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Tissue Engineering of the Corneal Endothelium: a Review of Carrier Materials

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Tissue Engineering of the Corneal Endothelium: a Review of Carrier Materials J. Funct. Biomater. 2013, 4, 178-208; doi:10.3390/jfb4040178 Journal of Functional Biomaterials ISSN 2079-4983 www.mdpi.com/journal/jfb/ Review Tissue Engineering of the Corneal Endothelium: A Review of Carrier Materials Juliane Teichmann 1,2,†, Monika Valtink 2,†,*, Mirko Nitschke 1, Stefan Gramm 1, Richard H.W. Funk 2,3, Katrin Engelmann 3,4 and Carsten Werner 1,3 1 Leibniz Institute of Polymer Research Dresden, Max Bergmann Center of Biomaterials, Institute of Biofunctional Polymer Materials, Hohe Straße 6, Dresden 01069, Germany; E-Mails: [email protected] (J.T.); [email protected] (M.N.); [email protected] (S.G.); [email protected] (C.W.) 2 Institute of Anatomy, Medical Faculty Carl Gustav Carus, Technische Universität Dresden, Fetscherstraße 74, Dresden 01307, Germany; E-Mail: [email protected] 3 CRTD/DFG-Center for Regenerative Therapies Dresden—Cluster of Excellence, Fetscherstraße 105, Dresden 01307, Germany 4 Department of Ophthalmology, Klinikum Chemnitz gGmbH, Flemmingstraße 2, Chemnitz 09116, Germany; E-Mail: [email protected] † These authors contribute equally to the work. * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +49-351-458-6124; Fax: +49-351-458-6303. Received: 3 August 2013; in revised form: 13 September 2013 / Accepted: 24 September 2013 / Published: 22 October 2013 Abstract: Functional impairment of the human corneal endothelium can lead to corneal blindness. In order to meet the high demand for transplants with an appropriate human corneal endothelial cell density as a prerequisite for corneal function, several tissue engineering techniques have been developed to generate transplantable endothelial cell sheets. These approaches range from the use of natural membranes, biological polymers and biosynthetic material compositions, to completely synthetic materials as matrices for corneal endothelial cell sheet generation. This review gives an overview about currently used materials for the generation of transplantable corneal endothelial cell sheets with a special focus on thermo-responsive polymer coatings. J. Funct. Biomater. 2013, 4 179 Keywords: tissue engineering; corneal endothelium; corneal endothelial cell sheets; natural membranes; biological polymers; thermo-responsive polymers; physicochemical properties; biomolecular functionalization 1. Introduction 1.1. The Cornea The cornea is a transparent tissue with a diffractive capacity of about 43 diopters that allows light to enter the eye and evoke a visual sensation in the retina. It is a bradytrophic tissue, meaning that it is avascular and not nourished via the blood system, but subsists on tear film and aqueous humor [1]. The cornea has a dome-shaped structure with a strictly layered architecture (Figure 1) [2,3]. Its average central thickness ranges from 530 µm to 550 µm [4] and increases to an average peripheral thickness of up to 670 µm [5]. The anterior cornea is covered by a stratified squamous epithelium, which produces mucins to retain the tear film. Epithelial progenitor cells reside in niches at the anterior corneal limbus and ensure a continuous regeneration of the epithelium. These limbal stem cells proliferate and migrate centripetally, thereby stratifying and differentiating [6,7]. The basal epithelial cells reside on Bowman’s membrane, a condensed layer comprised of mainly collagen type I that cannot be reproduced once it is destroyed. This membrane is penetrated by sensitive nerve fibers that form a subepithelial plexus in the central cornea [8,9]. The corneal stroma makes up to about 90% of the corneal thickness and is predominantly composed of collagen type I layers. These layers are interspersed with keratocytes that produce mainly proteoglycans and collagen. The collagen fibers are arranged in parallel in lamellae, and the direction of fibers in neighboring lamellae is mostly perpendicular [10–12]. Corneal avascularity and the lattice structure of stromal collagen fibers contribute to corneal transparency. Hydration of the corneal stromal plays a major role in maintaining this transparency, as glycosaminoglycan side chains of the proteoglycans, such as keratan sulfate, dermatan sulfate and chondroitin sulfate, exert a high swell pressure due to their water binding capacity. Excessive fluid imbibition by the stroma, usually due to a dysfunction in the corneal hydration regulatory system, can lead to corneal edema with haze or opacification and can eventually result in blindness. The posterior side of the corneal stroma is lined by Descemet’s membrane, the basal membrane of the posterior epithelium of the cornea, the corneal endothelium. Descemet’s membrane can grow up to 10–12 µm in thickness and is mainly comprised of collagens type IV and VIII [11], but does also contain fibronectin, vitronectin and various laminins [13]. This membrane adheres only weakly to the stroma and can be detracted. The corneal endothelium is a monolayer of hexagonal, squamous cells lining the posterior cornea [14]. Contrasting to the anterior epithelium, the posterior corneal endothelium cannot be regenerated, although limited regeneration has been observed in some cases after graft failure in younger patients (own clinical observation), and a recent report suggests that a minor regenerative capacity may exist in the utmost periphery in close proximity to the trabecular meshwork [15]. J. Funct. Biomater. 2013, 4 180 Figure 1. Structure of the human cornea (A) The cornea is composed of three cellular layers: (1) epithelium; (2) stroma; and (3) endothelium; (B) The epithelial layer resides on Bowman’s membrane (*); and (C) the endothelial layer resides on Descemet’s membrane (**). 1.2. The Corneal Endothelium With age, central corneal endothelial cell density gradually declines at a rate of 0.5%–0.6% per year [16]. This endothelial cell loss is compensated for by migration and enlargement of neighboring cells, but not by regeneration [14,17], resulting in an endothelium with a reduced number of hexagonal cells (pleomorphism) and greater variations in cell size (polymegathism) [10,18]. Proliferation of corneal endothelial cells in situ is suppressed by contact inhibition and by transforming growth factor β2 (TGF-β2), which is secreted into the aqueous humor and prevents entry into the S-phase of the cell cycle [19]. In addition, structural and compositional differences in adult versus embryonic Descemet’s membrane may contribute to G1-phase arrest of human corneal endothelial cells (HCEC) [19], e.g., collagen type III may stimulate cell proliferation, but interacts directly with HCEC only during embryonic development. Although cell division is inhibited in vivo, the cells are able to proliferate in vitro. Primary HCEC were shown to react towards mitogens and growth factors in an age-dependent mode, being more sensitive from younger than from older donors [20–22]. It could also be shown that wounding could induce a limited proliferative capacity in peripheral HCEC in situ and ex vivo, which was, to some extent, dependent on donor age [22]. In this context, it was also demonstrated that p53, a negative cell cycle regulator, is predominantly expressed in the central cornea [23]. These findings led to the hypothesis that the posterior periphery of the cornea may harbor a stem cell niche for corneal endothelial and trabecular meshwork cells, similarly to the anterior limbal stem cell niche of the corneal epithelium [15,24]. This hypothesis is supported by the observation that the stem cell markers, nestin, alkaline phosphatase and telomerase, were found in the posterior corneal periphery in the endothelium and trabecular meshwork around Schwalbe’s line. Upon wounding, additional stem cell markers, Oct-3/4 and Wnt-1, and differentiation markers, Pax-6 and Sox-2, were upregulated in the posterior periphery [15]. Such posterior stem cells might contribute to endothelial regeneration and repair in the periphery. J. Funct. Biomater. 2013, 4 181 Corneal endothelial cells form an anatomical and physiological barrier between the corneal stroma and the anterior chamber. They sustain an active transport of fluid at a rate of 3.5–6 µL cm−2 h−1 from the stroma into the anterior chamber, thereby regulating stromal hydration [10,25]. The so-called “pump-leak” hypothesis describes the mechanisms by which corneal hydration is maintained constant [10,26–28]. Tight junctions in the corneal endothelium are required to seal off both the stroma and the anterior chamber. Although the tight junctions are interrupted and the seal is not complete, they control the directed, extracellular diffusion of ions and water from the anterior chamber into the corneal stroma (“leaky” barrier [29]). Intact tight junctions also restrict a lateral diffusion of membrane proteins, such as ion transporters, which are required for an active transport of ions and water from the stroma into the anterior chamber (“pump” function). This delimitation supports the apical-basal polarity of the endothelial cells (“fence” function) and the difference of potential across the endothelial layer, thereby sustaining a local osmotic gradient by preventing an uncontrolled paracellular reflux (“gate” function). Two main theories describe the coaction of ion pumps and ion channels involved in transendothelial transport of ions and water. The theory set up by David M. Maurice [25,30] states that basolaterally localized Na+/K+-ATPase [10,31] actively sustains an osmotic gradient that drives secondary transporters [32]. A passive flow of water from the stroma
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