280 N.A. McBrien

noted that staphyloma formation, although also related to eye size, is often a later occurrence in human high myopes.44,40 Our knowledge of the biomechanical properties of the sclera now dictates that these markedly thinned regional areas, with reduced glycosaminoglycan content and small collagen fibrils, have a reduced resistance to intraocular pressure. Furthermore, the relatively annular organization of these bundles of collagen, around the optic nerve insertion and macular area, result in a local area that is particularly susceptible to the expansive force of the normal intraocular pressure. Other scleral regions, where collagen fiber bundle organization is relatively anisotropic, would be relatively protected against such an occurrence. Indeed, the anatomical area described above corresponds remarkably well with the area of formation of by far the most common type of staphyloma, the type I posterior staphyloma.40 The data presented in this review are consistent with the hypothesis that the localized thinning of the sclera and glycosaminoglycan loss, in conjunction with the scleral collagen fibril diameter changes, interact with the localized orientation of fibril bundles to result in the development of staphyloma.

Biochemical Changes in the Sclera of Myopic Eyes

The marked changes in structure reported in the sclera of eyes with high myopia, and the evidence that this is occurring due to tissue loss, indicate that changes occur in the biochemistry of the sclera. Studies to test this hypothesis have broadly concentrated on investigating three specific aspects of the biochemical processes in the sclera, namely: i) the biochemistry of the structural components of the sclera; ii) the regulation of the degradative processes in the sclera; and iii) the cellular changes that ultimately regulate the structural and biochemical alterations.

Structural biochemistry of the sclera in myopia

The majority of investigations into the biochemistry of scleral structure in myopia have concentrated on the main structural components, namely collagen and proteoglycans. The importance of scleral collagen biochemistry in the myopic eye is illustrated by the results of a study that prevented tropocollagen cross-linking in the sclera, through the use of beta-aminopropionitrile, which inhibits lysyl oxidase activity.18 The

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treatment was found to result in a significant increase in myopia development and significantly increased scleral thinning in the posterior region of form-deprived myopic eyes, indicating that altered scleral biochemistry made the eye markedly more susceptible to the normal expansive intraocular forces. However, there was found to be no observable effect on eye growth in normal eyes, indicating the importance of other underlying changes in scleral properties in myopia development.

Studies in tree shrews and humans have found a reduced collagen content at the posterior pole of the sclera of highly myopic eyes.15,45 Recently, studies have demonstrated reductions of up to 35% in collagen type I mRNA expression in myopic eyes, also suggesting that collagen accumulation in the sclera is reduced due to a decrease in production.46,47 Furthermore, confirmation that the incorporation of the radiolabelled collagen precursor, [3H]-proline, was reduced by a similar magnitude was demonstrated when data was normalized to the extractable collagen content of the sclera, confirming that collagen synthesis is reduced early in myopia development.47 Subsequent investigations demonstrated that [3H]-proline elimination from the sclera was enhanced in a magnitude consistent with the previously reported scleral dry weight changes.47 The above data further strengthens the argument that scleral dry weight loss in myopia development is primarily a result of reduced collagen accumulation. Collectively, one can conclude from the above data that reduced collagen accumulation in the posterior sclera of myopic eyes is driven by both reduced collagen synthesis and increased collagen degradation (see later). Furthermore, these studies demonstrate that the reduction in collagen accumulation is greatest during the early stages of myopia development, which is consistent with the time dependent response of scleral thinning and scleral tissue (dry weight) loss.19,43

A recent study investigated scleral expression of the quantitatively minor fibrillar collagen subtypes III and V. Type V collagen, in particular, is important in the control of fibril diameter in the cornea, and therefore represents a candidate for the regulation of the collagen diameter changes found in the sclera of myopic eyes. Studies have found that although collagen type I mRNA expression is reduced in eyes developing myopia, there are no changes in collagen types III and V expression between myopic and control eyes.47 The reduced type I collagen production and stable type III and V levels result in a 20% increase in the type V/type I and type III/type I collagen ratio. Thus, in relative terms, newly synthesised fibrils in myopic eyes may contain 20% more type V collagen. Previous reports in the

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whether proteoglycan content is reduced in conjunction with glycosaminoglycan content, the important role that the negative charge density plays in the control of extracellular matrix mechanics51 implicates glycosaminoglycans in the mediation of the earliest biomechanical changes in myopic eyes. These observations are consistent with the hypothesis that scleral glycosaminoglycan content is a major factor underlying the early changes in the visco-elastic properties of the sclera that are characteristic of myopic eyes.

Degradative processes in the sclera of myopic eyes

Studies have shown a change in the activity of collagen degrading enzymes in the sclera of myopic eyes. Matrix metalloproteinase-2 (MMP-2) has been shown to be important both in the degradation of native collagen fibrils and in the further degradation of their breakdown products.52 The enzyme is secreted in a pro-enzyme, or latent, form and is then activated at the cell membrane by cleavage of the latency-conferring terminus of the pro-enzyme.53 Studies in mammalian models have shown that MMP-2 activity is increased in the sclera of myopic eyes (Fig. 6A).26 Protein analyses of the latent and active forms of the enzyme show a supplementation of the latent pools of the enzyme in the sclera, but the major change during myopia development is a three-fold increase in levels of the active form of the enzyme (Fig. 6B).26 Subsequent studies of MMP-2 confirm a small increase in the mRNA expression of latent MMP-2.27 However, this increase does not match the relative increase in levels of active MMP-2 in the sclera of myopic eyes, indicating the major change is related to activation of latent MMP-2 and not increased production of MMP-2 (Fig. 6C).

Findings to date are consistent with the hypothesis that increased degradative activity, and possibly net reduction in the inhibition of this activity, drives the scleral thinning and tissue loss seen in myopic eyes. However, it should be remembered that biochemical data also indicates there is a concomitant decrease in the synthesis of new structural material in the extracellular matrix, which contributes to the reduced accumulation of the scleral matrix.

Cellular changes in the sclera in myopia

Reduced DNA synthesis accompanies tissue remodelling in the sclera of myopic eyes, implicating cellular changes of the scleral fibroblasts, which