CELL SCIENCE AT A GLANCE ARTICLE SERIES: STEM CELLS

Corneal epithelial stem cells and their niche at a glanceCraig S. Nowell1,* and Freddy Radtke2

ABSTRACTThecorneal epitheliumacts asaprotective barrier on the anterior ocularsurface and is essential for maintaining transparency of the cornea andthus visual acuity. During both homeostasis and repair, the cornealepithelium is maintained by self-renewing stem cells, which persistthroughout the lifetime of the organism. Importantly, as in other self-renewing tissues, the functional activity of corneal epithelial stem cells(CSCEs) is tightly regulated by the surrounding microenvironment, orniche, which provides a range of cues that maintain the stem cellpopulation. ThisCell Science at aGlance article and the accompanyingposter will therefore aim to summarise our current understanding ofthe corneal epithelial stem cell niche and its role in regulating stem cellactivity during homeostasis, repair and disease.

KEY WORDS: Cornea epithelial stem cell, Stem cell niche, Tissuerepair

Introduction: the cornea and the corneal epitheliumThe cornea is the transparent region of the ocular surface and isessential for maintaining vision, as it enables light to enter the eyeand stimulate the photoreceptor cells of the retina (Notara et al.,2010a). It also acts as a physical barrier between the internalstructures of the eye and the outside world, thus protecting the eyefrom environmental damage (Notara et al., 2010a).

Structurally, the cornea consists of an avascular, collagen-richstromal tissue that is lined by a self-renewing, stratified, non-keratinisng squamous epithelium (Daniels et al., 2001) (see poster).The transparent nature of the cornea is largely due to specificfeatures of the corneal stroma. Particularly important characteristicsin this respect include the absence of blood vessels, the distinctorganisation of collagen fibres and the low numbers of stromal cells(Xuan et al., 2016). The corneal epithelium lines the external surfaceof the stroma and protects it from environmental insults. It istherefore essential for the maintenance of the attributes of the stromathat enable transparency. Furthermore, unlike keratinising epithelia

1CMU, Department for Pathology and Immunology, Rue Michel Servet, 1211Geneva, Switzerland. 2Ecole Polytechnique Federale de Lausanne, School of LifeSciences, Swiss Institute for Experimental Cancer Research, Lausanne, Vaud,1015, Switzerland.

*Author for correspondence (Craig.Nowell@hcuge.ch)

C.S.N., 0000-0002-6535-2643

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such as the epidermis, in which the outermost cell layers replacetheir cytoplasm with keratin proteins, the corneal epitheliummaintains living cells at the surface, further aiding transparency.Anatomically, the corneal epithelium is continuous with the

epithelium that lines the conjunctiva (Dziasko and Daniels, 2016).However, they are separated by a junctional zone known as thelimbus and are phenotypically distinct, as they express distinctcytokeratins (cytokeratin 3 and 12 in the cornea, cytokeratin 13 inthe conjunctiva) (Notara et al., 2010a) (see poster). In addition, theconjunctiva contains mucin-producing goblet cells, which areabsent from the cornea (Notara et al., 2010a). The unique attributesof the corneal epithelium are essential in allowing it to functionas an effective barrier, while simultaneously remaining fullytransparent.Given the barrier function that the corneal epithelium performs

and the range of insults it is exposed to, its long-term maintenance iscritical and is mediated by epithelial stem cells that reside within thetissue. Presently, our understanding of how corneal epithelial stemcells (CESCs) are regulated during homeostasis, repair and diseaseremains incomplete, and further elucidation of the cellular andmolecular mechanisms that control CESC function will haveimportant clinical implications. In particular, this will advance ourknowledge of a variety of disorders of the ocular surface that cancause blindness and will potentially lead to new therapeuticstrategies that can restore the corneal epithelium and thus visualacuity. This Cell Science at a Glance article will summarise ourcurrent understanding of the biology of CESCs, with a particularfocus on the role the stromal microenvironment, or niche, plays inregulating stem cell function.

Identity and location of corneal epithelial stem cellsThe location of CESCs has been intensively investigated for anumber of years and remains a highly active and somewhatcontroversial area of research. The prevailing and widely acceptedmodel is that CESCs reside exclusively at the limbus, which is at thejunction between the cornea and the conjunctiva (see poster). Theevidence supporting this is based on a variety of studies. Firstly,epithelial cells within the basal layer of the limbal epithelium exhibitcharacteristics of immature, undifferentiated cells that are consistentwith the presence of stem cells (Daniels et al., 2001; Dziasko andDaniels, 2016; Notara et al., 2010a). Specifically, epithelial cellswithin this location lack expression of cytokeratin 3 and 12, whichare expressed by mature, differentiated corneal epithelial cells,while they retain the expression of cytokeratin 14, which isexpressed by immature stem or progenitor cells in the basal layer ofa variety of stratified epithelia (Daniels et al., 2001; Dziasko andDaniels, 2016; Notara et al., 2010a). In addition, many cells withinthe limbus also express putative stem cell markers. These includethe ΔN isoform of p63 (also known as TP63), which is expressed byproliferative stem or progenitor cells in several stratified epithelia,and the transporter protein ABCG2, which confers the so-called‘side-population’ phenotype and is often considered to be auniversal stem cell marker (Daniels et al., 2001; Dziasko andDaniels, 2016; Notara et al., 2010a). Other putative stem cellmarkers expressed by cells within this region include N-cadherinand Fzd7 (Dziasko and Daniels, 2016). Furthermore, it has beendemonstrated that the limbal epithelium contains a high proportionof quiescent cells that rarely divide, a property that is exhibited bylong-lived stem cells in a variety of other tissues (Cotsarelis et al.,1989). Although the expression pattern of these markers is generallyconsistent with the presence of stem cells, it is important toemphasise that a definitive phenotype for corneal epithelial stem

cells, which correlates with bona fide stem cell activity, remains tobe determined.

In light of this, the most convincing evidence supporting thepresence of stem cells in the limbus is the demonstration that cellsisolated from this region can readily generate long-term proliferativeclones in vitro (holoclones) and can reconstitute the cornea upongrafting (Daniels et al., 2001, 2007). Indeed, the utility of stem cellsthat have been isolated from the limbus is powerfully demonstratedby their use in the clinic, as they can be used to regenerate the corneain patients who have suffered severe damage to the ocular surfacefollowing injury or disease (Daniels et al., 2001, 2007).

While the above data supports the hypothesis that stem cellscapable of long-term maintenance of the corneal epithelium arepresent in the limbus, whether or not CESCs reside exclusively inthe limbus remains an open question. A recent investigation foundevidence for stem cell activity throughout the ocular surface in avariety of mammalian species, including mouse, pig and rabbit(Majo et al., 2008). In this study, stem cells could be isolated fromseveral regions of the cornea in addition to the limbus, although theywere present in higher numbers within the limbal epithelium andperipheral cornea. Furthermore, a lineage-tracing experimentdemonstrated that cells derived from the limbus only contribute tothe cornea during repair, and that they remain dormant duringhomeostasis (Majo et al., 2008). These results therefore suggest thatstem cells in the limbus do not significantly contribute to thehomeostasis of the corneal epithelium, but do perform an importantregenerative function following injury.

One possible explanation for these findings may be that theCESCs in the limbus are a dormant population that becomesactivated only during wound healing, whereas CESCs present in thecorneal epithelium perform the bulk of the task of routinehomeostatic maintenance. The presence of multiple populations ofstem cells is consistent with findings obtained from other self-renewing tissues, such as the epidermis, intestine and bone marrow(Arwert et al., 2012; Tian et al., 2011; Wilson et al., 2008).Interestingly, in these tissues, there is considerable evidence thatdifferent stem cell compartments contribute differentially to tissuemaintenance depending on the circumstances. For example, in boththe intestine and the bone marrow, dormant populations of stemcells have been identified, which only become active during woundhealing, whereas a separate stem cell compartment mediates thehomeostatic maintenance of the tissue. It is therefore possible thatlimbal stem cells are a dormant population of stem cells thatfunction only in extreme circumstances, whereas maintenanceduring normal homeostasis is performed by other stem or progenitorcells that are distributed throughout the cornea. Further work isneeded to definitively establish the relative contributions of limbaland non-limbal stem cells during both homeostasis and repair of theocular surface. However, it is clear that the corneal epitheliumcontains stem cells that exhibit remarkable regenerative capacity.

The corneal stem cell nicheThe activity of stem cells in any system needs to be tightly regulatedin order to ensure that a tissue remains in equilibrium. In this respect,the tissue microenvironment, or niche, in which stem cells resideplays a critical role in regulating stem cell fate decisions (Watt andHogan, 2000). The stem cell niche thus represents a highlyspecialised, anatomically distinct region of a tissue that provides theappropriate microenvironmental cues required to maintain apopulation of cells that is capable of meeting the regenerativedemands of a tissue at any given time. Examples of stem cell nichesinclude the crypt base of the small intestine, in which intestinal

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epithelial stem cells reside (Sato et al., 2011), and the bulge regionof hair follicles (see Box 1), where cutaneous epithelial stem cellsare found (Arwert et al., 2012). Within these specialisedmicroenvironments, a variety of signals are provided, whichensure appropriate stem cell activity. However, the precise cellularand molecular mechanisms by which the niche regulates stem cellbehaviour is only beginning to be elucidated. Nevertheless, it isbecoming clear that a variety of niche components, such asvasculature, mesenchymal cells and the extracellular matrix (ECM),play a critical role in providing a range of cues that influence stemcell fate decisions (Watt and Hogan, 2000; Watt and Huck, 2013).These include soluble biochemical factors, mechanical cues andmetabolic factors, as well as cell-contact-dependent signals (Wattand Hogan, 2000).With respect to the cornea, despite the uncertainty surrounding

the location and identity of CESCs in the ocular surface, all of theavailable experimental evidence indicates that they are highlyenriched in the limbal epithelium. The limbus is thus a region ofconsiderable interest with regards to identifying the nichecomponents that regulate CESCs (see poster).In the human ocular surface, there are several features of the

limbus that distinguish it from both the conjunctiva and the cornea.Perhaps most strikingly, the stromal tissue in the limbus formspapillae-like invaginations known as the ‘Palisades of Vogt’, inbetween which are limbal epithelial crypts (see poster) (Dziasko andDaniels, 2016). Within these crypt structures, a high proportion of

basal epithelial cells express putative stem cell markers, such asFzd7, N-cadherin and ABCG2, which is consistent with the notionthat the limbus provides a specialised stromal environment that iscapable of supporting CESCs (Dziasko and Daniels, 2016). Inaddition, the limbal stroma is heavily vascularised and containsdistinct ECM components compared to the corneal stroma (α1 andα2 collagen IV, β2 laminin and vitronectin) (Dziasko and Daniels,2016), all of which may be critical in maintaining CESCs. Thereis also evidence that direct physical interactions betweenmesenchymal cells in the limbal stroma and epithelial cells in thelimbal crypts are important in maintaining the stem cell population(Dziasko and Daniels, 2016).

The molecular mechanisms by which the various components ofthe limbal stroma may regulate the CESC population require furtherelucidation. However, the expression of specific biochemicalfactors by limbal stromal cells has been implicated in stem cellmaintenance. Examples include Wnt ligands (Dziasko and Daniels,2016; Nakatsu et al., 2011; Ouyang et al., 2014), which areimportant in other stem cell niches such as the intestinal crypt(Clevers et al., 2014), and cytokines and chemokines such as IL-6(Notara et al., 2010b). Cell-contact-dependent pathways, such as theNotch signalling cascade, as well as direct interactions with ECMcomponents and vasculature, may also be important (Dziasko andDaniels, 2016).

An additional feature that has been shown to be important inregulating stem cell function is the mechanical properties of thesurrounding tissue. Factors, such as elasticity and topography,have been shown to influence how a cell responds to othermicroenvironmental cues, such as growth factors and/or cytokines(Aragona et al., 2013). In this regard, stem cell niches often exhibita distinct topography and are composed of specific ECMcomponents, each of which will endow the niche with particularmechanical traits (Hsu et al., 2014; Watt and Huck, 2013). In thisregard, the dome-like shape of the cornea is likely to impose distinctmechanical forces at different regions of the tissue (Majo et al.,2008), which may favour stem cell maintenance at specificlocations. Furthermore, the specific ECM composition of thelimbal stroma may also confer distinct mechanical properties.Consistent with this, there is some evidence that the limbus isslightly stiffer than the central cornea (Nowell et al., 2016). Theniche also maintains appropriate metabolic conditions for stem cellmaintenance, including nutrient availability and oxygen tension,each of which can have a profound effect on stem cell function(Rovida et al., 2014).

Thus, the distinct characteristics of the limbal stroma, such asECM composition, vascularisation and growth factor expression,are likely to play an important role in maintaining a functionalpopulation of CESCs. Insight into the cellular and molecularmechanisms by which niche components regulate CESCs may beprovided by examining how cutaneous epithelial stem cells areregulated by their microenvironment (see Box 1). In this tissue,biochemical factors secreted by mesenchymal cells located withinthe stem cell niche have been shown to regulate processes such asstem cell quiescence and activation. For example, expression ofbone morphogenetic proteins (BMPs) by mesenchymal nichecomponents promotes quiescence of cutaneous epithelial stemcells (Plikus et al., 2008), whereas expression of fibroblast growthfactors (FGFs), TGF-β and the BMP inhibitor noggin promote theiractivation and proliferation (Greco et al., 2009; Oshimori and Fuchs,2012). Functional studies in mice have also indicated that thevasculature plays an important role in the activation of cutaneousepithelial stem cells, although the mechanisms remain to be

Box 1. Cutaneous epithelial stem cells and their nicheCutaneous epithelial stem cells maintain the epithelium of the skin andits appendages, as has been reviewed extensively elsewhere (e.g. Hsuet al., 2014). The skin broadly consists of an interfollicular epidermis,which is a stratified, keratinising squamous epithelium, which forms aprotective barrier to the outside world, and its associated appendages,such as the hair follicles. Underlying the epidermis and its appendages isthe dermis, which consists of fibroblasts, nerves, ECM components andblood vessels. Epithelial stem cells have been identified in multiplelocations, including the basal layer of the epidermis, the bulge region ofthe hair follicle and the hair germ. These regions therefore representdistinct stem cell niches. During homeostasis, stem cells in the epidermisrespond to signals from both the niche and from the neighbouringepithelium, and these cells modulate their proliferative activity in order tomaintain the tissue in a state of equilibrium. Following injury, the activityof epidermal stem cells increases significantly, enabling rapid re-epithelialisation and wound closure.In contrast to the epidermis, hair follicles undergo repeated cycles ofregression and regeneration. Each hair cycle consists of a growth phasetermed anagen, a regression phase termed catagen and a resting phasetermed telogen. Epithelial stem cells in the hair follicles reside in twodistinct locations, the bulge region and the hair germ, which is locatedjust below the bulge. Stem cells in the bulge and hair germ are similar atthe molecular level, although stem cells in the hair germ aremore likely toproliferate. During telogen, hair follicle stem cells in both locations displaylow activity and are quiescent. During the early stages of anagen, stemcells in the hair germ become active and proliferate to generate transit-amplifying cells that will ultimately differentiate to form the hair shaft. Thisonset of activity is induced by signals derived from the dermal papilla,which lies just beneath the hair germ and is an essential component ofthe hair follicle stem cell niche, and consists of fibroblasts, ECM andblood vessels. Stem cell activity in the bulge is initiated after that of thehair germ, and may in part be induced by cues derived from the progenyof the hair germ stem cells, in addition to signals from dermal fibroblasts.Because stem cells in the hair follicles undergo cyclic rounds of activityand quiescence, they are an excellent model for studying how the nicheregulates stem cell behaviour and function.

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elucidated (Hsu et al., 2014). Sonic hedgehog (SHH) secretion byother epithelial components of the skin has also been shown topromote cell cycle entry in quiescent cutaneous epithelial stem cells(Hsu et al., 2014). Furthermore, other components present withinthe cutaneous stem cell niche, such as peripheral nerves, immunecells and ECM components have also been implicated in regulatingstem cell activity (Hsu et al., 2014). It will be interesting todetermine whether similar cellular and molecular mechanismsoperate in the cornea to control the function of CESCs. In addition,as referred to above, there are multiple stem cell compartments in theskin that each have their own distinct niche. Therefore, given thatstem cell activity has been demonstrated at multiple locations on theocular surface, it will be important to establish whether distinctniches are also present to support this organ.

Corneal epithelial stem cells and pathologies of the ocularsurfaceThere are several ocular surface disorders in which aberrant functionof CESCs results in pathological changes in the corneal epithelium.One of the most common manifestations is conjunctivalisation ofthe cornea, whereby the corneal epithelium is replaced byconjunctival epithelial cells (Notara et al., 2010a). This is thoughtto occur due to depletion of CESCs in the limbus (limbal stem celldeficiency), resulting in migration of the conjunctiva onto thecorneal surface (Notara et al., 2010a). Because of the significantphenotypic differences between the conjunctival and cornealepithelium, a range of abnormalities ensue, including opacification,neovascularisation and increased susceptibility to injury, all of whichcause considerable pain and can result in corneal blindness (Notaraet al., 2010a). Conditions that cause a depletion of corneal epithelialstem cells include severe chemical or thermal injury, chronic limbitis(inflammation of the limbus), contact lens keratopathy and Stevens–Johnson syndrome (an acute inflammatory disease) (Puangsricharernand Tseng, 1995). In each case, the depletion in stem cells is thoughtto arise from either the primary injury or the resulting inflammatoryresponse.Primary genetic disorders may also induce CESC deficiency. For

example, in the hereditary condition aniridia, haploinsufficiency ofPax6, which is essential for normal ocular development, results in avariety of ocular surface defects, including impaired cornealdifferentiation, conjunctavilisation and neovascularisation (Liet al., 2008). Studies using mice have suggested that the cornealdefects observed in aniridia may result from a reduced activity ofCESCs and thus represent a form of limbal stem cell deficiency(Collinson et al., 2004).Another pathological condition of the corneal epithelium is

squamous cell metaplasia, in which the normally non-keratinisingcorneal epithelium adopts a keratinised phenotype similar to that ofthe epidermis (Notara et al., 2010a). This condition frequentlyoccurs following chronic inflammation of the ocular surface andalso severely impairs visual acuity (Chen et al., 2009). Theunderlying cellular basis of this phenomenon is thought to be thetransdifferentiaion of CESCs or their immediate progeny into akeratinising epithelia, a notion that is supported by several studiesusing mouse models (Mukhopadhyay et al., 2006; Vauclair et al.,2007). Indeed, it is likely that CESCs, like other epithelial stemcells, are multipotent (Bonfanti et al., 2010; Donati and Watt, 2015)and thus have the ability to generate a variety of epithelial lineages,in addition to the cornea. Interestingly, stem or progenitor cellsisolated from the corneal epithelium have been shown to give rise toconjunctival-like cells in vitro (Majo et al., 2008), which could berelevant for corneal conjunctivalisation.

Aberrant stem cell function and changes in the nicheThe loss of normal CESC function during disease may be aconsequence of pathological changes that occur in the stem cellniche (see poster). As described above, many of the pathologies thatultimately result in stem cell deficiency or depletion involveabnormal immune or inflammatory responses. Such abnormalimmune responses can induce profound changes in themicroenvironment to which CESCs are exposed and thus impairtheir maintenance and/or survival. For example, immune-mediatedpathology can damage niche components and induce conditionssuch as fibrosis in the sub-epithelial stroma (Aceves and Ackerman,2009; Bonnans et al., 2014; Notara et al., 2010a). Significantmetabolic changes in the niche can also occur (e.g. changes inoxygen tension and nutrient availability) (Rovida et al., 2014), andthe expression of a variety of growth factors and cytokines can beinduced in mesenchymal stromal cells (Bonnans et al., 2014;Grivennikov et al., 2010). Similar damage to the niche may alsooccur following chemical or thermal injury, either as a directconsequence of the insult or in response to the subsequentinflammatory response induced. Primary genetic disorders mayalso impair stem cell function through alterations to the niche. Forexample, in aniridia, there is evidence that Pax6 deficiency impairsthe interaction between CESCs and mesenchymal cells in the limbalstroma (Ramaesh et al., 2005). Thus, the initial effect of ocularsurface disease or injury might be the destruction or alteration of theniche, which subsequently results in the loss of microenvironmentalcues that would normally maintain CESCs (see poster).

Abnormal function of corneal epithelial stem cells during diseaseor injury might also be a consequence of changes that occur in theniche. For example, squamous cell metaplasia of the ocular surfacethat occurs during chronic inflammation might be a result of nicheremodelling. Indeed, our recent study has shown that chronicinflammation of the ocular surface induces increased ECMdeposition in the corneal stroma, which subsequently leads toincreased tissue stiffness (Nowell et al., 2016). This in turn elicitsaberrant mechanotransduction in corneal epithelial stem and/orprogenitor cells and promotes their conversion into a keratinisingepithelium that resembles the epidermis (see poster). Thus, theabnormal differentiation of corneal epithelial stem and/or progenitorcells is an indirect consequence of microenvironmental changesinduced by chronic inflammation.

In addition to improving our understanding of ocular surfacedisease, further elucidation of how exactly the niche changes duringdisease or injury, and how this subsequently affects CSCE function,will potentially improve the efficacy of CESC transplantation. Theuse of limbus-derived stem cells to restore the corneal surfacefollowing disease or injury is an important clinical application ofCESCs. However, its success can be limited in patients in which thecorneal stroma is severely damaged (Liang et al., 2009; Samsonet al., 2002). Thus, an improved understanding of the factors thatdifferentiate a ‘healthy’ niche from a diseased or dysfunctionalniche may enable the development of clinical strategies that canimprove the outcome of CESC transplantation.

PerspectivesCESCs, like other stem cells, are exquisitely sensitive to themicroenvironment they are exposed to and inappropriatemicroenvironmental cues can result in their depletion and/orimpaired function. Therefore, a more detailed understanding ofhow CESCs are regulated by the niche and how niche componentschange during disease or injury has the potential to result inimproved therapeutic strategies for the treatment of a variety of

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ocular surface disorders. To this end, the identification of molecularmarkers that unambiguously identify CESCs will be a significantadvance, as it will enable researchers to study the interactionsbetween CESCs and their microenvironment in a more refined andcomprehensive manner.

Competing interestsThe authors declare no competing or financial interests.

FundingThe work in the authors’ laboratories was funded in part by OptiSTEM SeventhFramework Programme, the Swiss National Science Foundation and the SwissCancer League.

Cell science at a glanceA high-resolution version of the poster and individual poster panels are available fordownloading at http://jcs.biologists.org/lookup/doi/10.1242/jcs.198119.supplemental

ReferencesAceves, S. S. and Ackerman, S. J. (2009). Relationships between eosinophilicinflammation, tissue remodeling, and fibrosis in eosinophilic esophagitis.Immunol. Allergy Clin. North Am. 29, 197-211, xiii-xiv.

Aragona, M., Panciera, T., Manfrin, A., Giulitti, S., Michielin, F., Elvassore, N.,Dupont, S. and Piccolo, S. (2013). A mechanical checkpoint controlsmulticellular growth through YAP/TAZ regulation by actin-processing factors.Cell 154, 1047-1059.

Arwert, E. N., Hoste, E. andWatt, F. M. (2012). Epithelial stem cells, wound healingand cancer. Nat. Rev. Cancer 12, 170-180.

Bonfanti, P., Claudinot, S., Amici, A. W., Farley, A., Blackburn, C. C. andBarrandon, Y. (2010). Microenvironmental reprogramming of thymic epithelialcells to skin multipotent stem cells. Nature 466, 978-982.

Bonnans, C., Chou, J. andWerb, Z. (2014). Remodelling the extracellular matrix indevelopment and disease. Nat. Rev. Mol. Cell Biol. 15, 786-801.

Chen, Y. T., Li, S., Nikulina, K., Porco, T., Gallup, M. and McNamara, N. (2009).Immune profile of squamous metaplasia development in autoimmune regulator-deficient dry eye. Mol. Vis. 15, 563-576.

Clevers, H., Loh, K. M. and Nusse, R. (2014). Stem cell signaling. An integralprogram for tissue renewal and regeneration: Wnt signaling and stem cell control.Science 346, 1248012.

Collinson, J. M., Chanas, S. A., Hill, R. E. and West, J. D. (2004). Cornealdevelopment, limbal stem cell function, and corneal epithelial cell migration in thePax6+/- mouse. Invest Opthalmol Vis. Sci. 45, 1101-1108.

Cotsarelis, G., Cheng, S.-Z., Dong, G., Sun, T.-T. and Lavker, R. M. (1989).Existence of slow-cycling limbal epithelial basal cells that can be preferentiallystimulated to proliferate: implications on epithelial stem cells. Cell 57, 201-209.

Daniels, J. T., Dart, J. K., Tuft, S. J. and Khaw, P. T. (2001). Corneal stem cells inreview. Wound Repair. Regen. 9, 483-494.

Daniels, J. T., Notara, M., Shortt, A. J., Secker, G., Harris, A. and Tuft, S. J.(2007). Limbal epithelial stem cell therapy. Expert Opin Biol. Ther. 7, 1-3.

Donati, G. andWatt, F. M. (2015). Stem cell heterogeneity and plasticity in epithelia.Cell Stem Cell 16, 465-476.

Dziasko, M. A. and Daniels, J. T. (2016). Anatomical features and cell-cellinteractions in the human limbal epithelial stem cell niche.Ocul. Surf. 14, 322-330.

Greco, V., Chen, T., Rendl, M., Schober, M., Pasolli, H. A., Stokes, N., dela Cruz-Racelis, J. and Fuchs, E. (2009). A two-step mechanism for stem cell activationduring hair regeneration. Cell Stem Cell 4, 155-169.

Grivennikov, S. I., Greten, F. R. and Karin, M. (2010). Immunity, inflammation, andcancer. Cell 140, 883-899.

Hsu, Y.-C., Li, L. and Fuchs, E. (2014). Emerging interactions between skin stemcells and their niches. Nat. Med. 20, 847-856.

Li, W., Chen, Y.-T., Hayashida, Y., Blanco, G., Kheirkah, A., He, H., Chen, S.-Y.,Liu, C.-Y. and Tseng, S. C. G. (2008). Down-regulation of Pax6 is associated with

abnormal differentiation of corneal epithelial surface diseases. J. Pathol. 214,114-122.

Liang, L., Sheha, H., Li, J. and Tseng, S. C. G. (2009). Limbal stem celltransplantation: new progresses and challenges. Eye 23, 1946-1953.

Majo, F., Rochat, A., Nicolas, M., Jaoude, G. A. and Barrandon, Y. (2008).Oligopotent stem cells are distributed throughout the mammalian ocular surface.Nature 456, 250-254.

Mukhopadhyay, M., Gorivodsky, M., Shtrom, S., Grinberg, A., Niehrs, C.,Morasso, M. I. and Westphal, H. (2006). Dkk2 plays an essential role in thecorneal fate of the ocular surface epithelium. Development 133, 2149-2154.

Nakatsu, M. N., Ding, Z., Ng, M. Y., Truong, T. T., Yu, F. and Deng, S. X. (2011).Wnt/β-catenin signaling regulates proliferation of human cornea epithelial stem/progenitor cells. Invest. Ophthalmol. Vis. Sci. 52, 4734-4741.

Notara, M., Alatza, A., Gilfillan, J., Harris, A. R., Levis, H. J., Schrader, S.,Vernon, A. and Daniels, J. T. (2010a). In sickness and in health: cornealepithelial stem cell biology, pathology and therapy. Exp. Eye Res. 90, 188-195.

Notara, M., Shortt, A. J., Galatowicz, G., Calder, V. and Daniels, J. T. (2010b). IL-6 and the human limbal stem cell niche: a mediator of epithelial-stromalinteraction. Stem Cell Res 5, 188-200.

Nowell, C. S., Odermatt, P. D., Azzolin, L., Hohnel, S., Wagner, E. F., Fantner,G. E., Lutolf, M. P., Barrandon, Y., Piccolo, S. and Radtke, F. (2016). Chronicinflammation imposes aberrant cell fate in regenerating epithelia throughmechanotransduction. Nat. Cell Biol. 18, 168-180.

Oshimori, N. and Fuchs, E. (2012). Paracrine TGF-β signaling counterbalancesBMP-mediated repression in hair follicle stem cell activation. Cell Stem Cell 10,63-75.

Ouyang, H., Xue, Y., Lin, Y., Zhang, X., Xi, L., Patel, S., Cai, H., Luo, J., Zhang,M.,Zhang, M. et al. (2014). WNT7A and PAX6 define corneal epitheliumhomeostasis and pathogenesis. Nature 511, 358-361.

Plikus, M. V., Mayer, J. A., de la Cruz, D., Baker, R. E., Maini, P. K., Maxson, R.and Chuong, C.-M. (2008). Cyclic dermal BMP signalling regulates stem cellactivation during hair regeneration. Nature 451, 340-344.

Puangsricharern, V. and Tseng, S. C. G. (1995). Cytologic evidence of cornealdiseases with limbal stem cell deficiency. Opthalmology 102, 1476-1485.

Ramaesh, K., Ramaesh, T., Dutton, G. N. and Dhillon, B. (2005). Evolvingconcepts on the pathogenic mechanisms of aniridia related keratopathy.Int. J. Biochem. Cell Biol. 37, 547-557.

Rovida, E., Peppicelli, S., Bono, S., Bianchini, F., Tusa, I., Cheloni, G., Marzi, I.,Cipolleschi, M. G., Calorini, L. and Sbarba, P. D. (2014). The metabolically-modulated stem cell niche: a dynamic scenario regulating cancer cell phenotypeand resistance to therapy. Cell Cycle 13, 3169-3175.

Samson, C. M., Nduaguba, C., Baltatzis, S. and Foster, C. S. (2002). Limbal stemcell transplantation in chronic inflammatory eye disease. Ophthalmology 109,862-868.

Sato, T., van Es, J. H., Snippert, H. J., Stange, D. E., Vries, R. G., van den Born,M., Barker, N., Shroyer, N. F., van de Wetering, M. and Clevers, H. (2011).Paneth cells constitute the niche for Lgr5 stem cells in intestinal crypts. Nature469, 415-418.

Tian, H., Biehs, B., Warming, S., Leong, K. G., Rangell, L., Klein, O. D. and deSauvage, F. J. (2011). A reserve stem cell population in small intestine rendersLgr5-positive cells dispensable. Nature 478, 255-259.

Vauclair, S., Majo, F., Durham, A.-D., Ghyselinck, N. B., Barrandon, Y. andRadtke, F. (2007). Corneal epithelial cell fate is maintained during repair byNotch1 signaling via the regulation of vitamin A metabolism. Dev. Cell 13,242-253.

Watt, F. M. and Hogan, B. L. (2000). Out of Eden: stem cells and their niches.Science 287, 1427-1430.

Watt, F. M. and Huck, W. T. S. (2013). Role of the extracellular matrix in regulatingstem cell fate. Nat. Rev. Mol. Cell Biol. 14, 467-473.

Wilson, A., Laurenti, E., Oser, G., van der Wath, R. C., Blanco-Bose, W.,Jaworski, M., Offner, S., Dunant, C. F., Eshkind, L., Bockamp, E. et al. (2008).Hematopoietic stem cells reversibly switch from dormancy to self-renewal duringhomeostasis and repair. Cell 135, 1118-1129.

Xuan, M., Wang, S., Liu, X., He, Y., Li, Y. and Zhang, Y. (2016). Proteins of thecorneal stroma: importance in visual function. Cell Tissue Res. 364, 9-16.

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