Differentiation status of limbal epithelial cells cultured on intact and denuded amniotic membrane before and after air-lifting

Single-cell atlas of keratoconus corneas revealed aberrant transcriptional signatures and implicated mechanical stretch as a trigger for keratoconus pathogenesis

Keratoconus is a common ectatic corneal disorder in adolescents and young adults that can lead to progressive visual impairment or even legal blindness. Despite the high prevalence, its etiology is not fully understood. In this study, we performed single-cell RNA sequencing (scRNA-Seq) analysis on 39,214 cells from central corneas of patients with keratoconus and healthy individuals, to define the involvement of each cell type during disease progression. We confirmed the central role of corneal stromal cells in this disease, where dysregulation of collagen and extracellular matrix (ECM) occurred. Differential gene expression and histological analyses revealed two potential novel markers for keratoconus stromal cells, namely CTSD and CTSK. Intriguingly, we detected elevated levels of YAP1 and TEAD1, the master regulators of biomechanical homeostasis, in keratoconus stromal cells. Cyclical mechanical experiments implicated the mechanical stretch in prompting protease production in corneal stromal cells during keratoconus progression. In the epithelial cells of keratoconus corneas, we observed reduced basal cells and abnormally differentiated superficial cells, unraveling the corneal epithelial lesions that were usually neglected in clinical diagnosis. In addition, several elevated cytokines in immune cells of keratoconus samples supported the involvement of inflammatory response in the progression of keratoconus. Finally, we revealed the dysregulated cell-cell communications in keratoconus, and found that only few ligand-receptor interactions were gained but a large fraction of interactional pairs was erased in keratoconus, especially for those related to protease inhibition and anti-inflammatory process. Taken together, this study facilitates the understanding of molecular mechanisms underlying keratoconus pathogenesis, providing insights into keratoconus diagnosis and potential interventions.

Assessment of corneal substrate biomechanics and its effect on epithelial stem cell maintenance and differentiation

Whilst demonstrated extensively in vitro, the control of cell behaviour via modulation of substrate compliance in live tissues has not been accomplished to date. Here we propose that stem cells can be regulated solely through in situ modulation of tissue biomechanics. By first establishing, via high-resolution Brillouin spectro-microscopy, that the outer edge (limbus) of live human corneas has a substantially lower bulk modulus compared to their centre, we then demonstrate that this difference is associated with limbal epithelial stem cell (LESC) residence and YAP-dependent mechanotransduction. This phenotype-through-biomechanics correlation is further explored in vivo using a rabbit alkali burn model. Specifically, we show that treating the burnt surface of the cornea with collagenase effectively restores the tissue's mechanical properties and its capacity to support LESCs through mechanisms involving YAP suppression. Overall, these findings have extended implications for understanding stem cell niche biomechanics and its impact on tissue regeneration.

In Vitro Cultivation of Limbal Epithelial Stem Cells on Surface-Modified Crosslinked Collagen Scaffolds

Purpose

To investigate the efficacy of recombinant human collagen type I (RHC I) and collagen-like peptide (CLP) hydrogels as alternative carrier substrates for the cultivation of limbal epithelial stem cells (LESC) under xeno-free culture conditions.

Methods

Human LESC were cultivated on seven different collagen-derived hydrogels: (1) unmodified RHC I, (2) fibronectin-patterned RHC I, (3) carbodiimide-crosslinked CLP (CLP-12 EDC), (4) DMTMM- (4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-morpholinium-) crosslinked CLP (CLP-12), (5) fibronectin-patterned CLP-12, (6) “3D limbal niche-mimicking” CLP-12, and (7) DMTMM-crosslinked CLP made from higher CLP concentration solution. Cell proliferation, cell morphology, and expression of LESC markers were analyzed. All data were compared to cultures on human amniotic membrane (HAM).

Results

Human LESC were successfully cultivated on six out of seven hydrogel formulations, with primary cell cultures on CLP-12 EDC being deemed unsuccessful since the area of outgrowth did not meet quality standards (i.e., inconsistence in outgrowth and confluence) after 14 days of culture. Upon confluence, primary LESC showed high expression of the stem cell marker ΔNp63, proliferation marker cytokeratin (KRT) 14, adhesion markers integrin-β4 and E-cadherin, and LESC-specific extracellular matrix proteins laminin-α1, and collagen type IV. Cells showed low expression of differentiation markers KRT3 and desmoglein 3 (DSG3). Significantly higher gene expression of KRT3 was observed for cells cultured on CLP hydrogels compared to RHC I and HAM. Surface patterning of hydrogels influenced the pattern of proliferation but had no significant effect on the phenotype or genotype of cultures. Overall, the performance of RHC I and DMTMM-crosslinked CLP hydrogels was equivalent to that of HAM.

Conclusion

RHC I and DMTMM-crosslinked CLP hydrogels, irrespective of surface modification, support successful cultivation of primary human LESC using a xeno-free cultivation protocol. The regenerated epithelium maintained similar characteristics to HAM-based cultures.

A completely human-derived biomaterial mimicking limbal niche: Platelet-rich fibrin gel

Platelet-rich fibrin (PRF) is a natural biomaterial and has excellent biochemical and physical properties with a history of proven biocompatibility in the field of tissue engineering and regenerative medicine. Recent reports of fibrin-based matrices have offered new opportunities to apply PRF as a supplement for in vitro cell culture. Here, custom-modified human-derived PRF (HPRF) was produced via different centrifugation protocols, then, characterized by morphologically and chemically and utilized as a substrate and as a conditioned medium for limbal explant culture for the first time. It was found that the HPRF released significantly higher levels of growth factors which are essential for epithelial cell growth. The enhanced physicochemical properties of the HPRF were also proven in the limbal explant cultures in terms of cell growth, migration, viability, and stemness in comparison with the conventional limbal explant culture on human-derived amniotic membrane. Consequently, HPRF hydrogels are appealing natural biomaterials for the purpose of mimicking limbal niche and the discovery elucidates this new, xeno-chemical-free, completely human-derived biomaterial can be utilized as a supplement to promote epithelial cell behaviour in vitro.

Native and synthetic scaffolds for limbal epithelial stem cell transplantation

Limbal stem cell deficiency (LSCD) is a complex blinding disease of the cornea, which cannot be treated with conventional corneal transplants. Instead, a stem cell (SC) graft is required to replenish the limbal epithelial stem cell (LESC) reservoir, which is ultimately responsible for regenerating the corneal epithelium. Current therapies utilize limbal tissue biopsies that harbor LESCs as well as tissue culture expanded cells. Typically, this tissue is placed on a scaffold that supports the formation of corneal epithelial cell sheets, which are then transferred to diseased eyes. A wide range of biological and synthetic materials have been identified as carrier substrates for LESC, some of which have been used in the clinic, including amniotic membrane, fibrin, and silicon hydrogel contact lenses, each with their own advantages and limitations. This review will provide a brief background of LSCD, focusing on bio-scaffolds that have been utilized in limbal stem cell transplantation (LSCT) and materials that are being developed as potentially novel therapeutics for patients with this disease.

STATEMENT OF SIGNIFICANCE

The outcome of patients with corneal blindness that receive stem cell grafts to restore eye health and correct vision varies considerably and may be due to the different biological and synthetic scaffolds used to deliver these cells to the ocular surface. This review will highlight the positive attributes and limitations of the myriad of carriers developed for clinical use as well as those that are being trialled in pre-clinical models. The overall focus is on developing a standardized therapy for patients, however due to the multiple causes of corneal blindness, a personal regenerative medicine approach may be the best option.

The application of decellularized human term fetal membranes in tissue engineering and regenerative medicine (TERM)

Tissue engineering and regenerative medicine (TERM) is a field that applies biology and engineering principles to "restore, maintain or repair a tissue after injury". Besides the potential to treat various diseases, these endeavours increase our understanding of fundamental cell biology. Although TERM has progressed rapidly, engineering a whole organ is still beyond our skills, primarily due to the complexity of tissues. Material science and current manufacturing methods are not capable of mimicking this complexity. Therefore, many researchers explore the use of naturally derived materials that maintain important biochemical, structural and mechanical properties of tissues. Consequently, employing non-cellular components of tissues, particularly the extracellular matrix, has emerged as an alternative to synthetic materials. Because of their complexity, decellularized tissues are not as well defined as synthetic materials but they provide cells with a microenvironment that resembles their natural niche. Decellularized tissues are produced from a variety of sources, among which the fetal membranes are excellent candidates since their supply is virtually unlimited, they are readily accessible with minimum ethical concerns and are often discarded as a biological waste. In this review, we will discuss various applications of decellularized fetal membranes as substrates for the expansion of stem cells, their use as two and three-dimensional scaffolds for tissue regeneration, and their use as cell delivery systems. We conclude that fetal membranes have great potential for use in TERM.

Effect of Amniotic Membrane Suspension (AMS) and Amniotic Membrane Homogenate (AMH) on Human Corneal Epithelial Cell Viability, Migration and Proliferation In Vitro

PURPOSE

To analyze the effects of different concentrations of amniotic membrane suspension (AMS) or amniotic membrane homogenate (AMH) on human corneal epithelial cell (HCEC) viability, migration and proliferation.

METHODS

Amniotic membranes (AMs) of 13 placentas were prepared and stored at -80°C. For AMS preparation, following de-freezing, AM pieces were inserted in six-well plates and 5 ml Dulbecco's Modified Eagle's Medium (DMEM)/F12 (with 5% fetal bovine serum, FBS) per gram tissue was added for 96 h. After removal of the AM, the remaining supernatant was collected for experiments. For AMH preparation, following de-freezing, AMs were homogenized in liquid nitrogen and 5 ml DMEM/F12 (with 5% FBS) per gram tissue was added. Following centrifugation, the supernatant was collected for experiments. HCECs were expanded and incubated in DMEM/F12, 5% FBS supplemented by 15%, 30% or 100% AMS or 15% or 30% AMH. Viability was analyzed using Cell Proliferation Kit XTT, migration using wound healing assay and proliferation by the cell proliferation ELISA BrdU kit.

RESULTS

HCEC viability remained unchanged using 15% or 30% AMS (p = 1.0 for both); however, it decreased significantly using 100% AMS (p < 0.001) or 15% (p = 0.041) or 30% AMH (p < 0.001), compared to controls. Using 15% or 30% AMS, HCEC migration increased significantly (p < 0.001 for both). Using 15% or 30% AMH (p = 0.153; p = 0.083), HCEC migration remained unchanged and 100% AMS inhibited HCEC migration (p < 0.001). Next, 15% and 30% AMS had no effect on HCEC proliferation (p = 0.454 and p = 0.119), but 100% AMS (p < 0.001) and 15% (p = 0.002) and 30% AMH (p = 0.001) inhibited HCEC proliferation significantly.

CONCLUSION

With unchanged HCEC viability and proliferation and increased HCEC migration, 15% and 30% AMS application seems to be the most appropriate method to support epithelial healing.

Concise review: transplantation of cultured oral mucosal epithelial cells for treating limbal stem cell deficiency-current status and future perspectives

A number of diseases and external factors can deplete limbal stem cells, causing pain and visual loss. Ten years have passed since the first transplantation of cultured oral mucosal epithelial cells in humans, representing the first autologous cell-based therapy for severe bilateral limbal stem cell deficiency. Its steady increase in popularity since then can be attributed to the accumulating evidence of its efficacy in reverting limbal stem cell deficiency. In this review, the focus is on clinical, and to a lesser degree laboratory, features of cultured oral mucosal epithelial transplants over the past 10 years. Comparisons with other available technologies are made. Avenues for research to stimulate further improvements in clinical results and allow worldwide distribution of limbal stem cell therapy based on oral mucosal cells are discussed. These include storage and transportation of cultured oral mucosal epithelial sheets and in vivo culture of oral mucosal epithelial cells.

Synthetic vs natural scaffolds for human limbal stem cells

Aim

To investigate the impact of synthetic electrospun polyurethane (PU) and polycaprolactone (PCL) nanoscaffolds, before and after hydrolytic surface modification, on viability and differentiation of cultured human eye epithelial cells, in comparison with natural scaffolds: fibrin and human amniotic membrane.

Methods

Human placenta was taken at elective cesarean delivery. Fibrin scaffolds were prepared from commercial fibrin glue kits. Nanoscaffolds were fabricated by electrospinning. Limbal cells were isolated from surpluses of human cadaveric cornea and seeded on feeder 3T3 cells. The scaffolds used for viability testing and immunofluorescence analysis were amniotic membrane, fibrin, PU, and PCL nanoscaffolds, with or without prior NaOH treatment.

Results

Scanning electron microscope photographs of all tested scaffolds showed good colony spreading of seeded limbal cells. There was a significant difference in viability performance between cells with highest viability cultured on tissue culture plastic and cells cultured on all other scaffolds. On the other hand, electrospun PU, PCL, and electrospun PCL treated with NaOH had more than 80% of limbal cells positive for stem cell marker p63 compared to only 27%of p63 positive cells on fibrin.

Conclusion

Natural scaffolds, fibrin and amniotic membrane, showed better cell viability than electrospun scaffolds. On the contrary, high percentages of p63 positive cells obtained on these scaffolds still makes them good candidates for efficient delivery systems for therapeutic purposes.

Reconstruction of auto-tissue-engineered lamellar cornea by dynamic culture for transplantation: a rabbit model

To construct an auto-tissue-engineered lamellar cornea (ATELC) for transplantation, based on acellular porcine corneal stroma and autologous corneal limbal explants, a dynamic culture process, which composed of a submersion culture, a perfusion culture and a dynamic air-liquid interface culture, was performed using appropriate parameters. The results showed that the ATELC-Dynamic possessed histological structure and DNA content that were similar to native lamellar cornea (NLC, p>0.05). Compared to NLC, the protein contents of zonula occludens-1, desmocollin-2 and integrin β4 in ATELC-Dynamic reached 93%, 89% and 73%, respectively. The basal cells of ATELC-Dynamic showed a better differentiation phenotype (K3⁻, P63⁺, ABCG2⁺) compared with that of ATELC in static air-lift culture (ATELC-Static, K3⁺, P63⁻, ABCG2⁻). Accordingly, the cell-cloning efficiency of ATELC-Dynamic (9.72±3.5%) was significantly higher than that of ATELC-Static (2.13±1.46%, p<0.05). The levels of trans-epithelial electrical resistance, light transmittance and areal modulus variation in ATELC-Dynamic all reached those of NLC (p>0.05). Rabbit lamellar keratoplasty showed that the barrier function of ATELC-Dynamic was intact, and there were no signs of epithelial shedding or neovascularization. Furthermore, the ATELC-Dynamic group had similar optical properties and wound healing processes compared with the NLC group. Thus, the sequential dynamic culture process that was designed according to corneal physiological characteristics could successfully reconstruct an auto-lamellar cornea with favorable morphological characteristics and satisfactory physiological function.

A simple alkaline method for decellularizing human amniotic membrane for cell culture

Human amniotic membrane is a standard substratum used to culture limbal epithelial stem cells for transplantation to patients with limbal stem cell deficiency. Various methods were developed to decellularize amniotic membrane, because denuded membrane is poorly immunogenic and better supports repopulation by dissociated limbal epithelial cells. Amniotic membrane denuding usually involves treatment with EDTA and/or proteolytic enzymes; in many cases additional mechanical scraping is required. Although ensuring limbal cell proliferation, these methods are not standardized, require relatively long treatment times and can result in membrane damage. We propose to use 0.5 M NaOH to reliably remove amniotic cells from the membrane. This method was used before to lyse cells for DNA isolation and radioactivity counting. Gently rubbing a cotton swab soaked in NaOH over the epithelial side of amniotic membrane leads to nearly complete and easy removal of adherent cells in less than a minute. The denuded membrane is subsequently washed in a neutral buffer. Cell removal was more thorough and uniform than with EDTA, or EDTA plus mechanical scraping with an electric toothbrush, or n-heptanol plus EDTA treatment. NaOH-denuded amniotic membrane did not show any perforations compared with mechanical or thermolysin denuding, and showed excellent preservation of immunoreactivity for major basement membrane components including laminin α2, γ1-γ3 chains, α1/α2 and α6 type IV collagen chains, fibronectin, nidogen-2, and perlecan. Sodium hydroxide treatment was efficient with fresh or cryopreserved (10% dimethyl sulfoxide or 50% glycerol) amniotic membrane. The latter method is a common way of membrane storage for subsequent grafting in the European Union. NaOH-denuded amniotic membrane supported growth of human limbal epithelial cells, immortalized corneal epithelial cells, and induced pluripotent stem cells. This simple, fast and reliable method can be used to standardize decellularized amniotic membrane preparations for expansion of limbal stem cells in vitro before transplantation to patients.

Oxidized alginate hydrogels as niche environments for corneal epithelial cells

Chemical and biochemical modification of hydrogels is one strategy to create physiological constructs that maintain cell function. The aim of this study was to apply oxidised alginate hydrogels as a basis for development of a biomimetic niche for limbal epithelial stem cells that may be applied to treating corneal dysfunction. The stem phenotype of bovine limbal epithelial cells (LEC) and the viability of corneal epithelial cells (CEC) were examined in oxidised alginate gels containing collagen IV over a 3-day culture period. Oxidation increased cell viability (P ≤ 0.05) and this improved further with addition of collagen IV (P ≤ 0.01). Oxidised gels presented larger internal pores (diameter: 0.2-0.8 µm) than unmodified gels (pore diameter: 0.05-0.1 µm) and were significantly less stiff (P ≤ 0.001), indicating that an increase in pore size and a decrease in stiffness contributed to improved cell viability. The diffusion of collagen IV from oxidised alginate gels was similar to that of unmodified gels suggesting that oxidation may not affect the retention of extracellular matrix proteins in alginate gels. These data demonstrate that oxidised alginate gels containing corneal extracellular matrix proteins can influence corneal epithelial cell function in a manner that may impact beneficially on corneal wound healing therapy.

Human corneal epithelial subpopulations: oxygen dependent ex vivo expansion and transcriptional profiling

Corneal epithelium is being regenerated throughout life by limbal epithelial stem cells (LESCs) believed to be located in histologically defined stem cell niches in corneal limbus. Defective or dysfunctional LESCs result in limbal stem cell deficiency (LSCD) causing pain and decreased visual acuity. Since the first successful treatment of LSCD by transplantation of ex vivo expanded LESCs in 1997, many attempts have been carried out to optimize culture conditions to improve the outcome of surgery. To date, progress in this field of bioengineering is substantially hindered by both the lack of specific biomarkers of LESCs and the lack of a precise molecular characterization of in situ epithelial subpopulations. The aim of this dissertation was to optimize culture systems with regard to the environmental oxygen concentration for selective ex vivo expansion of LESCs and to analyse in situ subpopulations in human corneal epithelium using a combination of laser capture microdissection and RNA sequencing for global transcriptomic profiling. We compared dissociation cultures, using either expansion on γ-irradiated NIH/3T3 feeder cells in serum-rich medium or expansion directly on plastic in serum-free EpiLife medium, using a range of physiologically relevant oxygen concentrations (2%, 5%, 10%, 15% and 20%). Using immunocytochemistry and advanced fluorescence microscopy, cells were characterized regarding growth, cell cycle distribution, colony-forming efficiency (CFE), phenotypes and cytomorphometry. Limbal epithelial cells expanded in 2% O2 exhibited slow growth, low fraction of cells in S/G2 , high CFE, high expression of stem cell markers ABCG2 and p63α, and low fraction of differentiation marker CK3 resembling a LESC phenotype. The effect of hypoxia to maintain LESCs in culture was not dependent on the system used for propagation (Bath et al. 2013a). Laser capture microdissection was used to isolate cellular subpopulations in situ from the spatially defined differentiation pathway in human corneal epithelium according to an optimized protocol for maintenance of expression profiles. Isolated total RNA from basal limbal crypts (BLCs), superficial limbal crypts (SLCs), paracentral/central cornea and limbal stroma was amplified and converted to fragmented cDNA libraries for use in deep paired-end next-generation sequencing. Global transcriptional profiling was carried out using bioinformatics. The location of primitive cells in BLCs, migratory and activated cells in SLCs and differentiated cells in paracentral/central cornea was evident from mapping of significantly upregulated genes in each compartment to the gene ontology (GO). Interestingly, many GO terms in BLCs were also involved in neurogenic processes, whereas many GO terms in SLCs were related to vasculature. Mapping upregulated genes in BLCs to pathway annotations in Kyoto Encyclopedia of Genes and Genomes described many active pathways as signalling and cancer-associated pathways. We supply extensive information on possible novel biomarkers, reveal insight into both active pathways and novel regulators of LESCs such as Lrig1 and SOX9 and provide an immense amount of data for future exploration (Bath et al. 2013b). Selective ex vivo expansion of LESCs in hypoxia and the comprehensive molecular characterization of corneal epithelial subpopulations in situ are expected to be beneficial for the future treatment of LSCD by cultured limbal epithelial transplantation.

Transcriptional dissection of human limbal niche compartments by massive parallel sequencing

Corneal epithelium is maintained throughout life by well-orchestrated proliferation of limbal epithelial stem cells (LESCs), followed by migration and maturation centripetally towards the ocular surface. Disturbance of LESCs can potentially lead to a blinding condition, which can be reversed by reconstitution of a functional LESC pool. The current clinical procedures are effective to some degree, however, deeper knowledge of the molecular interplay within the limbal niche is necessary to achieve a fully satisfactory patient outcome. The present study was thus undertaken to carry out a comprehensive transcriptome analysis of four distinct human limbal compartments, including basal limbal crypts (BLCs), superficial limbal crypts (SLCs), cornea, and the supporting stroma, with the aid of laser capture microdissection and deep RNA sequencing. The tissue harvest pipeline was rigorously optimized so that the exposure to cold ischemia would be less than five minutes. The global gene ontology analysis confirmed existence of primitive cells in BLCs, migratory and activated cells in SLCs, and differentiated cells in cornea. Interestingly, many significantly upregulated genes in SLCs mapped to processes involved in regulation of vasculature, such as sFLT1. In contrast, BLCs exhibited many genes mapping to neurogenic processes and processes related to cell development. The primitive nature of BLCs was, furthermore, confirmed by the KEGG pathway analysis, and some potential regulators of LESCs were revealed, such as Lrig1 and SOX9. The analysis also yielded comprehensive lists of uniquely expressed genes in both BLCs and cornea, which may be useful to identify possible biomarkers. In conclusion, the current investigation provides new insight into the relationship between distinct cell populations within the limbal niche, identifies candidates to be verified for novel biological functions, and yields a wealth of information for prospective data mining.

The culture of limbal epithelial cells

The transplantation of cultured limbal epithelial cells (LEC) has since its first application in 1997 emerged as a promising technique for treating limbal stem cell deficiency. The culture methods hitherto used vary with respect to preparation of the harvested tissue, choice of culture medium, culture time, culture substrates, and supplementary techniques. In this chapter, we describe a procedure for establishing human LEC cultures using a feeder-free explant culture technique with human amniotic membrane (AM) as the culture substrate.

Enhanced viability of corneal epithelial cells for efficient transport/storage using a structurally modified calcium alginate hydrogel

AIMS

Therapeutic limbal epithelial stem cells could be managed more efficiently if clinically validated batches were transported for 'on-demand' use.

MATERIALS & METHODS

In this study, corneal epithelial cell viability in calcium alginate hydrogels was examined under cell culture, ambient and chilled conditions for up to 7 days.

RESULTS

Cell viability improved as gel internal pore size increased, and was further enhanced with modification of the gel from a mass to a thin disc. Ambient storage conditions were optimal for supporting cell viability in gel discs. Cell viability in gel discs was significantly enhanced with increases in pore size mediated by hydroxyethyl cellulose.

CONCLUSION

Our novel methodology of controlling alginate gel shape and pore size together provides a more practical and economical alternative to established corneal tissue/cell storage methods.

Silk fibroin as a biomaterial substrate for corneal epithelial cell sheet generation

PURPOSE

To evaluate a silk fibroin (SF) biomaterial as a substrate for corneal epithelial cell proliferation, differentiation, and stratification in vitro compared with denuded human amniotic membrane (AM).

METHODS

Primary human and rabbit corneal epithelial cells and immortalized human corneal limbal epithelial cells were cultured on the SF and denuded AM, respectively. The biological cell behavior, including the morphology, proliferation, differentiation, and stratification, on the two substrates was compared and analyzed.

RESULTS

Corneal epithelial cells can adhere and proliferate on the SF and denuded AM with a cobblestone appearance, abundant microvilli on the surface, and wide connection with the adjacent cells. MTT assay showed that cell proliferation on denuded AM was statistically higher than that on SF at 24 and 72 hours after plating (P = 0.001 and 0.0005, respectively). Expression of ΔNp63a and keratin 3/12 was detected in primary cell cultures on the two substrates with no statistical difference. When cultured at the air-liquid interface for 7 days, cells on SF could form a comparable stratified graft with a 2- to 3-cell layering, which compared similarly to AM cultures.

CONCLUSIONS

SF, a novel biomaterial, could support corneal epithelial cells to proliferate, differentiate, and stratify, retaining the normal characteristic epithelium phenotype. Compared with AM, its unique features, including the transparency, ease of handling, and transfer, and inherent freedom from disease transmission, make it a promising substrate for corneal wound repair and tissue-engineering purposes.

Ocular epithelial transplantation: current uses and future potential

Visual loss may be caused by a variety of ocular diseases and places a significant burden on society. Replacing or regenerating epithelial structures in the eye has been demonstrated to recover visual loss in a number of such diseases. Several types of cells (e.g., embryonic stem cells, adult stem/progenitor/differentiated epithelial cells and induced pluripotent cells) have generated much interest and research into their potential in restoring vision in a variety of conditions: from ocular surface disease to age-related macular degeneration. While there has been some success in clinical transplantation of conjunctival and particularly corneal epithelium utilizing ocular stem cells, in particular, from the limbus, the replacement of the retinal pigment epithelium by utilizing stem cell sources has yet to reach the clinic. Advances in our understanding of all of these cell types, their differentiation and subsequent optimization of culture conditions and development of suitable substrates for their transplantation will enable us to overcome current clinical obstacles. This article addresses the current status of knowledge concerning the biology of stem cells, their progeny and the use of differentiated epithelial cells to replace ocular epithelial cells. It will highlight the clinical outcomes to date and their potential for future clinical use.

Photochemical cross-linking of plastically compressed collagen gel produces an optimal scaffold for corneal tissue engineering

The experiments were designed to use photochemically cross-linked plastically compressed collagen (PCPCC) gel to support corneal epithelial cells. A plastically compressed collagen (PCC) scaffold was photo cross-linked by UVA in the presence of riboflavin to form a biomaterial with optimal mechanical properties. The breaking force, rheology, surgical suture strength, transparency, ultrastructure, and cell-based biocompatibility were compared between PCPCC and PCC gels. The breaking force increased proportionally with an increased concentration of riboflavin. The stress required to reach breaking point of the PCPCC scaffolds was over two times higher compared to the stress necessary to break PCC scaffolds in the presence of 0.1% riboflavin. Rheology results indicated that the structural properties of PCC remain unaltered after UVA cross-linking. The PCC gels were more easily broken than PCPCC gels when sutured on to bovine corneas. The optical density values of PCPCC and PCC showed no significant differences (p > 0.05). SEM analyses showed that the collagen fibres within the PCPCC gels were similar in morphology to PCC gels. No difference in cell-based biocompatibility was seen between the PCPCC and PCC scaffolds in terms of their ability to support the ex vivo expansion of corneal epithelial cells or their subsequent differentiation evidenced by similar levels of cytokeratin 14. In conclusion, PCPCC scaffold is an optimal biomaterial for use in therapeutic tissue engineering of the cornea.

Investigation of K14/K5 as a stem cell marker in the limbal region of the bovine cornea

Background

Identification of stem cells from a corneal epithelial cell population by specific molecular markers has been investigated previously. Expressions of P63, ABCG2 and K14/K5 have all been linked to mammalian corneal epithelial stem cells. Here we report on the limitations of K14/K5 as a limbal stem cell marker.

Methodology/Principal Findings

K14/K5 expression was measured by immunohistochemistry, Western blotting and Real time PCR and compared between bovine epithelial cells in the limbus and central cornea. A functional study was also included to investigate changes in K5/14 expression within cultured limbal epithelial cells undergoing forced differentiation. K14 expression (or its partner K5) was detected in quiescent epithelial cells from both the limbal area and central cornea. K14 was localized predominantly to basal epithelial cells in the limbus and suprabasal epithelial cells in the central cornea. Western blotting revealed K14 expression in both limbus and central cornea (higher levels in the limbus). Similarly, quantitative real time PCR found K5, partner to K14, to be expressed in both the central cornea and limbus. Following forced differentiation in culture the limbal epithelial cells revealed an increase in K5/14 gene/protein expression levels in concert with a predictable rise in a known differentiation marker.

Conclusions/Significance

K14 and its partner K5 are limited not only to the limbus but also to the central bovine cornea epithelial cells suggesting K14/K5 is not limbal specific in situ. Furthermore K14/K5 expression levels were not lowered (in fact they increased) within a limbal epithelial cell culture undergoing forced differentiation suggesting K14/K5 is an unreliable maker for undifferentiated cells ex vivo.