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ultimately regulate the process of scleral remodelling.54 However, these studies also report that total DNA content of the sclera is not significantly altered after periods of myopia development. Thus, although DNA synthesis is reduced in myopic eyes, there is no net change in the number of scleral fibroblasts.45,54 There is, however, found to be an increase in the number of scleral cells per unit dry weight in the sclera of myopic eyes, which might be expected if cell numbers remain the same but matrix tissue is lost. One possible explanation is that the reduced overall metabolic demand on the scleral cells of myopic eyes results in a concurrent reduction in the number of cells committing to apoptosis, thus offsetting the reduced number of mitotic events in the sclera.54

Biomechanical Changes in the Sclera of Myopic Eyes

As a major component influencing the mechanical properties of any biological tissue is thickness, one would anticipate changes in the biomechanical strength of the sclera based solely on the reported structural changes in myopia.19 Indeed, studies have shown that strips of scleral tissue from myopic human and tree shrew eyes have a greater initial extensibility (elasticity) in response to an imposed load than control tissues from normal eyes, and this occurs in both the posterior sclera and equatorial sclera.18,38,55 This difference is predominantly a result of the thinner sclera of the myopic eye, rather than a change in the particular properties of the sclera, since the modulus of elasticity was found to be similar between myopic and control eyes. Importantly, however, finite element modelling, using the scleral properties determined in this experiment, suggested that this simple elastic stretch could account for no more than 20% of the increase in eye size in myopia.38 This finding demonstrates that scleral thinning alone cannot account for the majority of ocular enlargement that occurs in myopic eyes and strongly implicates there is a significant contribution from other material properties of the sclera in facilitating changes in eye size during myopia development.

More recent studies have investigated the visco-elastic, time-dependent response (creep) of the sclera from myopic tree shrew eyes to a constant load over time. The data demonstrated that scleral creep rates were higher in samples from the posterior sclera of myopic eyes, even when correction is made for the cross-sectional area of the tissue samples (Figs. 7A and B).56

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Findings from equatorial scleral samples in tree shrews also show increased creep rates in myopic eyes.56 This indicates that the visco-elastic properties of the sclera are indeed markedly altered in myopic eyes.55,56 These changes occur as early as four days into the process of myopia development, when the sclera displays a creep rate in excess of 200% that of control eyes.55 Of particular significance is the finding that there is a strong correlation between the actual degree of myopia induced and the creep properties of the mammalian sclera (Figs. 7C and D).56 These findings demonstrate a direct relationship between the degree of myopia and the material properties of the sclera in mammalian eyes. The data also indicates that in an eye with a weakened sclera due to a change in material properties and given sufficient time, physiological intraocular pressures may be sufficient to induce continuing progressive ocular enlargement.

The data from ultrastructural and biomechanical studies of the sclera from myopic eyes consistently demonstrate that, in early myopia development, scleral tissue loss rapidly results in scleral thinning. This scleral thinning contributes to, but cannot account for, the majority of the early changes in eye size. Furthermore, collagen fibril diameter, a structural element that influences the elastic modulus of the collagen matrix in a number of tissues, is only found to be reduced after myopia has been present for an extended period. These findings demonstrate that changes in other properties of the scleral matrix, such as glycosaminoglycan charge density and/or tissue hydration, may be more important in the early alterations of scleral biomechanical properties found in myopic eyes.

Unlike data from the in vitro studies described earlier, changes in axial length act as a surrogate for the extensibility of the eye, and allow estimates of both the elastic and creep behavior of the eye to be determined in vivo. When intraocular pressure is increased, both avian and mammalian eyes exhibit an initial elastic response to the imposed intraocular pressure rise, followed in the avian eye by a gradual, creep extension (Fig. 8A).35 However, in the mammalian eye, this initial elastic enlargement of the eye is subsequently offset by a gradual shortening of the eye, yielding negative creep values (eye gets shorter). When the intraocular pressure was returned to normal, the eye had become shorter than its original starting value (Fig. 8B). The rapidity of the shortening response (less than one hour) cannot be explained in terms of scleral matrix remodelling, implicating the scleral cells themselves in the physical

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Figure 7. Scleral creep extension curves for samples from myopic, fellow control, and normal eyes, and the relation between scleral creep rates and vitreous chamber elongation and myopia progression. A. Complete scleral creep extension vs. time curves from a highly myopic and fellow control eye of a tree shrew, following 12 days of myopia progression. B. Averaged creep extension vs. time curves from highly myopic, fellow control, and agematched normal eyes in tree shrews following 12 days of myopia development. For each sample, creep extension is the percentage of sample length at 300 seconds after the application of the 5 g load. n = 10 animals in each group. C. Interocular difference in vitreous chamber depth vs. interocular difference in creep extensibility between highly myopic and fellow control eyes of tree shrews following 12 days of myopia progression. r = 0.75, p < 0.05. n = 10 animals. D. Interocular difference in refractive error vs. interocular difference in creep extensibility between highly myopic and fellow control eyes of tree shrews following 12 days of myopia progression. r = 0.79, p < 0.01. n = 10 animals. (Reproduced with permission from Phillips et al., 2000 © Association for Research in Vision and Ophthalmology.)

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Figure 8. The effect of intraocular pressure elevation on axial length. A. Change in the axial eye length of 10 normal chick eyes on raising IOP to 100 mm Hg. Initial values of axial length at 15 mm Hg are shown at time = −5 min. IOP was raised to 100 mm Hg at Time = 0 and remained at 100 mm Hg for one hour (horizontal bar) after which it was returned to 15 mm Hg. In all chick eyes axial length increased during the period of elevated pressure. The mean curve is shown as open circles. B. Change in the axial eye length of 10 normal tree shrew eyes on raising IOP to 100 mm Hg for one hour. Experimental procedures were the same as those for chick eyes. However, in all tree shrew eyes, axial length progressively decreased during the period of elevated pressure. The mean curve is shown as filled circles. (Reproduced with permission from Phillips and McBrien, 2004 © Association for Research in Vision and Ophthalmology.)

process of ocular shortening. Further investigation of scleral cells in the mammalian sclera via immunohistochemistry showed the sclera to contain a subset of cells that express a protein known as alpha-smooth muscle actin (α-SMA), namely myofibroblasts.35 Given that α-SMA is typically found in muscles, this finding has led to the hypothesis that the scleral cells have an active role in the mechanical properties of the mammalian sclera, and that this may be of importance in a number of physiological and pathological ocular functions.

TGF-β is of primary importance in the regulation of extracellular matrix turnover and the three mammalian isoforms of TGF-β have been demonstrated to be present in the sclera and to regulate collagen production via fibroblasts.57 Furthermore, TGF-β isoform changes in myopia development occur within 24 hours of the initiation of myopia development and have been linked to the altered regulation of ECM production found in the sclera of eyes developing myopia.57 mRNA expression levels