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Gross Scleral Anatomy
The mature sclera forms a spheroidal shell and accounts for some 85% of the total ocular surface. It is enclosed by the episclera, a loose connective tissue connecting the sclera with the overlying conjunctiva anteriorly and generally continuous with the tissue of Tenon’s capsule elsewhere on the globe. In humans, the sclera gradually thickens from the anterior/equatorial regions towards the posterior to reach a maximum thickness of approximately 1 mm at the posterior pole. Although it is essentially continuous, the sclera undergoes a number of specific regional modifications to its gross structure to facilitate rectus muscle insertions, the exit of the optic nerve fiber bundles at the lamina cribosa, also acting as a conduit for the central retinal artery and vein and a number of other nerves and vessels en route to anterior ocular structures. Post-natal scleral growth displays a characteristic anterior–posterior growth axis, as is the case in the embryonic sclera.20
Structural organization of the sclera
The sclera is a typical fibrous connective tissue predominantly consisting of collagen. In mammals, collagen accounts for as much as 90% of the scleral dry weight and the vast majority of this collagen (as much as 99%) has been estimated to be type I collagen.21 However, low levels of other fibrillar collagen subtypes, including type III and V have also been reported in the mammalian sclera, and it is possible to attribute likely roles to each of these subtypes.22,11 Scleral collagen fibrils are largely heterologous. Collagen type V has been found to be important in regulating fibril diameter during fibrillogenesis, as evidenced by the very high collagen type V concentration in the cornea to produce a uniform collagen fibril diameter.23 Other reported collagen subtypes of the sclera include types VI and XII, both of which are considered fibril-associated collagens, and the nonfibril forming collagen types VIII and XIII.
Proteoglycans are also a major component of the scleral extracellular matrix. A number of different proteoglycans, all consisting of a genetically distinct core protein and one or more attached glycosaminoglycan side chains, have been reported within the mammalian sclera. The mammalian sclera is rich in hyaluronan, a unique, non-sulphated glycosaminoglycan that does not associate with a core protein of its own. The sclera also contains large amounts of dermatan and chondroitin sulphate-based
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proteoglycans, particularly the small proteoglycans, decorin, and biglycan.16 These small proteoglycans play an important role in regulating collagen fibril assembly and interaction.24 In addition to these proteoglycans, larger proteoglycans, such as aggrecan, are also present in the scleral extracellular matrix. These ‘aggregating’ proteoglycans, with many glycosaminoglycan side chains, are likely to be important in the regulation of scleral hydration.
Remodelling of the structural matrix of the sclera has been shown to be mediated by a number of protease enzymes, the most extensively studied of these being the matrix metalloproteinase (MMP) family. Members of the gelatinase (MMP-2 and MMP-9) and stromelysin (MMP-3) families are present in the sclera and are involved in scleral remodelling during growth and development, since these enzymes are all known to be involved in the breakdown of collagen.25–27 Members of the collagenase family, most notably MMP-1, are also present in the sclera, particularly in anterior regions of the primate sclera, where they are thought to play a role in mediating the uveoscleral aqueous outflow pathway.28 At least two of the four natural regulators of MMPs, the tissue inhibitors of matrix metalloproteinase (TIMPs), are also present in the sclera with reports of TIMP-1 and TIMP-2 in mammalian species.29,27
Cellular content of the sclera
The structural organization of the sclera is largely reliant on the activity of the major extracellular matrix-producing cell, the fibroblast. Other cells, such as melanocytes and the normal transient population of inflammatory response cells are found in the mammalian sclera and are thought to derive from the choroid.30 The scleral fibroblasts, which reside between the collagen fiber bundle lamellae, are typically described as having a flattened spindle shape with a flattened nucleus. They have long branching processes that reach across relatively long distances.
Scleral fibroblasts, like many other cell types, express integrins, such as the α1, α2, and β1 subtypes.31 It is likely that clustering of integrin receptors, mediate scleral fibroblast communication with the extracellular matrix. Cell–cell communication within the scleral extracellular matrix is mediated through a complex cascade of growth factors, and among those currently identified within the scleral extracellular matrix are members of the insulin-like growth factor (IGF-I and IGF-II), transforming growth factor-beta (TGF-β1, 2 and 3), and fibroblast growth factor (FGF-2)
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families.11 In addition, a high-affinity FGF-2 receptor, FGFR-1, has also been found to be expressed.32 More recent studies have demonstrated that, in addition to expressing the expected collagen, MMP and TIMP subtypes, fibroblasts express mRNA for the muscarinic receptor subtypes M1, M2, M3, M4, and M5.33 Another finding of particular interest in terms of the biomechanical strength of the sclera is the fact that many scleral fibroblasts display a myofibroblastic phenotype in that they express α-smooth muscle actin, organized within the cytoskeletal architecture.34–36 The sclera is one of the few structures in the body that has a constant population of myofribroblasts.
Mechanical properties of the sclera
The biomechanical properties of the sclera are dependent upon a number of aspects of the scleral extracellular matrix, which can broadly be discussed within three categories. The first is the scleral structure itself, namely its thickness, the collagen fibril parameters, namely the organization of the collagen fiber bundles and the rate at which the scleral extracellular matrix is turned over. Another important determinant of the sclera’s mechanical properties is its level of hydration, which, in the absence of a barrier of epithelial or endothelial cells, is likely to be controlled by the hydrophylic carbohydrates, particularly the glycosaminoglycans. The final contributor to the mechanical properties of the sclera is the scleral fibroblasts themselves, which have recently been shown to display a myofibroblastic phenotype, thus endowing them with contractile ability.37 Myofibroblasts are generally defined as highly contractile cells that express the smooth muscle protein, α-SMA.34 Typically arising from fibroblast differentiation, these cells are capable of rapid contractile responses to imposed tissue stress, thus relieving tension within, and limiting expansion of, the surrounding matrix.38,39 These cells also control their local environment through remodelling of the surrounding extracellular matrix, strengthening it and relieving cellular stress. Characterization of the myofibroblast population of the sclera has thus far been limited, although the presence of myofibroblasts in the sclera has to date been demonstrated in all of the mammalian species assessed. Studies in human, monkey, tree shrew, and guinea pig sclera suggest that myofibroblasts comprise a subset of scleral cells, with one study suggesting an age-dependant increase in the proportion of myofibroblasts.34,35 These findings imply that scleral myofibroblasts are less prevalent when the eye is growing most rapidly (the