Stem cells and differentiation stages in the limbo-corneal epithelium

The Corneal Epithelial Stem Cell Niche

In recent years, it has become generally accepted that the corneal epithelial stem cells are localized in the basal cell layer of the limbal epithelium. However, a number of questions remain regarding the number, markers, generation, and maintenance of the corneal epithelial stem cells. One of the key questions concerns what makes up the microenvironment or niche that is responsible for allowing the stem cells to remain and function throughout the life of the tissue. This review will consider the unique aspects of the limbus and compare these to what is known about other stem cell niches.

Stem cells in the eye

In the adult organism, all tissue renewal and regeneration depends ultimately on somatic stem cells, and the eye is no exception. The importance of limbal stem cells in the maintenance of the corneal epithelium has long been recognised, and such cells are now used clinically for repair of a severely damaged cornea. The slow cycling nature of lens epithelial cells and their ability to terminally differentiate into fiber cells are suggestive of a stem cell lineage. Furthermore, recent studies have identified progenitor cells in the retina and ocular vasculature which may have important implications in health and disease. Although the recent literature has become flooded with articles discussing aspects of stem cells in a variety of tissues our understanding of stem cell biology, especially in the eye, remains limited. For instance, there is no definitive marker for ocular stem cells despite a number of claims in the literature, the patterns of stem cell growth and amplification are poorly understood and the microenvironments important for stem cell regulation and differentiation pathways are only now being elucidated. A greater understanding of ocular stem cell biology is essential if the clinical potential for stem cells is to be realised. For instance; How do we treat stem cell deficiencies? How do we use stem cells to regenerate damaged retinal tissue? How do we prevent stem cell lineages contributing to retinal vascular disease? This review will briefly consider the principal stem cells in the mature eye but will focus in depth on limbal stem cells and corneal epithelium. It will further discuss their role in pathology and their potential for therapeutic intervention.

Phenotypic characterization of human corneal epithelial cells expanded ex vivo from limbal explant and single cell cultures

Cultivated human corneal epithelial cells have been successfully used for corneal reconstruction. Explant and single cell systems are currently used for human corneal epithelial cultivation. This study was conducted to characterize the phenotypes of human corneal epithelial cells expanded ex vivo by these two culture systems with regard to their growth potential, morphology and antigen expression patterns. Human corneal epithelial cells were expanded by limbal explant culture or limbal single cell suspension culture on a mitomycin C treated 3T3 fibroblast feeder layer. The phenotypes of primary cultured cells were evaluated by morphology and immunohistochemical staining with antibodies for proposed keratinocyte stem cell markers (p63, EGFR, K19 and integrin beta1) and differentiation markers (K3, involucrin and gap junction protein connexin 43). BrdU labeling was performed to identify the label-retaining cells. Human corneal epithelial cells were grown from limbal tissues preserved as long as 16 days by both culture systems. The growth rate depended on the tissue freshness, the time from death to preservation and the time from death to culture, but not on the donor age. Cell growth was observed in 96.2% (n = 43) of single cell suspension cultures and in 90.8% (n = 213) of explant cultures. The cell expansion was confluent in 10-14 days in single cell suspension cultures and 14-21 days in explant cultures. The cell morphology in single cell suspension culture was smaller, more compact and uniform than that in explant culture. Immunostaining showed a greater number of the small cells expressing p63, EGFR, K19 and integrin beta1, while more larger cells stained positively for K3, involucrin and connexin 43 in both culture systems. BrdU-label retaining cells were identified in 2.3+/-0.7% of explant cultures and 3.73+/-1.5% of single cell cultures chased for 21 days. In conclusion, the limbal rims are a great treasure for ex vivo expansion of human corneal epithelial cells. The phenotypes of corneal epithelial cells, ranging from basal cells to superficial differentiated cells, are well maintained in both culture systems. Slow-cycling BrdU-label retaining cells, that are characteristic of stem cells, were identified in the cultures.

Keratolimbal allograft in corneal reconstruction

The replenishment of corneal epithelial SC is a crucial step for reconstructing the ocular surface in patients suffering from devastating ocular surface diseases manifesting with total LSCD. KLAL is one of such procedures and has a long track record and a long follow-up for patients with bilateral total LSCD. This review summarizes the literature experiences and outline new strategies that are important to enhance the success of this procedure. Further research is needed to fully understand the biological processes involved in allogeneic tissue transplantation for preserving epithelial SC adhesion, migration, and survival.

Epithelial cell characteristics of cultured human limbal explants

AIM

To determine the immunohistochemical characteristics of putative corneal epithelial stem cells remaining on limbal explants maintained in culture.

METHODS

Human limbal explant cultures were generated from 25 residual corneoscleral donor rims following penetrating keratoplasty. Serial sections of these explants were studied using immunohistochemical techniques with a panel of antibodies, on day 0 and 1, 2, and 3 weeks.

RESULTS

The number of epithelial cells expressing cytokeratin 19 and vimentin increased with duration in culture, while the number of cells expressing cytokeratin 3 decreased. Connexin 43 expression was lost by 1 week in culture. p63 was expressed by cells that had migrated around the explant and the number of p63 positive cells decreased with longer duration in culture. The explants were initially negative for Ki67, but the epithelial cells were positive at 1 week, and expression of Ki67 was progressively lost with increasing duration in culture. The initial uniform staining of the epithelium for epidermal growth factor receptor and alpha enolase remained unchanged at 3 weeks.

CONCLUSIONS

There is an expansion of less differentiated (cytokeratin 3 negative and CK19/vimentin positive) epithelial cells on corneoscleral explants maintained in culture for 3 weeks. The pattern of expression of p63 noted in this study does not support the suggestion that it is a marker of limbal stem cells. The decline in p63 and Ki67 expression among the epithelial cells of the cultured explant button implies that as the epithelial sheet outgrowing from the explant button reaches confluence, the proliferative status of the cells remaining on the explant button declines. These findings are of clinical relevance as explants of limbal tissue are used in limbal stem cell transplantation. There is no information available to date on the fate of epithelial cells on such explants. This study provides some insight into this and suggests that an expansion of the stem cell pool or its progeny may occur in limbal explants.

Stem cell differentiation and the effects of deficiency

Stem cells have several unique attributes, the key features being their potency and plasticity. They have the ability to give rise to multiple cell lineages and to transdifferentiate into totally different cell type(s) when relocated to a novel stem cell niche. Most self-renewing tissues are served by stem cells. At the ocular surface, the corneo-scleral limbus is believed to provide the niche for corneal epithelial stem cells. A large body of circumstantial evidence, both clinical and basic, supports this view. However, specific identification of limbal stem cells has proved elusive. Cytokeratin markers, vimentin, epidermal growth factor receptors, p63, and others have been used to identify epithelial cell populations at the limbus, which could harbour putative stem cells. In contrast, none of the known haematopoietic stem cell markers namely, CD34 and CD133, stain any specific subset of corneal or limbal epithelial cells. Singly or collectively, none of these markers point to any unique cell(s) that could be regarded as stem cells, supporting the notion that the corneal epithelium is served by 'committed progenitors' rather than by stem cells. Disease or destruction of the corneo-scleral limbus is associated with consequential events that eventually lead to visual impairment or blindness. Conjunctivalisation and vascularisation of the corneal surface and persistent or recurring epithelial defects are hallmarks of limbal deficiency.

Ex vivo expansion of limbal epithelial stem cells: amniotic membrane serving as a stem cell niche

Identification, maintenance, and expansion of stem cells for subsequent transplantation has become a new strategy for treating many diseases in most medical subspecialties. The stem cells of the corneal epithelium are located in the limbal basal layer and are the ultimate source for constant corneal epithelial renewal. Like those in other tissues, limbal stem cells are supported by a unique stromal microenvironment called the stem cell niche, which consists of certain extracellular matrix components, cell membrane-associated molecules, and cytokine dialogues. Destructive loss of limbal stem cells or dysfunction of their stromal environment renders many corneas with a clinical entity called limbal stem cell deficiency, which is characterized by variable extents of conjunctival ingrowth depending on the severity of limbal damage. A new strategy of treating limbal stem cell deficiency is to transplant a bio-engineered graft by expanding limbal epithelial stem cells ex vivo on amniotic membrane. This review summarizes the published literature data collectively explaining how amniotic membrane is an ideal biological substrate that can help maintain and support the expansion of limbal epithelial stem cells.

Phenotypic study of a case receiving a keratolimbal allograft and amniotic membrane for total limbal stem cell deficiency

PURPOSE

To report the expression pattern of key molecules by the reconstructed corneal epithelium after a keratolimbal allograft (KLAL) and amniotic membrane transplantation (AMT) for total limbal stem cell deficiency.

DESIGN

Interventional case report.

METHOD

A 50-year-old woman with severe chemical burns in both eyes received an AMT as a temporary patch at the acute stage, and a KLAL with AMT as a graft at the chronic stage for total limbal stem cell deficiency. The corneal button removed during subsequent corneal transplantation was submitted for immunofluorescence staining with monoclonal antibodies against keratin K3, MUC5AC, connexin 43, integrins alpha3beta1 and alpha6beta4, and laminin 5 for comparison with a normal cornea.

RESULTS

Histologically, a normal stratified corneal epithelium has five to six cell layers that lay on the thick amniotic membrane basement membrane. The phenotype was of a corneal origin, based on expression of positive keratin K3, negative MUC5AC, and positive connexin 43. Furthermore, intact basement membrane complexes were present, evidenced by positive staining to integrins alpha3beta1 and alpha6beta4 and to laminin 5.

CONCLUSIONS

A normal corneal epithelial phenotype with normal basement membrane complexes was restored after a KLAL and AMT in a case with total limbal stem cell deficiency.

Phenotypic study of a case with successful transplantation of ex vivo expanded human limbal epithelium for unilateral total limbal stem cell deficiency

OBJECTIVE

To minimize the risk to the donor eye when a conjunctival limbal autograft is performed for unilateral total limbal stem cell deficiency (LSCD), a new approach has been reported of expanding limbal epithelial progenitor cells from a small limbal biopsy cultured on amniotic membrane (AM). Herein, we present for the first time the morphologic and phenotypic outcome of one such patient.

DESIGN

Interventional case report.

METHODS

A 31-year-old male with a severe acid burn to his left eye received AM transplantation at the acute stage and a keratolimbal allograft (KLAL) at the chronic stage for total LSCD. As an alternative to combat the failed KLAL, the above-mentioned new surgical procedure was performed. The corneal button, obtained after a penetrating keratoplasty performed 5.5 months later, and a normal corneal button as a control were submitted to hematoxylin-eosin and immunofluorescence staining for keratin K3, connexin 43, goblet-cell mucin MUC 5AC, laminin 5, and integrins alpha3beta1 and alpha6beta4.

MAIN OUTCOME MEASURES

Clinical and immunohistologic features.

RESULTS

The resultant epithelium was stratified with five to six cell layers and anchored to laminin 5 of the amniotic basement membrane via integrins alpha3beta1 and alpha6beta4 in a manner similar to the normal corneal epithelium. Intriguingly, the epithelial phenotype was limbal and not corneal, based on the negative expression of keratin K3 and connexin 43 of the basal epithelium.

CONCLUSIONS

The technique described ensures the preservation of amniotic basement membrane, which allows formation of adhesion complexes and maintains normal corneal architecture. The preservation of a limbal epithelial phenotype on the reconstructed corneal surface indicates that AM provides a unique stromal environment conducive to the preservation and expansion of limbal epithelial progenitor cells.

The corneal epithelial stem cell

The aim of this paper was to develop a GFP-expressing transgenic mouse model for the keratoepithelioplasty and to use this to follow the outcome of this form of graft, when placed on an inflamed corneal surface. Further aims were to characterize both the graft and the epithelial surface of the mouse and rat cornea using putative stem cell markers (P63 and Telomerase) and marker of cell differentiation (14-3-3 sigma). Keratepithelioplasty was carried out using a GFP transgenic mouse cornea as donor tissue. Fluorescent epithelial outgrowth from each keratepithelioplasty was scored and quantified. Donor corneal graft tissue was obtained from the paracentral region or the anatomical limbal region of murine corneas. Paracentral donor grafts (n = 20) consistently demonstrated a significant increase in proliferative potential compared to grafts obtained from the anatomical limbal region of the mouse cornea (n = 25) (P = 0.000, Mann-Whitney U). Correspondingly, P63 expression was maximal in the paracentral region of the mouse cornea, in keeping with the demonstrated increased proliferative potential of donor grafts harvested from this region of the cornea. The murine corneal epithelium demonstrated decreased rather than increased cellular layers at the limbal region, in contrast to that of the rat or human epithelium. In addition, as a general finding in all species tested, there was an apparent increase noted in P63 expression in basal corneal epithelial cells in regions that had increased cellular layers (limbus in humans and rats and the paracentral corneal region in the mouse). Epithelium, which had migrated from donor grafts onto recipient corneas, retained P63 expression for the period of time examined (up to 3 days postengraftment). In addition, the conjunctival surface of an injured conjunctivalized displayed an abnormal pattern of P63 expression. Telomerase expression was widespread throughout many layers of both the murine and rat corneal epithelium. In the mouse and rat corneal epithelium P63 expression was maximal in areas of increased proliferative potential. Its expression, however, was not confined to stem cells alone. Migrating cells from transplanted keratoepithelial grafts retained P63 expression at least in the early stages post-transplantation. Finally, damaged conjunctivalized corneas displayed an abnormal P63 expression pattern when compared to either normal conjunctiva or normal cornea.

Characteristics of the human ocular surface epithelium

An appreciation of the biological characteristics of the human ocular surface epithelium affords us a great insight into the physiology of the human ocular surface in health and disease. Here, we review five important aspects of the human ocular surface epithelium. First, we recognize the discovery of corneal epithelial stem cells, and note how the palisades of Vogt have been suggested as a clinical marker of their presence. Second, we introduce the concept of the gene expression profile of the ocular surface epithelium as arrived at using a new strategy for the systematic analysis of active genes. We also provide a summary of several genes abundantly or uniquely expressed in the human corneal epithelium, namely clusterin, keratin 3, keratin 12, aldehyde dehydrogenase 3 (ALDH3), troponin-I fast-twitch isoform, ssig-h3, cathepsin L2 (cathepsin V), uroplakin Ib, and Ca(2+)-activated chloride channel. Genes related to limbal and conjunctival epithelia are also described. Third, we touch upon the genetic abnormalities thought to be involved with epithelial dysfunction in Meesmann's dystrophy, gelatinous drop-like corneal dystrophy, and the ssig-h3-mutated corneal dystrophies. Fourth, we provide an update regarding the current state of knowledge of the role of cytokines, growth factors and apoptosis in relation to ocular surface homeostasis and tissue reconstruction; the main factors being epidermal growth factor (EGF), keratinocyte growth factor (KGF), hepatocyte growth factor (HGF), transforming growth factor-ss (TGF-ss), and some inflammatory cytokines. Fifth, corneal epithelial barrier function and dysfunction as measured by fluorophotometry is remarked upon, with an explanation of the FL-500 fluorophotometer and its ability to detect corneal epithelial dysfunction at a subclinical level. The research described in this review has undoubtedly generated a complete understanding of corneal epithelial pathophysiology-an understanding that, directly or indirectly, has helped advance the development of new therapeutic modalities for ocular surface reconstruction.

Role of the Lewis(x) glycan determinant in corneal epithelial cell adhesion and differentiation

In this study we demonstrate that in corneal epithelium there is cell-cell contact-regulated expression of a 145-kDa glycoprotein (GP) bearing the glycan determinant Lewis(x) (Le(x)) (Galbeta(1,4)[Fucalpha(1,3)]GlcNAc). This glycoprotein (Le(x)-GP) was expressed in confluent/contact-inhibited cultures but not in sparse cultures of corneal epithelium. In contrast, a 135-kDa glycoprotein bearing precursor, unfucosylated, lactosamine-containing glycans (Galbeta1-4GlcNAcbeta1-R) was expressed in sparse cultures. Immunofluorescence staining and confocal microscopy of confluent cultures revealed that in corneal epithelium, Le(x) antigen is located in high density at sites of cell-cell adhesion. In in vitro cell-cell adhesion assays, anti-Le(x), but not anti-sialyl-Le(x) monoclonal antibodies, inhibited the formation of corneal epithelial cell-cell adhesion. Also, when added to confluent cultures, antibodies to Le(x) disrupted the monolayer and caused tightly packed polygonal cells to round up. Analysis of the expression of Fut genes that encode alpha-1,3-fucosyltransferases, the enzymes that generate the Le(x) determinant, revealed that confluent/contact-inhibited cultures of rabbit corneal epithelium contain markedly elevated levels of Fut4 and Fut3/5/6 gene transcripts compared with sparse cultures. These data suggest that the Fut4 and Fut3/5/6 genes are targets of cell-cell contact-regulated signals and that Fut gene products direct cell-cell contact-associated expression of Le(x) on the Le(x)-GP in corneal epithelium. Immunohistochemical analysis revealed that the expression of Le(x) antigen in the epithelium of adult and developing corneas is related to the stage of differentiation of the cells. Although early differentiated cells robustly expressed Le(x), relatively undifferentiated cells did not, and the expression level was relatively low in terminally differentiated cells. Overall, these data provide evidence that a Le(x)-bearing glycoprotein plays a role through the Le(x) determinant in corneal epithelial cell-cell adhesion, and these data suggest that Le(x)-mediated cell-cell interactions contribute to mechanisms that mediate corneal epithelial cell differentiation.

Effects of adenine nucleosides and nucleotides on the isolated heart of the snail Helix aspersa and the slug Arion ater

1. Adenine nucleosides and nucleotides were examined for pharmacological activity in hearts isolated from the snail Helix aspersa and the slug Arion ater. 2. Adenosine, AMP, ADP and ATP (above 100 microM) produced either an excitation or an inhibition in the isolated hearts of the snail and slug. 3. 2-Chloroadenosine, alpha, beta-methylene ATP and 2-methylthio ATP were inactive at concentrations up to 1 mM. 4. Responses were not blocked by any commonly accepted vertebrate purinoceptor antagonists, indicating that these purinoceptors are dissimilar to vertebrate purinoceptors and cannot be classified according to accepted purinoceptor classifications. 5. Electrical field stimulation of the snail heart produced frequency-dependent responses: 1-4 Hz produced predominantly excitation, 8-32 Hz predominantly inhibition. These responses were unaffected by the purines up 3 mM.