The presence and distribution of elastin in the posterior and retrobulbar regions of the mouse eye

Abstract

The Presence and distribution of elastin in the posterior and retrobulbar regions of the mouse eye was investigated. Mice of two strains (C57/BL6 and DBA/2J) were studied at 2 months and 8–12 months of age. Light, confocal, and transmission electron microscopy were used to identify elastin, using immunohistochemical techniques and ultrastructural evaluation. Elastin was found in the following ocular structures: conjunctiva, muscle tendons, sclera, choroid, and meninges. The elastin in the sclera was most dense in a ring surrounding the peripapillary optic nerve head, with its presence in the inner sclera declining with greater distance from the nerve head. Elastin fibers were oriented in the sclera along what would be expected to be the principal stress directions generated from the intraocular pressure, though actual biomechanical measurements have not yet been made in the mouse sclera. Elastin comprises a portion of the mouse sclera and its distribution in the peripapillary area is similar to that in human eyes.

Introduction

The internal structures of the eye are enclosed within the corneoscleral shell, made up of connective tissues that serve as both the primary refracting surface and load-bearing structure. The sclera has a central role in maintaining the mechanical integrity of the pressurized eye. Changes in intraocular pressure (IOP) cause scleral deformation that is transmitted to the optic nerve head, the site of glaucoma damage (Quigley et al., 1981), causing potentially damaging deformation (Sigal et al., 2005). An alteration in the structure or the material properties of the sclera will affect the deformation response to a given level of IOP. IOP and ocular blood flow provide both constant and pulsating stress in the eyewall that must be tolerated without permanent damage to its principal elements: collagen, elastin, and glycosaminoglycans.

Collagens I and III impart inherent tensile strength to the human sclera, complemented by elastin, which permits elastic deformation and recovery (Kielty et al., 2002). This function is critical to the maintenance of scleral integrity given the repeated cycles of loading produced by the choroidal vessels (Faury, 2001). Elastin is a complex of deposited tropoelastin on a template of fibrillin-rich microfibrils (Mecham and Davis, 1994). Tropoelastin is a 60–70 kDa protein composed of alternating hydrophobic and lysine-containing cross-linking domains (Gray et al., 1973). Of approximately 40 lysine residues in the secreted tropoelastin monomer, many are cross-linked by lysyl oxidase producing great stability and insolubility of the protein. Elastin has a typical longevity equal to the human lifespan (Shapiro et al., 1991) and its multi-step assembly pattern is tissue-specific, indicating that renovation or replacement may be difficult in adult tissues (Wagensweil and Mecham, 2007). Mice lacking both alleles of the one elastin gene die at or before birth, while those with one normal allele live a short time, but have defective arteries with increased numbers of elastic lamellae (Dietz and Mecham, 2000).

The microfibrils surrounding the elastin core may participate in extensibility of the complex (Wang et al., 2009). Fibrillin interacts with transforming growth factor β, regulating its activation, and abnormality in this function is related to development of Marfan syndrome (Neptune et al., 2003). Fibrillin-containing microfilaments are also found without elastin in the lens zonules.

Alterations in elastin may be either a result of glaucomatous damage to the eye or may participate in its causation. Hernandez and colleagues detected changes in elastin in human glaucoma eyes (Hernandez et al., 1989, Hernandez, 1992, Pena et al., 1998). In studies of both human and monkey eyes with glaucoma damage, our laboratory found an altered appearance of elastin without actual loss of fibers (Quigley et al., 1991a, Quigley et al., 1991b, Quigley et al., 1994, Quigley et al., 1996). We speculated that elastin was seemingly disconnected from the remainder of the scleral and optic nerve head connective tissue matrix. Since African-derived persons have a greater prevalence of glaucoma compared to European-derived persons, it is interesting that they exhibit differences in elastin appearance in the optic nerve head region (Urban et al., 2007). Intriguingly, polymorphisms in the lysyl oxidase-like 1 gene are associated with exfoliation syndrome, the most common syndrome associated with open angle glaucoma (Thorleifsson et al., 2007). The protein coded by this gene participates in modification of elastin.

Animal models of glaucoma have added important information to our understanding of its pathogenesis, with recent use of both spontaneous (Jakobs et al., 2005) and induced (Grozdanic et al., 2003) mouse models of glaucoma. The relevance of mouse research in glaucoma depends upon the degree of homology of important ocular structures to the human eye. We did not find previous descriptions of the detailed structure of the sclera in the mouse, nor any prior mention of the presence or distribution of murine ocular elastin. Given its relevance to the biomechanical response of the eye to IOP, the present report presents the first description of the distribution of elastin in mouse sclera.