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This article was downloaded by:[Mt Sinai School of Medicine, Levy Library] On: 14 April 2008 Access Details: [subscription number 769426197] Publisher: Informa Healthcare Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Current Eye Research Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713618400 Compensation for spectacle lenses involves changes in proteoglycan synthesis in both the sclera and choroid Debora L. Nickla; Christine Wildsoet; Josh Wallman First Published on: 01 April 1997 To cite this Article: Nickla, Debora L., Wildsoet, Christine and Wallman, Josh (1997) 'Compensation for spectacle lenses involves changes in proteoglycan synthesis in both the sclera and choroid', Current Eye Research, 16:4, 320 - 326 To link to this article: DOI: 10.1076/ceyr.16.4.320.10697 URL: http://dx.doi.org/10.1076/ceyr.16.4.320.10697 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Downloaded By: [Mt Sinai School of Medicine, Levy Library] At: 17:18 14 April 2008 Current Eye Research Compensation for spectacle lenses involves changes in proteoglycan synthesis in both the sclera and choroid Debora L. Nickla1, Christine Wildsoet2 and Josh Wallman1 1 Biology Department, City College of CUNY, New York, USA and 2 School of Optometry, Queensland University of Technology, Brisbane, Queensland 4001, Australia Abstract Purpose. It has been demonstrated that chick eye growth compensates for defocus imposed by spectacle lenses: the eye elongates in response to hyperopic defocus imposed by negative lenses and slows its elongation in response to myopic defocus imposed by positive lenses. We ask whether the synthesis of scleral extracellular matrix, specifically glycosaminoglycans, changes in parallel with the changes in ocular elongation. In addition, there is a choroidal component to compensation for spectacle lenses; the choroid thickens in response to myopic defocus and thins in response to hyperopic defocus. We ask whether choroidal glycosaminoglycan synthesis changes in parallel with changes in choroidal thickness. Methods. Chicks wore either a 115 diopter (D) or 215 D spectacle lens over one eye, or they wore one lens of each power over each eye for 5 days. At the end of this period, we measured refractive errors and ocular dimensions by refractometry and A-scan ultrasonography, respectively. Pieces of the scleras and choroids from these eyes were put into culture and the synthesis of glycosaminoglycans was assessed by measuring the incorporation of radioactive inorganic sulfur. Results. We here report that the compensatory modulation of the length of the eye involves changes in the synthesis of glycosaminoglycans in the sclera, with synthesis increasing in eyes wearing 215 D spectacles lenses and decreasing in eyes wearing 115 D lenses. In addition, changes in the synthesis of glycosaminoglycans in the choroid are correlated with changes in choroidal thickness: eyes wearing 115 D lenses develop thicker choroids and these choroids synthesize more glycosaminoglycans than choroids from eyes wearing 215 D lenses. Conclusions. Changes in scleral glycosaminoglycan synthesis accompany lens-induced changes in the length of the eye. Furthermore, changes in the thickness of the choroid are also asso- Correspondence: Current affiliation: Debora L. Nickla, New England College of Optometry, 424 Beacon St., Boston, MA, 02115, USA ciated with changes in the synthesis of glycosaminoglycans. These results are consistent with the regulation of the growth of the eye being bidirectional, and with the retina being able to sense the sign of defocus. Curr. Eye Res. 16: 320–326, 1997. Key words: chicks; emmetropization; eye growth; hyperopia; myopia; proteoglycans Introduction During postnatal ocular development, the length of the eye usually becomes matched to the optics of the cornea and lens so that images of distant objects fall on the retina; that is, the eye becomes emmetropic. The notion that this optical “tuning” is visually guided has been extensively studied since the finding that depriving the eye of patterned input results in excessive growth and myopia: monkeys (1), chicks (2), tree shrews (3, 4) and cats (5, 6). Stronger evidence for the visual regulation of eye growth comes from more recent studies showing that the eyes of chicks, monkeys, tree shrews and guinea pigs all show compensatory changes in length and refractive error for defocus imposed by spectacle lenses: chicks (7–10), monkeys (11), tree shrews (12), guinea pigs (13). In the chick, at least, the retina appears to be able to determine not only the magnitude of imposed defocus, but also its direction, and to initiate compensation over a wide range of myopic and hyperopic defocus, although the “bidirectionality” of this compensatory mechanism is not universally accepted. In this paper we looked for biochemical correlates of the compensatory growth responses of eyes wearing positive and negative lenses. Changes in the rate of growth of the chick eye are correlated with changes in the rate of synthesis of scleral extracellular matrix; specifically, scleras of eyes that are rapidly elongating in response to form deprivation show an increase in synthesis of matrix glycosaminoglycans, the sulfated side chains of proteoglycans, whereas scleras of eyes that have slowed their elongation in response to removal of the diffusers show a Received on June 19, 1996; revised on October 30 and accepted on October 31, 1996 © Oxford University Press Downloaded By: [Mt Sinai School of Medicine, Levy Library] At: 17:18 14 April 2008 Spectacle lenses and proteoglycan synthesis in chick eyes decrease in synthesis of glycosaminoglycans (14–16). Furthermore, scleras from form-deprived eyes contain significantly more of the core protein of the cartilage proteoglycan, aggrecan, than do scleras from normal eyes (14), indicating that changes in the rate of synthesis of aggrecan underlie the changes in growth rate. We measured GAG synthesis in scleras of eyes wearing positive and negative spectacle lenses to see whether changes in the rate of synthesis might underlie the defocus-induced changes in the length of the eye. Furthermore, to make a start toward understanding the mechanism whereby the choroid changes its thickness in response to myopic or hyperopic defocus (i.e. positive or negative lenses), we also measured GAG synthesis in the choroids from the same eyes. We studied refractive error and 3 ocular dimensions: (1) The axial length (cornea to sclera), which we expected to be most closely related to scleral growth (2), the vitreous chamber depth, the distance of the retina from the lens, which should be well correlated with refractive error, and (3) the choroid 1 retinal thickness, because the choroid participates in compensation by becoming thicker in response to myopic defocus and thinner in response to hyperopic defocus, thereby moving the retina. Predictably, we found that eyes wearing negative lenses were longer than eyes wearing positive lenses. Our new findings are that the scleras of eyes wearing negative lenses synthesized more GAGs than the scleras of eyes wearing positive lenses, and furthermore, that the thicker choroids from eyes wearing positive lenses synthesized more GAGs than the thinner choroids of the negative lens treatment group. Part of this work has been presented as an abstract (17) and the data in Figure 3a have been published in Wallman et al., 1995 (9). Methods Subjects Subjects were White Leghorn chickens (Gallus gallus) obtained as day-old hatchlings from Truslow Farms (Chestertown, MD). Birds were housed in wire cages with raised floors within temperature controlled chambers. The light cycle was 14L/10D. Food was sifted to minimize dust accumulation on the lenses. Lenses were cleaned every 3–4 hours over the period during which the lights were on. All experiments conformed to the ARVO Resolution on the Use of Animals in Research. Lenses and experimental paradigms Lenses were made from PMMA material with a back optic radius of 7 mm. To attach the lenses to the chicks, lenses were first glued to plastic rings that were affixed to Velcro rings. A mating ring of Velcro was glued to the feathers around the eye. This system allowed for their easy removal for frequent cleaning. Lenses were put onto the eyes on day 5 after hatching. On the fifth day of lens wear, the refractive errors and ocular dimensions were measured by refractometry and A-scan ultrasonography respectively. The animals were then sacrificed with 321 an overdose of sodium pentobarbitol and the eyes dissected and put into culture (described below). Two groups of birds comprise the data in this paper. In one group (“binocular lenses”, n 5 8), one eye wore a 115 D lens and the other eye wore a 215 D lens. (The +15 D and 215 D lenses had effective powers at the cornea, of 116.5 D and 213.8 D respectively, based on an average vertex distance of 6 mm). A second group (“monocular lenses”, n 5 13) wore either a 115 D or 215 D lens over one eye (115 D, n 5 7; 215 D, n 5 6), while the fellow control eye wore a plano (0 D) lens. Ocular dimensions and refractive errors were measured in all 21 birds. Proteoglycan synthesis was measured in the scleras and choroids of the “binocular lenses” group, and in 4 birds of the “monocular lenses” group (n 5 2 for each lens power and 4 plano eyes). Statistical comparisons between groups used a two-tailed t-test, unless otherwise indicated. Because there were no statistically significant differences between the plano lens-wearing eyes paired with negative lenses versus those paired with positive lenses, these data were combined for all analyses. Ocular measurements For measurement of refractive error and ocular dimensions, birds were anaesthetized with chloropent, a mixture of chloral hydrate and sodium pentobarbitol. Mydriasis (and presumably cycloplegia) was obtained using 6 drops/eye of a mixture of vecuronium bromide (Norcuron, Organon, West Orange, NJ) and benzalkonium chloride (10 mg/ml and 0.26 mg/ml, respectively) in saline. Refractive error was measured using a Hartinger refractometer (Jena Coincidence Refractometer). The pupillary axis of the eye was aligned with the refractometer by centering in the pupil the corneal reflection of a ring of light mounted coaxially with the instrument. Two pairs of measurements along orthogonal meridians were made and the data averaged. The eye was realigned twice more, and two more pairs of measurements for each orthogonal axis were taken each time. The median of these 6 average spherical equivalent refractive errors per eye (2 measurements per 3 realignments) yielded the refractive data presented here. Refraction data were not corrected for the artifact of retinoscopy. We measured ocular dimensions by A-scan ultrasound. A 7.5 MHz transducer with an attached water-filled standoff aligned along the optic axis of the eye, and with an intervening layer of gel between the cornea and the standoff. The signals were recorded on diskettes by a Nicolet digital oscilloscope; four sweeps per eye were subsequently analyzed and averaged to get the data for each eye. Because this measuring system could not resolve the retinal/choroidal interface, the thickness of the retina and choroid combined was measured and is referred to in the text and graphs as “retina 1 choroid” thickness. Nonetheless, because we know that the thickness of the retina does not change over the 5 days of lens wear (based on unpublished observations using a high frequency ultrasound system which resolves the two layers), we can confidently state that the thickness changes observed are due solely to changes in the choroidal Downloaded By: [Mt Sinai School of Medicine, Levy Library] At: 17:18 14 April 2008 322 D. L. Nickla et al. layer. Vitreous chamber depth is defined as the distance between the posterior lens surface and the anterior retinal surface. Axial length is defined as the distance between the anterior cornea and the anterior sclera. Measurement of GAG synthesis To dissect out the choroid and sclera, eyes were placed in cold medium and the extraocular muscles were cleared from the globe. The eyes were then bisected along the ora serrata using a razor blade, and a 7 mm punch of tissue from the posterior pole of the eye was obtained using a trephine. The retina/retinal pigment epithelium (RPE) was separated from the underlying choroid and sclera and discarded. The choroid was then gently dissected away from the sclera, and any adhering RPE was removed using a small sable paint brush. Pieces of sclera and choroid were incubated in separate wells in medium labeled with Na235SO4 (30 mCi/ml) for 24 hours. The medium used for all experiments was the chemically defined medium (N2) of Bottenstein (18). This medium is a 1:1 ratio of Dulbecco’s Modified Eagle’s Medium and Ham’s mixture (F-12) with added sodium bicarbonate, sodium selenite, progesterone, putrescine, pyruvate, glutamine, transferrin, catalase and insulin. Antibiotics added were streptomycin, garamycin and fungizone. To assay newly synthesized glycosaminoglycans, tissue was digested in 0.05% proteinase-K (protease type XXVIII, Sigma, in 10 mM EDTA, 0.1M sodium phosphate, pH 6.5) overnight at 578C. This treatment results in complete digestion of the cartilaginous and fibrous sclera, and of the choroid. Glycosaminoglycans were precipitated by the addition of 0.5% cetylpyridinium chloride (CPC) in 2 mM Na2SO4 in the presence of unlabeled chondroitin sulfate (1 mg/ml in distilled H2O). Samples were incubated for 1 hour at 378C and the precipitated glycosaminoglycans were collected on Whatman filters (GF/A) using a Millipore 12-port manifold. Filters were rinsed 5 times with 0.1% CPC containing 0.05 M NaCl. Radioactivity in the filters was measured by liquid scintillation counting in 10 ml scintillation fluid (CytoScint, Fischer). Further details of this procedure are in Rada et al., 1992 (15). Results Efficacy of spectacle lens treatment Refractive compensation was observed in response to both the positive and negative spectacle lenses worn (albeit only partially for negative lenses, probably due to the relatively short duration of lens wear compared to other studies) (Fig. 1). Furthermore, the degree of compensation to the lenses did not differ depending on whether the bird wore two lenses of opposite sign, or one plano lens: Eyes wearing 115 D lenses became hyperopic (binocular lenses: mean 5 116 D; monocular lenses: mean 5 112 D), whereas those wearing 215 D lenses became myopic (mean 5 26 D for both groups (Fig. 1a); all comparisons between eyes wearing lenses of different powers are significantly different, p , 0.001). Corresponding to these refractive changes, the now hyperopic eyes that had worn 115 D lenses had significantly shorter vitreous chambers (back of lens to retina) than either the myopic eyes that had worn 215 D lenses, or the eyes that had worn plano lenses (Fig. 1b, all comparisons between lenses significantly different). For vitreous chamber depth, too, the compensation for the lenses was similar, regardless of the power of the lens worn on the fellow eye (compare bars in the 115 D and 215 D groups, Fig. 1b). The refractive error was highly correlated with the vitreous chamber depth (Fig. 1c, r = 20.71, p , 0.001), and linear regression analysis yielded a slope of 19 D/mm, implying that changes in the length of the vitreous chamber are largely responsible for the changes in refractive error. (The average length of these eyes was approximately 9mm. We estimated the refractive change per mm change in axial length in these eyes to be 17.5D/mm, based on the formula in Wallman et al., 1995 [9]). Scleral GAG synthesis and eye length We find that GAG synthesis in scleras from eyes wearing spectacle lenses is changed in the same direction as the eye length (Fig. 2). Specifically, the scleras of eyes wearing 115 D lenses, which are shortest, synthesize significantly fewer GAGs than the scleras of eyes wearing 215 D lenses, which are longest (means: 215 vs 456 mmoles sulfur per punch; p , 0.01, Fig. 2a, compare to Fig. 2b). The scleral GAG synthesis in control eyes wearing plano lenses is intermediate between the two (mean: 370 mmoles sulfur per punch) although not significantly different from either, probably because of the small number of eyes in this group. The facts that the axial length (cornea to sclera) predominantly reflects scleral growth, and that the eyes wearing negative lenses are significantly longer than eyes wearing positive lenses (binocular lenses: p , 0.01; monocular lenses: one-tailed t-test, p , 0.05) are consistent with the hypothesis that changes in the synthesis of scleral GAGs induced by spectacle lenses are causally related to changes in eye length. Choroidal GAG synthesis and choroid thickness We find that the choroid, too, shows changes in synthesis of GAGs in the direction appropriate for the changes in its thickness (Fig. 3). Specifically, the choroid 1 retinas in the eyes that are compensating for the myopic defocus are thicker than those from eyes compensating for hyperopic defocus (115 D vs 215 D lenses: binocular lenses: mean 5 0.85 vs 0.42 mm, p , 0.0005; monocular lenses: mean 5 0.68 vs 0.44 mm, p , 0.05, Fig. 3b) and thicker than in eyes wearing plano lenses (mean: 0.68 vs 0.45 mm, p , 0.05). These thicker choroids show significantly greater GAG synthesis than the choroids from eyes wearing negative lenses (mean: 19 vs 12 mmoles sulfur/punch, p , 0.001, Fig. 3a). In addition, although we (unlike Wildsoet and Wallman, 1995 [10]) did not detect a difference in the thickness of the choroid + retinas from eyes wearing plano lenses versus those wearing negative lenses (Fig. 3b), the choroids from eyes wearing negative lenses synthesized significantly fewer GAGs than those from control eyes (mean: 12 vs 17 mmoles sulfur/punch, p , 0.005, Fig. 3a). These results Downloaded By: [Mt Sinai School of Medicine, Levy Library] At: 17:18 14 April 2008 Spectacle lenses and proteoglycan synthesis in chick eyes 323 Figure 1. suggest that changes in glycosaminoglycan synthesis may be related to changes in choroidal thickness. Discussion This paper reports two main findings: First, scleral GAG synthesis changes in concert with eye length, both increasing with compensation for the hyperopic defocus imposed by negative lenses and both decreasing with compensation for the myopic defocus imposed by positive lenses. Second, the thickened choroids in eyes wearing positive lenses synthesize more GAGs than choroids in control eyes, and the choroids from eyes wearing negative lenses synthesize fewer GAGs. Much recent evidence indicates that the regulation of eye growth in the chick is at the level of the synthesis of scleral extracellular matrix molecules. Form deprivation results in an increase in scleral dry weight, as well as in the synthesis of protein, DNA and glycosaminoglycans relative to normal eyes (dry weight, protein and DNA: [19]; glycosaminoglycans [14]). Conversely, scleras of eyes that had normal vision restored and were recovering from form deprivation myopia showed decreased GAG synthesis (15, 16). Our finding of increased scleral GAG synthesis in eyes that were elongating more rapidly in response to negative lenses, and decreased synthesis in eyes that were elongating more slowly in response to positive lenses lends further support to the hypothesis that the regulation of eye length occurs via the modulation of scleral extracellular matrix synthesis. The chick sclera consists of two layers, an inner cartilaginous one and an outer fibrous one (20). The cartilaginous layer is composed of chondrocytes embedded in a matrix of collagen fibers and proteoglycans, predominantly aggrecan. The fibrous layer is composed of fibroblasts, collagen and predominantly small proteoglycans, like decorin. These two layers show opposite responses to both form-deprivation and negative spectacle lens wear in the rate of synthesis of GAGs: specifically, the cartilaginous layer shows increases in synthesis while the fibrous layer shows decreases in synthesis (21). Because the uptake of sulfur into proteoglycans in the cartilaginous layer is approximately 100 times greater than that in the fibrous layer, Downloaded By: [Mt Sinai School of Medicine, Levy Library] At: 17:18 14 April 2008 324 D. L. Nickla et al. Figure 2. analyses done using whole pieces of sclera predominantly reflect the response of the cartilaginous layer (21). Indeed, in whole scleral pieces, most of the incorporation of sulfur is into a proteoglycan of approximately the same size as the cartilage proteoglycan aggrecan (14, 22). Furthermore, the newlysulfated GAGs on the proteoglycans are cleaved by the enzymes chondroitinase and keratanase (14, 22) leading us to conclude that the molecules we are measuring are mostly aggrecan, and that the cells responsible for the production of these molecules are in large measure the chondrocytes in the cartilaginous layer of the sclera. However, because we are measuring sulfur incorporation into the GAG side chains, and because newly sulfated GAGs can be added to pre-existing core proteins, we cannot assert that changes in levels of newly synthesized molecules reflect changes in the rate of synthesis of intact proteoglycans, only that they reflect changes in the synthesis rate of the glycosaminoglycans. It should be noted that, in analyzing the biochemical data, we did not normalize the data to either DNA or protein, raising the possibility that the differences in synthesis between groups are an epiphenomenon of a change in cell number. There is evidence, however, that the density of the chondrocytes in the cartilage of scleras from form-deprived myopic eyes is lower than that in normal eyes (23), and that these scleras too show increased GAG synthesis (14). Furthermore, given the evidence that the GAG synthesis measured in scleral tissue predominantly reflects synthesis in the cartilaginous layer, we can conclude with some confidence that the greater synthesis in the scleras of eyes wearing negative lenses is not the result of an increase in the number of cells, but of an increase in the rate of synthesis of GAGs. The chick eye shows good compensation, in both refractive error and vitreous chamber depth, to lenses of different powers and opposite sign (7, 10). We here show that scleral GAG synthesis is higher in eyes wearing negative lenses and lower in eyes wearing positive lenses compared to eyes wearing plano lenses. One interpretation of these results is that the eye can either increase or decrease its growth in response to the sign of the defocus; that is, that there are two opposite signals arising from the retina, depending on whether the image plane is in front of or behind it, with the plano lens (or no lens) being the neutral point. An alternative interpretation is that there is only one signal, perhaps the same signal involved in form-deprivation myopia, the level of which depends on the amount of blur experienced over the day. Because the chick is presumed to spend much of its time viewing close objects, a positive lens may reduce the total amount of blur experienced, whereas negative lenses may increase the amount of blur. The weakness of this second interpretation is that it implicitly makes strong presumptions about the operation of accommodation. Either accommodation must not be used (as might be the case if the lens is applied monocularly in species with unyoked accommodation), or the defocus remaining in the presence of accommodation in eyes wearing negative lenses is adequate to cause myopia. Despite this weakness, this hypothesis is favored by some. One reason for its prevalence is that in mammals, the response to positive lenses is limited to low powers (11, 12), suggesting that the action of the lens is to correct the infantile hyperopia, rather than to produce a myopic refractive error to which the eye shows a compensatory response. However, recent findings suggest that rhesus monkeys, at least, can be induced to become very hyperopic in compensation for positive lenses, if the lens power is increased gradually (E. Smith of the University of Houston at the 1996 International Congress of Eye Research, Yokohama), suggesting that the eye can respond to positive lenses but only within a certain range of defocus. Although the evidence arguing for one or two mechanisms of lens compensation is equivocal at present, we would expect that, if the compensation for positive and negative lenses used the same mechanism as the response to diffusers, manipulations that affect the response to one would affect the responses Downloaded By: [Mt Sinai School of Medicine, Levy Library] At: 17:18 14 April 2008 Spectacle lenses and proteoglycan synthesis in chick eyes 325 Figure 3. to the others. Recent work describes five manipulations that reduce form-deprivation myopia: flickering light, constant light, treatment with atropine (a muscarinic acetylcholine antagonist), reserpine (a depleter of retinal serotonin and dopamine) or 6-hydroxydopamine; all of these also reduce the compensatory myopia resulting from negative lens wear, but have little effect on the compensatory hyperopia resulting from positive lens wear: flicker (24), atropine (25), reserpine (26), constant light (27), 6-hydroxydopamine (28); see also Wallman, 1995 (29). Furthermore, the response to negative lenses differs from the response both to diffusers and to positive lenses in that optic-nerve-section attenuates the former but not the latter two (10). We are inclined to interpret this set of findings as suggesting that two different mechanisms are involved in the compensation to positive and negative lenses, although the mechanism underlying compensation to negative lenses appears similar in most cases to that underlying form-deprivation induced myopia. The choroidal thickening response is the major component of the early phase of compensation to myopic defocus in the chick eye, serving to push the retina toward the image plane (9). This response occurs within hours (unpublished results), and in the absence of a connection to the brain (10, 30). In this paper, we report that the thicker choroids from eyes wearing positive lenses synthesize more glycosaminoglycans than the thinner choroids from both control eyes and eyes wearing negative lenses. We have also previously reported that the choroid thins significantly in response to form deprivation (22) and to negative lenses (9, 10), although in the present study we did not observe this thinning in eyes wearing negative lenses, presumably because of the lower resolution of the ultrasonography system used. Nonetheless, we found that the synthesis of GAGs by the choroids from eyes wearing negative lenses was significantly lower than in choroids from eyes wearing plano lenses. We raise the possibility that changes in the rate of synthesis of proteoglycans underlie the changes in choroidal thickness. Proteoglycans are large, highly sulfated, anionic macro- molecules that are abundant in the extracellular matrices of connective tissue (review: [31]). Because they are extremely hydrophilic, increased synthesis could cause tissue expansion by increasing movement of water into the tissue. Conversely, choroidal thinning could be affected by a decreased synthesis (and/or increased degradation) of proteoglycans. Nonetheless, our data do not preclude the alternative possibilities either that other mechanisms are involved, or that proteoglycans subserve some other function in the choroid. In summary, the compensatory responses of the eye to positive and negative lenses involve changes in glycosaminoglycan synthesis in both the sclera and the choroid. In the case of the sclera, the observed changes in synthesis of GAGs are consistent with, and may underlie, the lens-induced changes in ocular growth and thus indirectly, the compensatory changes in refraction. The changes in the choroidal GAG synthesis may play a role in regulating choroidal thickness in order to move the retina forward and back, thereby providing a fast component of the emmetropization mechanism. References 1. Wiesel, T. N. and Raviola, E. 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