cytokeratins 3 and 12. We propose the use of bioengineered collagen matrices as carriers for the clinically-relevant hPSC-derived LESC-like cells, as a novel tissue engineering approach for corneal reconstruction.