Scleral ultrastructure in young rhesus monkeys: A stereological approach

The sclera provides mechanical support to the globe and plays a key role in maintenance of ocular structural integrity. Evidence shows that scleral remodeling contributes to biomechanical weakening, increasing the risk of pathologies, such as glaucoma and posterior staphyloma in high myopia. A precise characterization of normal regional ultrastructure is needed for a better understanding of scleral remodeling. The purpose of this study was to develop and implement a stereological analysis method to characterize scleral ultrastructure in healthy young rhesus monkey eyes. Monkeys (N = 4 eyes) first underwent normal emmetropization in a primate nursery. At 150 days of age, scleral tissue was collected in 1 mm² sections from three regions, equatorial nasal, posterior, and equatorial temporal, and processed for transmission electron microscopy (TEM). Images were captured at three depths: outer, middle, and inner, for all regions. Scleral thickness, fibroblast volume fraction, volume fraction of fibrillin microfibrils, and collagen fibril diameters were quantified using stereology and manual segmentation. The sclera was thickest in the posterior region with a mean thickness of 217.1 ± 10.1 μm. A total of 103 micrographs were analyzed to compute the volume fraction of fibroblasts and fibrillin microfibrils. Median fibroblast volume fraction was 7.4 %, and fibrillin microfibril volume fraction was 1.8 %. A total of 3564 collagen fibrils, encompassing 99 fibrils per region and depth per eye, were analyzed. Median collagen fibril diameter was 79.8 nm and ranged from 23 nm to 205 nm. This stereological approach provides a foundation for future studies investigating myopic scleral remodeling, where axial elongation and scleral thinning are known to show regional differences.