Scleral structural alterations associated with chronic experimental intraocular pressure elevation in mice

PURPOSE To study changes in scleral structure induced by chronic experimental intraocular pressure elevation in mice. METHODS We studied the effect of chronic bead-induced glaucoma on scleral thickness, collagen lamellar structure, and collagen fibril diameter distribution in C57BL/6 (B6) and CD1 mice, and in collagen 8α2 mutant mice (Aca23) and their wild-type littermates (Aca23-WT) using electron and confocal microscopy. RESULTS In unfixed tissue, the control B6 peripapillary sclera was thicker than in CD1 mice (p<0.001). After 6 weeks of glaucoma, the unfixed CD1 and B6 sclera thinned by 9% and 12%, respectively (p<0.001). The fixed sclera, measured by electron microscopy, was significantly thicker in control Aca23 than in B6 or CD1 mice (p<0.05). The difference between fresh and fixed scleral thickness was nearly 68% in untreated control B6 and CD1 mice, but differed by only 10% or less in fresh/fixed glaucoma scleral comparisons. There were 39.3±9.6 lamellae (mean, standard deviation) in control sclera, categorized as 41% cross-section, 24% cellular, 20% oblique, and 15% longitudinal. After glaucoma, mean peripapillary thickness significantly increased in fixed specimens of all mouse strains by 10.3 ±4.8 µm (p=0.001) and the total number of lamellae increased by 18% (p=0.01). The number of cellular and cross-section lamellae increased in glaucoma eyes. After glaucoma, there were more small and fewer large collagen fibrils (p<0.0001). Second harmonic generation imaging showed that the normal circumferential pattern of collagen fibrils in the peripapillary sclera was altered in significantly damaged glaucomatous eyes. CONCLUSIONS Dynamic responses of the sclera to experimental mouse glaucoma may be more important than baseline anatomic features in explaining susceptibility to damage. These include decreases in nonfibrillar elements, alterations in lamellar orientation, an increased number of smaller collagen fibrils and fewer larger fibrils, and relative increase in the number of scleral fibroblast layers.