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TitleProgenitors for the corneal endothelium and trabecularmeshwork: a potential source for personalized stem cell therapyin corneal endothelial diseases and glaucoma
Author(s) Yu, WY; Sheridan, C; Grierson, I; Mason, S; Kearns, V; Lo, ACY;Wong, D
Citation Journal of Biomedicine and Biotechnology, 2011, v. 2011, articleno. 412743
Issued Date 2011
Rights This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
Hindawi Publishing CorporationJournal of Biomedicine and BiotechnologyVolume 2011, Article ID 412743, 13 pagesdoi:10.1155/2011/412743
Progenitors for the Corneal Endothelium and TrabecularMeshwork: A Potential Source for Personalized Stem Cell Therapyin Corneal Endothelial Diseases and Glaucoma
Wing Yan Yu,1 Carl Sheridan,2 Ian Grierson,2 Sharon Mason,2 Victoria Kearns,2
Amy Cheuk Yin Lo,1, 3 and David Wong1, 3, 4
1 Eye Institute, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong2 Department of Eye and Vision Science, Institute of Ageing and Chronic Disease, University of Liverpool,University Clinical Departments Building, The Duncan Building, Daulby Street, Liverpool L69 3GA, UK
3 Research Center of Heart, Brain, Hormone and Healthy Aging, Li Ka Shing Faculty of Medicine,The University of Hong Kong, Pokfulam, Hong Kong
4 St. Pauls Eye Unit, Royal Liverpool University Hospital, Prescot Street, Liverpool L7 8XP, UK
Correspondence should be addressed to Carl Sheridan, email@example.com and David Wong, firstname.lastname@example.org
Received 10 August 2011; Accepted 8 September 2011
Academic Editor: Ken-ichi Isobe
Copyright 2011 Wing Yan Yu et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Several adult stem cell types have been found in different parts of the eye, including the corneal epithelium, conjunctiva, andretina. In addition to these, there have been accumulating evidence that some stem-like cells reside in the transition area betweenthe peripheral corneal endothelium (CE) and the anterior nonfiltering portion of the trabecular meshwork (TM), which is knownas the Schwalbes Ring region. These stem/progenitor cells may supply new cells for the CE and TM. In fact, the CE and TM sharecertain similarities in terms of their embryonic origin and proliferative capacity in vivo. In this paper, we discuss the putativestem cell source which has the potential for replacement of lost and nonfunctional cells in CE diseases and glaucoma. The futuredevelopment of personalized stem cell therapies for the CE and TM may reduce the requirement of corneal grafts and surgicaltreatments in glaucoma.
Rapid progress in stem cell research in recent years providesnew hope for the treatment of various previously incurablediseases. The basic treatment principle is to replace lost ordamaged cells with healthy ones derived from stem cellsand/or stimulate endogenous regeneration via paracrineeffects mediated by the transplanted stem cells [1, 2].Stem cells can reasonably be categorized into three maintypes according to their origins: embryonic stem (ES) cells,induced pluripotent stem cells (iPSCs), and adult stem cells.The focus of this paper will concern a specific region ofthe eye, namely, the adult stem cells of the human cornealendothelium (CE) and neighbouring trabecular meshwork(TM), there will be considerable overlap in the techniquesapplied to the differentiation of cells and obstacles to beaddressed before regeneration therapies are available.
2. Stem Cell Overview
Whilst ES cells have numerous advantages in research suchas their unlimited capacity to self-renew and pluripotencyallowing them to differentiate into any cell type in the body,the fear of teratoma formation, immune-rejection issues, andethical concerns regarding the destruction of embryos haveslowed their progress towards clinical trials.
The research field of iPSCs has rapidly gained momen-tum since the discovery by Takahashi and Yamanaka .iPSCs afford an advantage in that an autologous approachmay be possible and as such circumventing the ethical andimmunological disadvantages of ES cells. However, thereare major safety concerns that involvement of retroviral orlentiviral vector integration in iPSCs engineering may causegenomic disruption and oncogenesis . Besides, afterthe reports proving iPSCs retained epigenetic memory from
2 Journal of Biomedicine and Biotechnology
the somatic cell of origin [7, 8], questions have been raisedwhether iPSCs are completely pluripotent like ES cells, ashow far back they are reprogrammed will influence theirdirected differentiation potential. Despite the bright futureiPSCs may have, extensive efforts and a measured scientificapproach are required to guarantee safety and productionquality, to understand more about the molecular signalingand pathways, and to find out reliable differentiation proto-cols of iPSCs before transplantation can be done in patients.
Adult stem or progenitor cells are also referred to assomatic stem cells. They reside in many adult tissues suchas the bone marrow, skeletal muscle, heart, brain, skin, andlimbus (summarized in ). Although they are not plurip-otent, they still retain high plasticity. Their ethical superior-ity over embryonic stem cells and autologous origin avoidingimmunological suppression after surgery makes them a pop-ular stem cell source for small-scale clinical application[10, 11]. Adult stem cells are enriched in locations that arevery close to the target tissue, as such, they may have moredirect and faster access to the site of injury when com-pared with other stem cell types . In addition, theyhave already undergone critical developmental stages, whichrender them comparatively easier to commit to the celltypes desired [12, 13]. Understandably, difficulties suchas their isolation, expansion efficiency due to limited cellnumbers and integration and survival in the host tissue stillremain to be unravelled. Adult stem cells, albeit with theirown limitations, may be a relatively safer and more ethicalalternative cell source for therapeutic use at present.
In the eye, the most successful stem-cell-based therapy todate has been the use of limbal epithelial stem cells toregenerate the corneal epithelium . Apart from limbalepithelial stem cells, intensive research has been done ondifferent niches of adult ocular stem cells, such as conjunc-tival epithelial and retinal stem cells, aiming for ocular repairand regeneration [13, 15, 16]. One niche of cells that hashad relatively limited attention and may be of considerableclinical value (which is the focus of this paper) are theprogenitor cells located in the transition zone between theperiphery of the CE and the anterior extension of the TM,which is known as the Schwalbes Ring region. In the recentdecade, more and more evidence emerged to support theidea that these progenitors may be able to provide new cellsfor the CE, TM, or possibly both. This opens up a newprospect on research using these intriguing progenitors totreat CE diseases and glaucoma. In this paper, we will reviewthe biological properties of CE and TM cells, summarize anddiscuss the evidence suggesting the presence of stem-like cellsin the transition area, and, in addition, outline how thesecells can be used for regeneration.
3. CE and TM: Structure, Function,and Embryology
3.1. Corneal Endothelium. Cornea is the transparent tissuelocated at the front of the eye which provides approximatelytwo-thirds of the total ocular refractive power (Figure 1(a)).It consists of five layers: the multilayered epithelium,
Bowmans membrane, the stroma, Descemets membrane,and the endothelium. CE is on the posterior surface of thecornea facing the anterior chamber. It is composed of asingle layer of regularly arranged hexagonal and pentagonalcells which are around 5 m thick and 20 m in diameter. The crucial function of the CE is to maintain cornealtransparency by regulating corneal hydration while allowingnutrients from the aqueous to diffuse back to the avascularcornea. The endothelium accomplishes this by a pump-leak model. It serves as a leaky barrier to permit selectivepermeability of the nutrients but prevents bulk fluid flowinto the stroma. At the same time, it actively removes excessfluid from the stroma into the anterior chamber throughionic fluid pumps to prevent corneal swelling. In additionto the barrier and pump functions, the endothelial cells areresponsible for the synthesis of Descemets membrane, whichis the basement membrane where the endothelium resides[18, 19].
3.2. Trabecular Meshwork. The anterior chamber of the eyeis bordered anteriorly by the CE and posteriorly by the iris.At the periphery of the chamber, there lie the TM, scleralspur, ciliary body, and iris root, which form the anteriorchamber angle (Figure 1(b)). The TM extends from ananatomical position called Schwalbes line, which marks theend of Descemets membrane, to the ciliary body and irisroot at their junction. There is a specific cell populationnear the transition area (Schwalbes line) between cornea andmeshwork, and this will be discussed in detail in Section 5.Together with Schlemms canal, the collector channels, andaqueous veins, the TM forms the major structure for aqueoushumour outflow . Aqueous humour is produced by theciliary body and passes through the pupil into the anteriorchamber. It subsequently leaves the eye through the TM intoSchlemms canal, then from there to the intrascleral plexus,and