Transplantation of corneas reconstructed with cultured adult human corneal endothelial cells in nude rats

The Innovative Biomaterials and Technologies for Developing Corneal Endothelium Tissue Engineering Scaffolds: A Review and Prospect

Corneal transplantation is the only treatment for corneal endothelial blindness. However, there is an urgent need to find substitutes for corneal endothelium grafts due to the global shortage of donor corneas. An emerging research field focuses on the construction of scaffold-based corneal endothelium tissue engineering (CETE). Long-term success in CETE transplantation may be achieved by selecting the appropriate biomaterials as scaffolds of corneal endothelial cells and adding bioactive materials to promote cell activity. This article reviews the research progress of CETE biomaterials in the past 20 years, describes the key characteristics required for corneal endothelial scaffolds, and summarizes the types of materials that have been reported. Based on these, we list feasible improvement strategies for biomaterials innovation. In addition, we describe the improved techniques for the scaffolds' surface topography and drug delivery system. Some promising technologies for constructing CETE are proposed. However, some questions have not been answered yet, and clinical trials and industrialization should be carried out with caution.

Biomaterials used for tissue engineering of barrier-forming cell monolayers in the eye

Cell monolayers that form a barrier between two structures play an important role for the maintenance of tissue functionality. In the anterior portion of the eye, the corneal endothelium forms a barrier that controls fluid exchange between the aqueous humor of the anterior chamber and the corneal stroma. This monolayer is central in the pathogenesis of Fuchs endothelial corneal dystrophy (FECD). FECD is a common corneal disease, in which corneal endothelial cells deposit extracellular matrix that increases the thickness of its basal membrane (Descemet's membrane), and forms excrescences (guttae). With time, there is a decrease in endothelial cell density that generates vision loss. Transplantation of a monolayer of healthy corneal endothelial cells on a Descemet membrane substitute could become an interesting alternative for the treatment of this pathology. In the back of the eye, the retinal pigment epithelium (RPE) forms the blood-retinal barrier, controlling fluid exchange between the choriocapillaris and the photoreceptors of the outer retina. In the retinal disease dry age-related macular degeneration (dry AMD), deposits (drusen) form between the RPE and its basal membrane (Bruch's membrane). These deposits hinder fluid exchange, resulting in progressive RPE cell death, which in turn generates photoreceptor cell death, and vision loss. Transplantation of a RPE monolayer on a Bruch's membrane/choroidal stromal substitute to replace the RPE before photoreceptor cell death could become a treatment alternative for this eye disease. This review will present the different biomaterials that are proposed for the engineering of a monolayer of corneal endothelium for the treatment of FECD, and a RPE monolayer for the treatment of dry AMD.

Research Progress of Polymer Biomaterials as Scaffolds for Corneal Endothelium Tissue Engineering

Nowadays, treating corneal diseases arising from injury to the corneal endothelium necessitates donor tissue, but these corneas are extremely scarce. As a result, researchers are dedicating significant efforts to exploring alternative approaches that do not rely on donor tissues. Among these, creating a tissue-engineered scaffold on which corneal endothelial cells can be transplanted holds particular fascination. Numerous functional materials, encompassing natural, semi-synthetic, and synthetic polymers, have already been studied in this regard. In this review, we present a comprehensive overview of recent advancements in using polymer biomaterials as scaffolds for corneal endothelium tissue engineering. Initially, we analyze and present the key properties necessary for an effective corneal endothelial implant utilizing polymer biomaterials. Subsequently, we focus on various emerging biomaterials as scaffolds for corneal endothelium tissue engineering. We discuss their modifications (including natural and synthetic composites) and analyze the effect of micro- and nano-topological morphology on corneal endothelial scaffolds. Lastly, we highlight the challenges and prospects of these materials in corneal endothelium tissue engineering.

New developments in corneal endothelial cell replacement

Corneal transplantation is currently the most effective treatment to restore corneal clarity in patients with endothelial disorders. Endothelial transplantation, either by Descemet membrane endothelial keratoplasty (DMEK) or by Descemet stripping (automated) endothelial keratoplasty (DS(A)EK), is a surgical approach that replaces diseased Descemet membrane and endothelium with tissue from a healthy donor eye. Its application, however, is limited by the availability of healthy donor tissue. To increase the pool of endothelial grafts, research has focused on developing new treatment options as alternatives to conventional corneal transplantation. These treatment options can be considered as either 'surgery-based', that is tissue-efficient modifications of the current techniques (e.g. Descemet stripping only (DSO)/Descemetorhexis without endothelial keratoplasty (DWEK) and Quarter-DMEK), or 'cell-based' approaches, which rely on in vitro expansion of human corneal endothelial cells (hCEC) (i.e. cultured corneal endothelial cell sheet transplantation and cell injection). In this review, we will focus on the most recent developments in the field of the 'cell-based' approaches. Starting with the description of aspects involved in the isolation of hCEC from donor tissue, we then describe the different natural and bioengineered carriers currently used in endothelial cell sheet transplantation, and finally, we discuss the current 'state of the art' in novel therapeutic approaches such as endothelial cell injection.

Approaches for corneal endothelium regenerative medicine

The state of the art therapy for treating corneal endothelial disease is transplantation. Advances in the reproducibility and accessibility of surgical techniques are increasing the number of corneal transplants, thereby causing a global deficit of donor corneas and leaving 12.7 million patients with addressable visual impairment. Approaches to regenerate the corneal endothelium offer a solution to the current tissue scarcity and a treatment to those in need. Methods for generating corneal endothelial cells into numbers that could address the current tissue shortage and the possible strategies used to deliver them have now become a therapeutic reality with clinical trials taking place in Japan, Singapore and Mexico. Nevertheless, there is still a long way before such therapies are approved by regulatory bodies and become clinical practice. Moreover, acellular corneal endothelial graft equivalents and certain drugs could provide a treatment option for specific disease conditions without the need of donor tissue or cells. Finally, with the emergence of gene modulation therapies to treat corneal endothelial disease, it would be possible to treat presymptomatic patients or those presenting early symptoms, drastically reducing the need for donor tissue. It is necessary to understand the most recent developments in this rapidly evolving field to know which conditions could be treated with which approach. This article provides an overview of the current and developing regenerative medicine therapies to treat corneal endothelial disease and provides the necessary guidance and understanding towards the treatment of corneal endothelial disease.

Evolution of therapies for the corneal endothelium: past, present and future approaches

Corneal endothelial diseases are leading indications for corneal transplantations. With significant advancement in medical science and surgical techniques, corneal transplant surgeries are now increasingly effective at restoring vision in patients with corneal diseases. In the last 15 years, the introduction of endothelial keratoplasty (EK) procedures, where diseased corneal endothelium (CE) are selectively replaced, has significantly transformed the field of corneal transplantation. Compared to traditional penetrating keratoplasty, EK procedures, namely Descemet's stripping automated endothelial keratoplasty (DSAEK) and Descemet membrane endothelial keratoplasty (DMEK), offer faster visual recovery, lower immunological rejection rates, and improved graft survival. Although these modern techniques can achieve high success, there are fundamental impediments to conventional transplantations. A lack of suitable donor corneas worldwide restricts the number of transplants that can be performed. Other barriers include the need for specialized expertise, high cost, and risks of graft rejection or failure. Research is underway to develop alternative treatments for corneal endothelial diseases, which are less dependent on the availability of allogeneic tissues - regenerative medicine and cell-based therapies. In this review, an overview of past and present transplantation procedures used to treat corneal endothelial diseases are described. Potential novel therapies that may be translated into clinical practice will also be presented.

Focus on cell therapy to treat corneal endothelial diseases

The cornea is a multi-layered structure which allows fine refraction and provides both resistance to external insults and adequate transparency. The corneal endothelium ensures stromal hydration, failure of which, such as in Fuchs endothelial corneal dystrophy, after trauma or in aging, may lead to loss of corneal transparency and induce blindness. Currently, no efficient therapeutic alternatives exist except for corneal grafting. Thus corneal tissue engineering represents a valuable alternative approach, which may overcome cornea donor shortage. Several studies describe protocols to isolate, differentiate, and cultivate corneal endothelial cells (CEnCs) in vitro. Two main in vitro strategies can be described: expansion of eye-native cell populations, such as CEnCs, or the production and expansion of CEnCs from non-eye native cell populations, such as induced Pluripotent Stem Cells (iPSCs). The challenge with these cells is to obtain a monolayer of CEnCs on a biocompatible carrier, with a specific morphology (flat hexagonal cells), and with specific functions such as programmed cell cycle arrest. Another issue for this cell culture methodology is to define the adapted protocol (media, trophic factors, timeframe) that can mimic physiological development. Additionally, contamination by other cell types still represents a huge problem. Thus, purification methods, such as Fluorescence Activated Cell Sorting (FACS), Magnetic Ativated Cell Sorting (MACS) or Sedimentation Field Flow Fractionation (SdFFF) are useful. Animal models are also crucial to provide a translational approach for these therapies, integrating macro- and microenvironment influences, systemic hormonal or immune responses, and exogenous interactions. Non-eye native cell graft protocols are constantly improving both in efficacy and safety, with the aim of being the most suitable candidate for corneal therapies in future routine practice. The aim of this work is to review these different aspects with a special focus on issues facing CEnC culture in vitro, and to highlight animal graft models adapted to screen the efficacy of these different protocols.

Magnetic Human Corneal Endothelial Cell Transplant: Delivery, Retention, and Short-Term Efficacy

Purpose

Corneal endothelial dysfunction leads to corneal edema, pain, and vision loss. Adequate animal models are needed to study the safety and efficacy of novel cell therapies as an alternative to corneal transplantation.

Methods

Primary human corneal endothelial cells (HCECs) were isolated from cadaveric donor corneas, expanded in vitro, transduced to express green fluorescent protein (GFP), loaded with superparamagnetic nanoparticles, and injected into the anterior chamber of adult rabbits immediately after endothelial cell or Descemet's membrane stripping. The same volume of balanced salt solution plus (BSS+) was injected in control eyes. We compared different models for inducing corneal edema in rabbits, and examined the ability of transplanted HCECs to reduce corneal edema over time by measuring central corneal thickness and tracking corneal clarity. GFP-positive donor cells were tracked in vivo using optical coherence tomography (OCT) fluorescence angiography module, and the transplanted cells were confirmed by human nuclei immunostaining.

Results

Magnetic HCECs integrated onto the recipient corneas with intact Descemet's membrane, and donor identity was confirmed by GFP expression and immunostaining for human nuclei marker. Donor HCECs formed a monolayer on the posterior corneal surface and expressed HCEC functional markers of tight junction formation. No GFP-positive cells were observed in the trabecular meshwork or on the iris, and intraocular pressure remained stable through the length of the study.

Conclusions

Our results demonstrate magnetic cell-based therapy efficiently delivers HCECs to restore corneal transparency without detectable toxicity or adverse effect on intraocular pressure. Magnetic delivery of HCECs may enhance corneal function and should be explored further for human therapies.

Corneal Endothelial Cells Over the Past Decade: Are We Missing the Mark(er)?

Corneal endothelial dysfunction is one of the leading causes of corneal edema and visual impairment, requiring corneal endothelial transplantation. The treatments are limited, however, by both logistics and a global donor shortage. As a result, corneal researchers are striving to develop tissue-engineered constructs as an alternative. Recently, the clinical results of the first patients treated using a novel corneal endothelial cell therapy were reported, and it is likely many more will follow shortly. As we move from lab to clinic, it is crucial that we establish accurate and robust methods of proving the cellular identity of these products, both in genotype and phenotype.

In this review, we summarized all of the markers and techniques that have been reported during the development of corneal endothelial cell therapies over the past decade. The results show the most frequently used markers were very general, namely Na⁺/K⁺ ATPase and zonula occludens-1 (ZO-1). While these markers are expressed in nearly every epithelial cell, it is the hexagonal morphology that points to cells being corneal endothelium in nature. Only 11% of articles aimed at discovering novel markers, while 30% were already developing cell therapies. Finally, we discuss the potential of functional testing of cell products to demonstrate potency in parallel with identity markers.

With this review, we would like to highlight that, while this is an exciting era in corneal endothelial cell therapies, there is still no accepted consensus on a unique endothelial marker panel. We must ask the question of whether or not we are getting ahead of ourselves and whether we need to refocus on basic science rather than enter clinics prematurely.

Experimental models of corneal endothelial cell therapy and translational challenges to clinical practice

The human corneal endothelium (CE) is a post-mitotic monolayer of endothelial cells, thought to be incapable of in vivo regeneration. Dysfunction of the CE is a commonly cited indication for corneal transplantation, with corneal blindness being the fifth most common cause of blindness globally. In 2012 alone 184,576 corneal transplants were performed in 116 countries (Gain et al., 2016). Presently, outcomes following human corneal transplantation have been reported to have over 97% success rate in restoring the recipient's vision (Patel et al., 2019). However, the continuing demand for cadaveric human corneas has driven research into alternative sources of CE and with the advent of protocols to produce cultured hCECs there is now the potential for cell therapy to regenerate the damaged CE. This review aims to examine the merits and limitations of different types of human and animal models used so far to test the concept of CE cell therapy.

Construction of a high cell density human corneal endothelial equivalent and its transplantation in primate models

BACKGROUND

Recently, many patients with corneal blindness caused by endothelial dysfunction have no opportunity to receive keratoplasty therapy because of the extremely limited number of donor corneas. Corneal tissue engineering opens a new path for in vitro reconstruction of tissue-engineered HCE which will cure the corneal endotheliopathy by clinical corneal transplantation. In this study, we construct a human corneal endothelium (HCE) equivalent with non-transfected monoclonal HCE (mcHCE) cells and modified denuded amniotic membrane (mdAM), and evaluate its functions in monkey models.

METHODS

Tissue-engineered HCE (TE-HCE) was constructed by culturing DiI-labeled mcHCE cells on mdAMs in 20% fetal bovine serum-containing DMEM/Ham's Nutrient Mixture F12 (1:1) medium and 5% CO₂ at 37°C on a 24-well culture plate. The constructed TE-HCE was transplanted into monkey corneas via penetrating keratoplasty with Descemet's membrane and endothelium stripped. The corneal transparency, thickness, and intraocular pressure were monitored in vivo, and the corneal morphology and histological structure were examined ex vivo 181 days after surgery.

RESULTS

The constructed TE-HCE, with an average density of 3602.22 ± 45.22 cells/mm² , mimicked its natural counterpart both in morphology and histological structure. In vivo, corneal transparency was maintained, and the corneal thickness gradually decreased to 567.33 ± 72.77 μm at day 181 after TE-HCE transplanted into monkey eyes, while intense corneal edema and turbid were found in mdAM-transplanted eyes with their corneal thicknesses maintained over 1000 μm during the monitoring period. Ex vivo, a monolayer of corneal endothelium, consisting of mcHCE cells at a density of 2795.65 ± 156.83 cells/mm² , was reconstructed in transplanted monkey eyes. The cells in the transplanted area had the hexagonal or polygonal morphology and normal ultrastructure, and established plenty of cell-cell and cell-stromal matrix junctions. Besides, huge membrane-bounded flat stacks with electric dense inclusions were found in mcHCE cells beneath the plasma membrane at the stromal side.

CONCLUSIONS

The constructed TE-HCE has normal histological property and functions well in monkey models. The TE-HCE could be used as a promising HCE equivalent in therapy of corneal endothelium dysfunction and corneal regenerative medicine.

Advances in culture, expansion and mechanistic studies of corneal endothelial cells: a systematic review

Human corneal endothelial cells are notorious for their restricted proliferative ability in vivo and in vitro. Hence, injury or dysfunction of these cells may easily result in blindness. Currently, the only treatment is to transplant a donor cornea that contains a healthy corneal endothelium. However there is a severe global shortage of donor corneas and there remains an unmet clinical need to engineer human corneal grafts with healthy corneal endothelium. In this review, we present current advances in the culture, expansion, and molecular understandings of corneal endothelial cells in vitro in order to help establish methods of engineering human corneal endothelial grafts.

Feasibility Study of Human Corneal Endothelial Cell Transplantation Using an In Vitro Human Corneal Model

PURPOSE

To test the feasibility of a cell therapy approach to treat corneal endothelial (CE) disorders using an in vitro model of human corneal decompensation.

METHODS

A CE decompensation model was established by removal of the Descemet membrane/endothelium complex from cadaveric human corneas in an air interface organ culture system (group 2) and compared with normal corneas (group 1). The posterior stroma of decompensated corneas was seeded with immortalized human corneal endothelial cells (HCEC-12) in group 3 and passage 0 primary human CE cells in group 4 corneas. Functional effects on stromal thickness were determined with histological analysis 3 to 10 days after cell therapy treatment.

RESULTS

Removal of the Descemet membrane/endothelium complex in group 2 corneas resulted in a stromal thickness of 903 ± 86 μm at 12 hours compared with 557 ± 72 μm in group 1 corneas. Stromal thickness reduced from 1218 ± 153 μm to 458 ± 90 μm (63% ± 6%, P = 0.001) after cell transplantation in group 3 and from 1100 ± 86 μm to 489 ± 94 μm (55% ± 7%, P = 0.00004) in group 4. Posttransplantation histology demonstrated formation of a monolayer of corneal endothelium attached to the posterior stromal surface.

CONCLUSIONS

Direct transplantation of cultured human CE cells and immortalized HCEC-12 to bare posterior corneal stroma resulted in formation of an endothelial monolayer and restoration of stromal hydration to physiological thickness, demonstrating the feasibility of cell therapy in treatment of CE decompensation in a human in vitro model.

In Vivo Functionality of a Corneal Endothelium Transplanted by Cell-Injection Therapy in a Feline Model

PURPOSE

To evaluate the functionality of a corneal endothelium reconstituted by injection of corneal endothelial cells (CEC) in the anterior chamber of a feline model.

METHODS

We operated the right eyes of 16 animals. Eight underwent central endothelial scraping and injection with 2 × 10(5) (n = 4) or 1 × 10(6) (n = 4) feline CEC supplemented with Y-27632 and labeled with 3,3'-Dioctadecyl-5,5'-Di(4-Sulfophenyl)Oxacarbocyanine (SP-DiOC18[3] or DiOC). After total endothelial scraping, two eyes were injected with 1 × 10(6) labeled CEC and Y-27632. The central (n = 3) or entire (n = 3) endothelium was scraped in six eyes followed by Y-27632 injection without CEC. Subjects were positioned eyes down for 3 hours. Outcomes included graft transparency, pachymetry, CEC morphometry, histology, electron microscopy, and function and wound healing-related protein immunostaining.

RESULTS

Postoperatively, corneas grafted with 2 × 10(5) CEC and centrally scraped controls displayed the best transparency and pachymetry. Corneas grafted with 1 × 10(6) CEC yielded intermediate results. Entirely scraped controls remained hazy and thick. Histopathology revealed a confluent endothelial monolayer expressing sodium-potassium adenosine triphosphatase (Na(+)/K(+)-ATPase) and zonula occludens-1 (ZO-1) in corneas grafted with 2 × 10(5) CEC and centrally scraped controls, a nonuniform endothelial multilayer without expression of functional proteins in centrally scraped corneas grafted with 1 × 10(6) CEC, and a nonfunctional fibrotic endothelium in entirely scraped grafts and controls. Expression of DiOC in grafts was scarce.

CONCLUSIONS

Injected CEC contributed little to the incompletely functional endothelium of grafted corneas. Y-27632 injection without CEC following scraping reconstituted the healthiest endothelium. Further studies investigating the therapeutic effect of Y-27632 alone are needed to validate these conclusions.

Effect of bone marrow and adipose tissue-derived mesenchymal stem cells on the natural course of corneal scarring after penetrating injury

In the present study, we investigate and compare the efficacy of bone marrow- and adipose tissue-derived mesenchymal stem cell (MSCs) in corneal wound healing. A penetrating injury was created in the right corneas of Wistar rats (n = 40). Ten microliters of phosphate-buffered solution (PBS) containing 2 × 10(5) green fluorescent protein (GFP) labeled bone-marrow-derived MSCs to group 1 (n = 15), 10 μl of PBS containing 2 × 10(5) GFP-labeled adipose-tissue-derived MSCs to group 2 (n = 15), 10 μl PBS was injected into anterior chamber in group 3 (n = 10, control). Corneal opacity scoring, in vivo confocal microscopy, and histopathological evaluation were done at the end of 8 weeks. Immunofluorescence sections were evaluated to detect transplanted cells. Immune staining was performed to measure the expression levels of keratocan, aldehyde dehydrogenase (ALDH) and CD34. The gene expression levels of tumor necrosis factor (TNF-α), the interleukin 6 receptor (IL-6R), interleukin 12b (IL-12b), and transforming growth factor beta (TGF-β1) was measured on corneas. The establishment of stem cells in the corneas of the transplanted groups was confirmed by immunofluorescence staining. The expression of keratocan, ALDH, and CD34 increased in the transplanted groups (p < 0.05). The density of keratocytes increased significantly in both transplanted groups according to the in vivo confocal microscopy data (p < 0.05). The expression of TNF-α, IL-6R, and IL-12b decreased significantly in the transplanted groups (p < 0.05). Based on our findings, we consider that allogeneic stem cells facilitate the regeneration of corneal stroma and can be a cell source for stromal repopulation in diseased cornea.

Substrates for Expansion of Corneal Endothelial Cells towards Bioengineering of Human Corneal Endothelium

Corneal endothelium is a single layer of specialized cells that lines the posterior surface of cornea and maintains corneal hydration and corneal transparency essential for vision. Currently, transplantation is the only therapeutic option for diseases affecting the corneal endothelium. Transplantation of corneal endothelium, called endothelial keratoplasty, is widely used for corneal endothelial diseases. However, corneal transplantation is limited by global donor shortage. Therefore, there is a need to overcome the deficiency of sufficient donor corneal tissue. New approaches are being explored to engineer corneal tissues such that sufficient amount of corneal endothelium becomes available to offset the present shortage of functional cornea. Although human corneal endothelial cells have limited proliferative capacity in vivo, several laboratories have been successful in in vitro expansion of human corneal endothelial cells. Here we provide a comprehensive analysis of different substrates employed for in vitro cultivation of human corneal endothelial cells. Advances and emerging challenges with ex vivo cultured corneal endothelial layer for the ultimate goal of therapeutic replacement of dysfunctional corneal endothelium in humans with functional corneal endothelium are also presented.

Engineering of Human Corneal Endothelial Grafts

Human corneal endothelial cells (HCEC) play a pivotal role in maintaining corneal transparency. Unlike in other species, HCEC are notorious for their limited proliferative capacity in vivo after diseases, injury, aging, and surgery. Persistent HCEC dysfunction leads to sight-threatening bullous keratopathy with either an insufficient cell density or retrocorneal membrane due to endothelial-mesenchymal transition (EMT). Presently, the only solution to restore vision in eyes inflicted with bullous keratopathy or retrocorneal membrane relies upon transplantation of a cadaver human donor cornea containing a healthy corneal endothelium. Due to a severe global shortage of donor corneas, in conjunction with an increasing trend toward endothelial keratoplasty, it is opportune to develop a tissue engineering strategy to produce HCEC grafts. Prior attempts of producing these grafts by unlocking the contact inhibition-mediated mitotic block using trypsin-EDTA and culturing of single HCEC in a bFGF-containing medium run the risk of losing the normal phenotype to EMT by activating canonical Wnt signaling and TGF-β signaling. Herein, we summarize our novel approach in engineering HCEC grafts based on selective activation of p120-Kaiso signaling that is coordinated with activation of Rho-ROCK-canonical BMP signaling to reprogram HCEC into neural crest progenitors. Successful commercialization of this engineering technology will not only fulfill the global unmet need but also encourage the scientific community to re-think how cell-cell junctions can be safely perturbed to uncover novel therapeutic potentials in other model systems.

Stem cell-based therapy of corneal epithelial and endothelial diseases

Corneal dysfunction is the second leading cause of blindness. Approximately 10 million patients worldwide are affected by some form of corneal disease. More than 50,000 cornea transplants are performed every year, but this procedure is limited by cornea donation availability. Recently, new cell replacement procedures have been developed to treat a variety of corneal diseases. This review will focus on the recent advances in the use of limbal epithelial stem cells (LESCs) to treat corneal epithelial cell deficiency and improvements in replacing dysfunctional corneal endothelial cells (CECs) with exogenous CECs. Several protocols have been developed to differentiate pluripotent stem cells into LESC- or CEC-like cells, potentially yielding an unlimited source for the cell replacement therapy of corneal diseases.

Fibrin-mediated delivery of an ovarian follicle pool in a mouse model of infertility

The cryopreservation and autotransplantation of ovarian tissue is emerging as a powerful approach for preserving fertility. However, for cancer patients, it may not be possible to transplant ovarian tissue due to the risk of re-seeding disease. We investigated strategies for transplantation of individually isolated follicles to minimize the risk of re-introducing cancer cells present within the vasculature of ovarian stroma. Procedures for large-scale isolation of early-stage follicles and their encapsulation into fibrin hydrogels were developed. For in vivo validation studies, mice were ovariectomized and transplanted with encapsulated follicles into the ovarian bursa. A substantial increase in the number of secondary follicles was observed in the graft at 9 days after transplantation, and antral follicles by day 21, demonstrating primordial follicle recruitment into the growing pool. Initially, elevated follicle-stimulating hormone levels declined substantially by day 21, indicating feedback from the graft; presence of corpora lutea showed the graft's capability of restoring hormone cyclicity. Taken together, the transplanted follicles were able to engraft, mature, and restore ovarian function in an infertile mouse. This biomaterial may, thus, provide a platform for follicle transplantation with a low risk of cancer contamination and for developing strategies that preserve fertility for women facing a cancer diagnosis.

Tissue engineering of the corneal endothelium: a review of carrier materials

Functional impairment of the human corneal endothelium can lead to corneal blindness. In order to meet the high demand for transplants with an appropriate human corneal endothelial cell density as a prerequisite for corneal function, several tissue engineering techniques have been developed to generate transplantable endothelial cell sheets. These approaches range from the use of natural membranes, biological polymers and biosynthetic material compositions, to completely synthetic materials as matrices for corneal endothelial cell sheet generation. This review gives an overview about currently used materials for the generation of transplantable corneal endothelial cell sheets with a special focus on thermo-responsive polymer coatings.

Successful transplantation of in vitro expanded human cadaver corneal endothelial precursor cells on to a cadaver bovine's eye using a nanocomposite gel sheet

PURPOSE

In vitro expansion of human corneal endothelial precursor (HCEP) cells has been reported via production of cell aggregated spheres. However, to translate this procedure in human patients warrants maintaining the position of the eyeballs facing down for 36 h, which is not feasible. In this study, we report a method using a nanocomposite (NC) gel sheet to accomplish the integration of HCEP cells to the endothelium of cadaver bovine's eyes.

MATERIALS AND METHODS

HCEP cells were isolated from the corneal endothelium of a cadaver human eye and then expanded using a thermoreversible gelation polymer (TGP) as reported earlier. For the study, three cadaver bovine eyes were used. The NC gel sheets were inserted into the bovine eyes', aligned and suture-fixed in position under the host endothelium. HCEP cells previously expanded in the TGP were harvested and injected using a 26-gauge syringe between the endothelium and the NC gel sheet. The eyes were left undisturbed for three hours following which the NC gel sheets were gently removed. The corneas were harvested and subjected to histopathological studies.

RESULTS

Histopathological studies showed that all the three corneas used for NC gel sheet implantation showed the presence of engrafted HCEP cells, seen as multi-layered cells over the native endothelium of the bovine cornea. Examination of the NC gel sheets used for implantation showed that only very few corneal endothelial cells remained on the sheets amounting to what could be considered negligible.

CONCLUSION

The use of the NC gel sheet makes HCEP cell transplantation feasible for human patients. Further in vitro basic studies followed by translational studies are necessary to bring this method for clinical application in appropriate indications.

The graft of autologous adipose-derived stem cells in the corneal stromal after mechanic damage

This study was designed to explore the feasibility of using autologous rabbit adipose derived stem cells (rASCs) as seed cells and polylactic-co-glycolic acid (PLGA) as a scaffold for repairing corneal stromal defects. rASCs isolated from rabbit nape adipose tissue were expanded and seeded on a PLGA scaffold to fabricate cell-scaffold constructs. After 1 week of cultivation in vitro, the cell-scaffold complexes were transplanted into corneal stromal defects in rabbits. In vivo, the autologous rASCs-PLGA constructed corneal stroma gradually became transparent without corneal neovascularization after 12 weeks. Hematoxylin and eosin staining and transmission electron microscopy examination revealed that their histological structure and collagen fibril distribution at 24 weeks after implantation were similar to native counterparts. As to the defect treated with PLGA alone, the stromal defects remained. And scar tissue was observed in the untreated-group. The implanted autologous ASCs survived up to 24 weeks post-transplantation and differentiated into functional keratocytes, as assessed by the expression of aldehyde-3-dehydrogenase1A1 (ALDH1A1) and cornea-specific proteoglycan keratocan. Our results revealed that autologous rASCs could be one of the cell sources for corneal stromal restoration in diseased corneas or for tissue engineering of a corneal equivalent.

Cultivation of human microvascular endothelial cells on topographical substrates to mimic the human corneal endothelium

Human corneal endothelial cells have a limited ability to replicate in vivo and in vitro. Allograft transplantation becomes necessary when an accident or trauma results in excessive cell loss. The reconstruction of the cornea endothelium using autologous cell sources is a promising alternative option for therapeutic or in vitro drug testing applications. The native corneal endothelium rests on the Descemet’s membrane, which has nanotopographies of fibers and pores. The use of synthetic topographies mimics the native environment, and it is hypothesized that this can direct the behavior and growth of human microvascular endothelial cells (HMVECs) to resemble the corneal endothelium. In this study, HMVECs are cultivated on substrates with micron and nano-scaled pillar and well topographies. Closely packed HMVEC monolayers with polygonal cells and well-developed tight junctions were formed on the topographical substrates. Sodium/potassium (Na⁺/K⁺) adenine triphosphatase (ATPase) expression was enhanced on the microwells substrate, which also promotes microvilli formation, while more hexagonal-like cells are found on the micropillars samples. The data obtained suggests that the use of optimized surface patterning, in particular, the microtopographies, can induce HMVECs to adopt a more corneal endothelium-like morphology with similar barrier and pump functions. The mechanism involved in cell contact guidance by the specific topographical features will be of interest for future studies.

Corneal endothelial regeneration and tissue engineering

Human corneal endothelial cells (HCECs) have a limited proliferative capacity. Descemet stripping with automated endothelial keratoplasty (DSAEK) has become the preferred method for the treatment of corneal endothelial deficiency, but it requires a donor cornea. To overcome the shortage of donor corneas, transplantation of cultured HCEC sheets has been attempted in experimental studies. This review summarizes current knowledge about the mechanisms of corneal endothelial wound healing and about tissue engineering for the corneal endothelium. We also discuss recent work on tissue engineering for DSAEK grafts using cultured HCECs and HCEC precursor cell isolation method (the sphere-forming assay). DSAEK grafts (HCEC sheets) were constructed by seeding cultured HCECs on human amniotic membrane, thin human corneal stroma, and collagen sheets. The pump function of the HCEC sheets thus obtained was approximately 75%-95% of that for human donor corneas. HCEC sheets were transplanted onto rabbit corneas after DSAEK. While the untransplanted control group displayed severe stromal edema, the transplanted group had clear corneas throughout the observation period. The sphere-forming assay using donor human corneal endothelium or cultured HCECs can achieved mass production of human corneal endothelial precursors. These findings indicate that cultured HCECs transplanted after DSAEK can perform effective corneal dehydration in vivo and suggest the feasibility of employing the transplantation of cultured HCECs to treat endothelial dysfunction. Additionally, corneal endothelial precursors may be an effective strategy for corneal endothelial regeneration.

Tissue engineering of corneal endothelium

Human corneal endothelial cells (HCECs) do not replicate after wounding. Therefore, corneal endothelial deficiency can result in irreversible corneal edema. Descemet stripping automated endothelial keratoplasty (DSAEK) allows selective replacement of the diseased corneal endothelium. However, DSAEK requires a donor cornea and the worldwide shortage of corneas limits its application. This review presents current knowledge on the tissue engineering of corneal endothelium using cultured HCECs. We also provide our recent work on tissue engineering for DSAEK grafts using cultured HCECs. We reconstructed DSAEK grafts by seeding cultured DiI-labelled HCECs on collagen sheets. Then HCEC sheets were transplanted onto the posterior stroma after descemetorhexis in the DSAEK group. Severe stromal edema was detected in the control group, but not in the DSAEK group throughout the observation period. Fluorescein microscopy one month after surgery showed numerous DiI-labelled cells on the posterior corneal surface in the DSAEK group. Frozen sections showed a monolayer of DiI-labelled cells on Descemet’s membrane. These findings indicate that cultured adult HCECs, transplanted with DSAEK surgery, maintain corneal transparency after transplantation and suggest the feasibility of performing DSAEK with HCECs to treat endothelial dysfunction.

Long-term cultivation of human corneal endothelial cells by telomerase expression

The objective of this study was to explore the potential role of human telomerase reverse transcriptase (TERT) in extending the proliferative lifespan of human corneal endothelial cells (HCECs) under long-term cultivation. A primary culture was initiated with a pure population of HCECs in DMEM/F12 media containing 10% fetal bovine serum and other various supplements. TERT gene was successfully transfected into normal HCECs. A stable HCECs cell line (TERT-HCECs) that expressed TERT was established. The cells could be subcultured for 36 passages. Within this line of cells, TERT not only extended proliferative lifespan and inhibited apoptosis but also enhanced the cell line remaining the normal characteristics similar to HCECs. There were no significantly differences in the expression of the pump function related proteins voltage dependent anion channel 3 (VDAC3), sodium bicarbonate cotransporter member 4 (SLC4A4), chloride channel protein 3 (CLCN3), Na(+)/K(+)-ATPase α1, and ZO-1 in the cell line TERT-HCECs and primary HCECs. TERT-HCECs formed a monolayer cell sheet, maintained similar cell junction formation and pump function with primary HCECs. Karyotype analysis exhibited normal chromosomal numbers. The soft agar colony assay and tumor formation in nude mice assay showed no malignant alterations in TERT-HCECs. Our findings indicated that we had established a cell line with its similar phenotype and properties to primary HCECs. Further study of the TERT-HCECs may be valuable in studying the function of the cells in vivo.

Self-assembled matrix by umbilical cord stem cells

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

Human corneal endothelial cell expansion for corneal endothelium transplantation: an overview

The monolayer of cells forming the human corneal endothelium is critical to the maintenance of corneal transparency and is not known to regenerate in vivo. Thus, dysfunction of these cells constitutes the most often cited reasons for the 150,000 or so corneal transplants performed yearly. Although current corneal transplantation is more than 90% successful at 1 year, longer term results are not as encouraging with approximately 70% success at 5 years. Nonimmunologic graft failure and allograft endothelial rejection are the main problems. Furthermore, the global shortage of donor corneas greatly restricts several corneal transplantations performed. With advances in understanding corneal endothelial cell biology, it is now possible to cultivate human corneal endothelial cells (HCECs) in vitro, thus providing new opportunities to develop novel tissue-engineered human corneal endothelium. This review will provide an overview of (a) the characteristics of human corneal endothelium; (b) past and present HCECs isolation and culture protocols; (c) various potential carriers for the generation of tissue-engineered corneal endothelium, together with some of the functional studies reported in various animal models; and (d) the current rapid advancements in surgical techniques for keratoplasty. A successful combination of tissue-engineered human corneal endothelium coupled with innovative and groundbreaking surgical procedures will bridge basic research involving cultured HCECs, bringing it from bench to bedside.

Enhanced survival in vitro of human corneal endothelial cells using mouse embryonic stem cell conditioned medium

Purpose

To determine whether mouse embryonic stem cell conditioned medium (ESC-CM) increases the proliferative capacity of human corneal endothelial cells (HCECs) in vitro.

Methods

Primary cultures of HCECs were established from explants of the endothelial cell layer, including the Descemet’s membrane. Cells were cultured in human corneal endothelium medium (CEM) containing 25% ESC-CM for the experimental group and CEM alone for the control group. Phase-contrast microscopy and reverse-transcription polymerase chain reaction (RT–PCR) were used to identify HCECs. The eruption time and HCEC morphology were observed under phase-contrast microscopy. We detected the protein expression of zona occludens protein-1 (ZO-1; a tight junction protein) and the Na⁺-K⁺-ATPase by western blot analysis and immunocytochemistry. The mRNA expression of the Na⁺-K⁺-ATPase, voltage-dependent anion channel 3 (VDAC3), solute carrier family 4, sodium bicarbonate cotransporter member 4 (SLC4A4), and chloride channel protein 3 (CLCN3) were detected by RT–PCR. To explore the proliferation capacity of HCECs, the colony forming efficiency (CFE) was determined by Giemsa staining and the cellular proliferation marker of Ki-67 protein (Ki-67) positive cells were detected by immunocytochemistry and flow cytometry. Progression of the cell cycle and apoptosis were analyzed by flow cytometry. Negative regulation of the cell cycle, as measured by cyclin-dependent kinase inhibitor p21 (p21) levels, was detected by western blot analysis and immunocytochemistry.

Results

In primary culture, HCECs in the 25%ESC-CM group erupted with polygonal appearance on day 2, while those in the CEM group erupted with slightly larger cells on day 3–4. HCECs in the 25%ESC-CM group could be subcultured until passage 6 without enlargement of cell volume, while those in the CEM group were enlarged and lost their polygonal appearance by passage 2. HCECs in both the 25%ESC-CM and CEM groups expressed ZO-1, Na⁺-K⁺-ATPase, VDAC3, SLC4A4, and CLCN3. The number of Ki67 positive cells, CFE, and percentage of cells entering the S and G₂ phases were higher in the 25%ESC-CM group than in the CEM group. The number of apoptotic cells and p21 protein expression both decreased in the 25%ESC-CM group.

Conclusions

Use of 25%ESC-CM significantly increased the number of proliferating cells. These effects may be achieved through inhibition of p21 expression and apoptosis. These results suggested that 25%ESC-CM may be a new tool for cultivating HCECs for transplantation.

Using Basement Membrane of Human Amniotic Membrane as a Cell Carrier for Cultivated Cat Corneal Endothelial Cell Transplantation

PURPOSE

To study the feasibility of using basement membrane of human amniotic membrane (BMHAM) as a carrier for transplantation of cultivated cat corneal endothelial cells (cCCECs).

METHODS

BHMAM was obtained by enzymic digestion. cCCECs were seeded on the BMHAM and cultivated traditionally. The resulting continuous monolayer of cCCECs was transplanted onto the cat corneal graft stripped of the Descemet membrane with endothelium. To determine whether the transplanted cCCECs were vital and functional in vivo, the corneal grafts were examined by slit-lamp microscope every day for 6 weeks, and corneal thickness was measured by ultrasonic pachymetry. Either in vivo or in vitro, the cCCEC sheets on BMHAMs were examined morphologically by light and electron microscope, and the cell density was measured.

RESULTS

Seven to 10 days after seeding on the BMHAM, the cCCECs were confluent and formed a continuous monolayer with 3486 +/- 53 cells/mm(2) cell density. Like normal corneal endothelial cells, the cCCECs were almost hexagonal, squamous, and uniform in size. After transplantation, most cells were vital and functional nearly enough to maintain corneal graft thickness and transparency without rejection for at least 6 weeks. Six weeks after operation, the average thickness of the transplanted corneal grafts was only slightly greater than that before operation. Compared with that in vitro, after transplantation there was 5% to 8% reduction per week in cell density, which lasted for almost 3 weeks. After that, the average cCCEC density of corneal grafts was 2837 +/- 57 cells/mm(2) and quite stable maintained.

CONCLUSIONS

This study demonstrated that BMHAM would be an ideal alternative for corneal Descemet membrane and a cell carrier for cCCEC transplantation.

Two clonal cell lines of immortalized human corneal endothelial cells show either differentiated or precursor cell characteristics

Access to primary human corneal endothelial cells (HCEC) is limited and donor-derived differences between cultures exacerbate the issue of data reproducibility, whereas cell lines can provide sufficient numbers of homogenous cells for multiple experiments. An immortalized HCEC population was adapted to serum-free culture medium and repeated cloning was performed. Clonally grown cells were propagated under serum-free conditions and growth curves were recorded. Cells were characterized immunocytochemically for junctional proteins, collagens, Na,K-ATPase and HCEC-specific 9.3.E-antigen. Ultrastructure was monitored by scanning and transmission electron microscopy. Two clonal cell lines, HCEC-B4G12 and HCEC-H9C1, could be isolated and expanded, which differed morphologically: B4G12 cells were polygonal, strongly adherent and formed a strict monolayer, H9C1 cells were less adherent and formed floating spheres. The generation time of B4G12 cells was 62.26 +/- 14.5 h and that of H9C1 cells 44.05 +/- 5.05 h. Scanning electron microscopy revealed that B4G12 cells had a smooth cell surface, while H9C1 cells had numerous thin filopodia. Both cell lines expressed ZO-1 and occludin adequately, and little but well detectable amounts of connexin-43. Expression of HCEC-specific 9.3.E-antigen was found commensurately in both cell lines, while expression of Na,K-ATPase alpha1 was higher in H9C1 cells than in B4G12 cells. B4G12 cells expressed collagen IV abundantly and almost no collagen III, while H9C1 cells expressed both collagens at reasonable amounts. It is concluded that the clonal cell line B4G12 represents an ideal model of differentiated HCEC, while H9C1 may reflect features of developing or transitional HCEC.

Adipose-derived stem cells are a source for cell therapy of the corneal stroma

Most corneal diseases affect corneal stroma and include immune or infectious diseases, ecstatic disorders, traumatic scars, and corneal dystrophies. Cell-based therapy is a promising therapeutic approach to overcome the current disadvantages of corneal transplantation. We intended to search for a cell source to repopulate and regenerate corneal stroma. We investigated the ability of human processed lipoaspirate derived (PLA) cells to regenerate corneal stroma in experimental animals. In the first set of experiments, we tested the biosafety and immunogenicity of human PLA stem cells transplanted into the corneal stroma of rabbits. No immune response was elicited even though we used immune-competent animals. PLA cells survived up to 10 weeks post-transplant, maintained their shape, and remained intermingled in the stroma without disrupting its histological pattern. Interestingly, transparency was preserved even 10 weeks after the transplant, when PLA cells formed a discontinuous layer in the stroma. In the second set of experiments, regeneration of the corneal stroma by PLA cells was assessed, creating a niche by partial ablation of the stroma. After 12 weeks, human cells were disposed following a multilayered pattern and differentiated into functional keratocytes, as assessed by the expression of aldehyde-3-dehydrogenase and cornea-specific proteoglycan keratocan. Based on our results, we believe that adipose-derived adult stem cells can be a cell source for stromal regeneration and repopulation in diseased corneas. The low health impact of the surgical procedure performed to obtain the PLA cells provides this cell source with an additional beneficial feature for its possible future autologous use in human patients.

Establishment of a novel corneal endothelial cell line from domestic rabbit, Oryctolagus curiculus

To develop a rabbit corneal endothelial (RCE) cell line, in vitro culture of RCE cells was initiated from Oryctolagus curiculus corneas and a novel RCE cell line was established in this study. To initiate the primary culture of RCE cells, corneas from rabbit eyes were sliced and attached into glutin-coated wells with endothelial cell surface down. After being cultured at a time-gradient interval from 48 to 6 h, the corneal slices were detached and reattached into new wells, respectively. Cells in the wells containing only a pure population of RCE cells were collected and cultured in 20% FBS-DMEM/F12 medium containing chondroitin sulfate, ocular extract, epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), carboxymethyl-chitosan, N-acetylglucosamine hydrochloride, glucosamine hydrochloride, culture medium of rabbit corneal stromal cells and oxidation-degradation products of chondroitin sulfate at 37 degrees C, 5% CO(2). The cultured RCE cells, in quadrangle and polygonal shapes, proliferated to confluence 3 weeks later. During the subsequent subculture, the shape of RCE cells changed gradually from polygonal to more fibroblastic. A novel RCE cell line, growing at a steady rate, with a population doubling time of 53.8 h, has been established and subcultured to passage 67. Chromosome analysis showed that the RCE cells exhibited chromosomal aneuploidy with the modal chromosome number of 44. The results of immuno-cytochemical staining with neuron specific enolase (NSE) confirmed that the RCE cells were in neuroectodermal origin. Combined with the results of vascular endothelial growth factor (VEGF) treatment and endothelial cell morphology recovery, it can be concluded that the cell line established here is an RCE cell line. This RCE cell line may serve as a useful tool in theoretical researches of mammalian corneal endothelial cells, and may also have potential application in artificial corneal endothelium development.

Thermo-responsive poly(NiPAAm-co-DEGMA) substrates for gentle harvest of human corneal endothelial cell sheets

Gentle harvesting of corneal endothelial cell sheets grown in culture is of interest for the development of cornea replacement strategies. Thin films of a fast responding copolymer of N-isopropylacrylamide (NiPAAm) and diethyleneglycol methacrylate (DEGMA) with a phase transition temperature of 32 degrees C were prepared and evaluated for that purpose. The polymer layers were immobilized onto fluorocarbon substrates using low pressure argon plasma treatment. Cell culture and detachment experiments were performed with L929 mouse fibroblasts and human corneal endothelial cells (HCEC) at standard conditions. The hydrogel-coated supports were found to permit adhesion, spreading, and proliferation of both cell types. Harvesting of cell sheets was achieved upon lowering the temperature to about 30 degrees C. The formation of a closed monolayer as a crucial prerequisite for maintaining ionic pump function in HCEC was proven by ZO-1 immunostainung. Labeling of fibronectin indicated that the vast majority of the extracellular matrix is detached from the hydrogel coatings together with the cell layer. Inspired by this result, the reuse of the hydrogel-coated culture carriers was investigated confirming the suitability of the substrates for repeated cell harvesting. Altogether, the introduced thermoresponsive coating was found advantageous for the efficient generation of HCEC sheets and will be further utilized in transplantation strategies.

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

Isolation of Human Corneal Endothelial Cell Precursors and Construction of Cell Sheets by Precursors

PURPOSE

To review recently published experimental data on human corneal endothelium (CE) precursors.

METHODS

A sphere-forming assay was used for the isolation of human CE precursors from human donor corneas and cultured human CE. CE morphology derived from precursors was compared with that from cultured human CE.

RESULTS

Human CE from donor corneas formed primary and secondary sphere colonies and expressed neural and mesenchymal proteins. The progeny of these colonies had a human-CE-like hexagonal shape and showed transport activity, suggesting that the isolated spheres were indeed precursors of human CE. Similar precursors were isolated from cultured human CE, which provided further evidence that the sphere-forming assay facilitates the mass production of human CE precursors. Cultured human CE obtained in this manner had a regular hexagonal morphology in contrast to passaged, cultured human CE.

CONCLUSION

The sphere-forming assay may become a powerful tool for the regeneration of human CE by precursor injection and the construction of cultured CE sheets.