Scleral remodeling during the development of and recovery from axial myopia in the tree shrew

PURPOSE. Recent investigations have suggested that scleral thinning in mamm alian eyes with axial myopia is a consequence of the loss of scleral tissue , rather than the redistribution of existing tissue as the eye enlarges. Th e present study investigated whether further changes in the distribution an d metabolism of scleral tissue occur during the process of recovery from ax ial myopia. Scleral glycosaminoglycan (GAG) synthesis and content as well a s scleral dry weight changes were monitored as indicators of remodeling in myopic and recovering tree shrew sclerae. METHODS. Myopia was induced in tree shrews by monocularly depriving them of pattern vision. Some animals then had the occluder removed and were allowe d to recover from the induced myopia for periods of 1, 3, 5, 7, and 9 days. Newly synthesized GAGs were radiolabeled in vivo with [S-35]sulfate. Sulfa te incorporation and total GAG content in the sclera was measured through s elective precipitation of GAGs from proteinase K digests with alcian blue d ye. Dry weights of the sclerae were also determined. Changes in ocular refr action and eye size were monitored using retinoscopy, keratometry, and ultr asonography. RESULTS. Eyes developing myopia showed a significant reduction in scleral G AG synthesis, particularly in the region of the posterior pole (-36% +/- 7% ) compared with contralateral control eyes. Scleral dry weight was also sig nificantly reduced in these eyes (-3.7% +/- 1.2%). In recovering eyes, sign ificant changes in GAG synthesis were apparent after 24 hours of recovery. After 3 days of recovery, significantly elevated levels of GAG synthesis we re found (+79% +/- 15%), returning to contralateral control eye values afte r 9 days of recovery. Interocular differences in scleral dry weight were sh own to follow a similar pattern to that observed for GAG synthesis. CONCLUSIONS. Active remodeling, resulting in either the loss or replacement of scleral tissue and not passive redistribution of scleral tissue, is ass ociated with changes in eye size during both myopia development and recover y. Regulatory changes in scleral metabolism can be rapidly evoked by a chan ge in visual conditions and the direction of regulation is related to the d irection of change in eye size.