Histochemical and morphological study of the regenerating corneal epithelium after limbus-to-limbus denudation

Stem cell-based therapy for treating limbal stem cells deficiency: A review of different strategies

The self renewal capability of limbal epithelial stem (LEST) cells is fundamental to the maintenance and healing of corneal epithelium. Limbal stem cell deficiency (LSCD), due to dysfunction or loss of LEST cells, therefore presents as persistent epithelial defects, corneal vascularization, conjunctivalization etc. Stem cell-based therapy, in its simplest form - limbal autograft, has been used successfully for more than a decade. For bilateral LSCD, similar approaches with limbal allografts have been unsuccessful largely due to strong immune rejection. Therefore, as an alternate strategy for treating bilateral LSCD, ex vivo expansion of the remaining LEST cells or autologous stem cells sourced from other potential sites is being explored. Different culture systems (with and without xenobiotic supplements) using substrates like amniotic membrane or fibrin gels have been used successfully for ex vivo LEST cell maintenance and reproduction by imitating the stem cell niche. This paper is organized into sections reviewing the LEST cells, LSCD and various stem cell-based approaches for treating LSCD and discussing future direction and challenges.

Lycopersicon esculentum lectin is a marker of transient amplifying cells in in vitro cultures of isolated limbal stem cells

The maintenance of a healthy corneal epithelium under both normal and wound healing conditions is achieved by a population of stem cells (SCs) located in the basal epithelium at the corneoscleral limbus. In the light of the development of strategies for reconstruction of the ocular surface in patients with limbal stem cell deficiency, a major challenge in corneal SCs biology remains the ability to identify stem cells in situ and in vitro. To date, not so much markers exist for the identification of different phenotypes. CESCs (corneal epithelial stem cells) isolated from limbal biopsies were maintained in primary culture for 14 days and stained with Hoechst and a panel of FITC-conjugated lectins. All lectins, with the exception of Lycopersicon esculentum, labelled CESCs irrespective of the degree of differentiation. Lycopersicon esculentum, that binds N-acetylglucosamine oligomers, labelled intensely only the surface of TACs (single corneal epithelial stem cells better than colonial cells). These results suggest that Lycopersicon esculentum lectin is a useful and easy-to-use marker for the in vitro identification of TACs (transient amplifying cells) in cultures of isolated CESCs.

Corneal epithelial stem cells: characterization, culture and transplantation

The epithelium covering the cornea at the front of the eye is maintained by stem cells located at its periphery, in a region known as the limbus. A lack or dysfunction of these so-called limbal stem cells (LSCs) results in the painful and blinding disease of LSC deficiency. In this review, current knowledge regarding the biology of these particular stem cells will be outlined, including recent advances that are enabling the gene expression analysis of these cells. The use of LSCs in therapeutic interventions for LSC deficiency will also be discussed, including the role for ex vivo expansion. In particular, the translation of basic science advances in LSC biology into therapeutic strategies will be highlighted.

Identification and characterization of limbal stem cells

The maintenance of a healthy corneal epithelium under both normal and wound healing conditions is achieved by a population of stem cells (SC) located in the basal epithelium at the corneoscleral limbus. In the light of the development of strategies for reconstruction of the ocular surface in patients with limbal stem cell deficiency, a major challenge in corneal SC biology remains the ability to identify stem cells in situ and in vitro. Until recently, the identification of limbal stem cells mainly has been based on general properties of stem cells, e.g. lack of differentiation, prolonged label-retaining, indefinite capacity of proliferation exemplified by the clonogenic assay as well as their special role in corneal wound healing. During the last years, a number of molecular markers for the limbal SC compartment has been proposed, however, their role in distinguishing limbal SC from their early progeny is still under debate. Data reported from the literature combined with our own recent observations suggest, that the basal epithelial cells of the human limbus contain ABCG2, K19, vimentin, KGF-R, metallothionein, and integrin alpha9, but do not stain for K3/K12, Cx43, involucrin, P-cadherin, integrins alpha2, alpha6, and beta4, and nestin, when compared to the basal cells of the corneal epithelium. A relatively higher expression level in basal limbal cells was observed for p63, alpha-enolase, K5/14, and HGF-R, whereas there were no significant differences in staining intensity for beta-catenin, integrins alphav, beta1, beta2, and beta5, CD71, EGF-R, TGF-beta-RI, TGF-beta-RII, and TrkA between limbal and corneal basal epithelial cells. Therefore, a combination of differentiation-associated markers (e.g. K3/K12, Cx43, or involucrin) and putative SC-associated markers (e.g. ABCG2, K19, vimentin, or integrin alpha9) may provide a suitable tool for identification of human limbal SC. While most putative SC markers label the majority of limbal basal cells and, therefore, may not distinguish SC from progenitor cells, only ABCG2 was strictly confined to small clusters of basal cells in the limbal epithelium. At present, ABCG2 therefore appears to be the most useful cell surface marker for the identification and isolation of corneal epithelial SC. Moreover, the characteristics of the specific microenvironment of corneal SC, as provided by growth factor activity and basement membrane heterogeneity in the limbal area, could serve as additional tools for their selective enrichment and in vitro expansion for the purpose of ocular surface reconstruction.

In search of markers for the stem cells of the corneal epithelium

The anterior one-fifth of the human eye is called the cornea. It consists of several specialized cell types that work together to give the cornea its unique optical properties. As a result of its smooth surface and clarity, light entering the cornea focuses on the neural retina allowing images to come into focus in the optical centres of the brain. When the cornea is not smooth or clear, vision is impaired. The surface of the cornea consists of a stratified squamous epithelium that must be continuously renewed. The cells that make up this outer covering come from an adult stem cell population located at the corneal periphery at a site called the corneal limbus. While engaging in the search for surface markers for corneal epithelial stem cells, vision scientists have obtained a better understanding of the healthy ocular surface. In this review, we summarize the current state of knowledge of the ocular surface and its adult stem cells, and analyse data as they now exist regarding putative corneal epithelial stem cell markers.

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.

Corneal stem cells in review

The cornea provides the eye with protection and the refractive properties essential for visual acuity. The transparent epithelium is highly specialized with basal and stratified squamous cells that are renewed throughout life from a stem cell population. The stem cells are thought to reside at the corneal limbus and may be maintained by a variety of intrinsic and extrinsic factors such as the local environment, survival factors, and cytokines. A number of markers have been localized to the limbus in an attempt to identify stem cells; however, definite stem cell identification remains elusive. During homeostasis and following injury to the corneal epithelium, the limbal stem cells divide to produce daughter transient amplifying cells that proliferate, migrate, and differentiate to replace lost cells. However, this cannot occur if the stem cell population is depleted. Limbal stem cell deficiency then results in corneal re-epithelialization by the neighboring conjunctiva, causing pain, poor vision, and even blindness. This review will focus on corneal epithelial stem cells in ocular surface repair and regeneration. The current knowledge of stem cell biology in the corneal epithelium, clinical consequences of stem cell deficiency, and therapeutic strategies aimed at reversing stem cell deficiency will be discussed.

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.

Limbal stem cells of the corneal epithelium

Stem cells have certain unique characteristics, which include longevity, high capacity of self-renewal with a long cell cycle time and a short S-phase duration, increased potential for error-free proliferation, and poor differentiation. The ocular surface is made up of two distinct types of epithelial cells, constituting the conjunctival and the corneal epithelia. Although anatomically continuous with each other at the corneoscleral limbus, the two cell phenotypes represent quite distinct subpopulations. Stem cells for the cornea reside at the corneoscleral limbus. The limbal palisades of Vogt and the interpalisade rete ridges are believed to be repositories of stem cells. The microenvironment of the limbus is considered to be important in maintaining the stemness of stem cells. Limbal stem cells also act as a "barrier" to conjunctival epithelial cells and normally prevent them from migrating on to the corneal surface. Under certain conditions, however, the limbal stem cells may be partially or totally depleted, resulting in varying degrees of stem cell deficiency with resulting abnormalities in the corneal surface. Such deficiency of limbal stem cells leads to "conjunctivalization" of the cornea with vascularization, appearance of goblet cells, and an irregular and unstable epithelium. This results in ocular discomfort and reduced vision. Partial stem cell deficiency can be managed by removing the abnormal epithelium and allowing the denuded cornea, especially the visual axis, to resurface with cells derived from the remaining intact limbal epithelium. In total stem cell deficiency, autologous limbus from the opposite normal eye or homologous limbus from living related or cadaveric donors can be transplanted on to the affected eye. With the latter option, systemic immunosuppression is required. Amniotic membrane transplantation is a useful adjunct to the above procedures in some instances.

Perpetuation of stem cells in the eye

In adult tissues, cell numbers are maintained through a subpopulation of cell termed stem cells, characterised in part by a high capacity of self-renewal, slow cell cycle, and resistance towards differentiation. Stem cells are capable of asymmetric division and able to maintain their position in a particular microenvironment or niche. In the cornea, epithelial stem cells are believed to reside in the basal cell layer of the limbal epithelium. We consider the question of how stem cells are perpetuated in the limbus without entering the pathway of terminal differentiation. This perpetuation could presumably be the result of extrinsic properties of the limbal zone creating a 'stem cell niche', or of intrinsic properties of the cells. For example, limbal basal cells contain four- to fivefold higher levels of epidermal growth factor receptor than central corneal basal cells, suggesting that high levels of epidermal growth factor receptor help maintain the limbal basal cells in an undifferentiated stem cell state.

Metabolic analysis of reepithelializing rabbit cornea using phosphorus-31 nuclear magnetic resonance spectroscopy

To investigate metabolic differences between the central and peripheral cornea the latter including the limbal area, corneas were dissected and examined using phosphorus-31 (31P) nuclear magnetic resonance spectroscopy. Since most 31P signals originate from the epithelium, 31P spectra of the cornea primarily represent the metabolic state of the epithelium. The spectra of the peripheral cornea showed all phosphorus resonances detected in the whole cornea; in contrast, the central cornea showed no phosphocreatine and glycerophosrylethanolamine, and only low levels of ATP. These results indicate that there is a higher metabolic activity in the peripheral epithelium, especially in the limbal area, than in the central epithelium. To evaluate the metabolic state of corneal epithelium during regeneration, we also examined corneas reepithelializing after 7 mm of central epithelial tissue had been removed by mechanical scraping. Rabbits were killed 24 and 48 h after scraping. The reepithelializing corneas clearly showed an increase in ATP, phosphocreatine, and sugar phosphates with time, although phosphorylcholine remained depressed. These findings suggest that the reepithelializing cornea has an elevated level of energy production and that it may have reached a higher steady state, thereby indicating accelerated metabolism of the epithelium during regeneration.

Growth characteristics of central and peripheral bovine corneal epithelial cells in vitro

Primary cultures of pure bovine corneal epithelial sheets, isolated from either central or peripheral areas of the cornea and grown on an extracellular matrix, exhibited major differences in relation to their respective growth characteristics and morphology. After 15 days in culture, cells of peripheral origin covered a 40% greater area and were 2.75 times more numerous than those of central origin. Most peripheral cells were small with a polygonal morphology, whereas central cells varied considerably in both size and shape, although areas consisting of large cells were regularly observed. Differences in the rates of proliferation between central and peripheral corneal epithelial cells were maintained throughout the first and second subcultures. However, the growth rates were considerably lower in second passage cultures of both central and peripheral cells when compared with those of first passage. The growth characteristics of primary cultures of pure epithelial sheets were confirmed by further studies on corneal buttons in culture. Our results in vitro strongly support the concept of a slow but steady physiological movement of increasingly differentiated cells from the periphery of the cornea towards the centre, resulting in a constant renewal of epithelial cells in vivo.

Increased cytochrome oxidase activity in alkali-burned corneas

Cytochrome oxidase (CO) activity on alkali-burned rabbit corneas was investigated histochemically to determine the metabolic change in inflamed corneas during wound healing. Cryostat sections of chemically burned and mechanically scraped corneas were stained for CO activity, which is regarded as an index of metabolic activity. Following chemical injury, positive CO activity was detected initially in the vascular endothelial cells of limbal blood vessels. Numerous active polymorphonuclear leukocytes (PMNs) and monocytes were found intravascularly and perivascularly. Fibroblasts that appeared at the wound site exhibited marked CO activity around the limbus. Over a period of 13 days, PMNs gradually invaded the central cornea, followed by fibroblasts of high metabolic activity. The areas of PMN infiltration were the same areas in which fibroblasts showed intense staining, suggesting that a PMN-derived mediator or secondary products might affect the activation of fibroblasts. Epithelial resurfacing was delayed in the chemically burned corneas, although reepithelialization was completed within two to three days in the scraped corneas. Limbal epithelial cells, which recently were suggested as the source of epithelial renewal, showed a remarkable increase of metabolic activity in response to chemical inflammatory stimulation, whereas those in the scraped model did not. This suggests that epithelial cell renewal at the limbus was accelerated in the presence of disturbed reepithelialization.