Site-dependent differences in collagen lamellae in the corneal substantia propria of beagle dogs

The fine structure in the center and periphery of the cornea of 16 beagle dogs were characterized and compared. The central cornea (about 540 microm) was apparently thinner than the peripheral cornea (about 720 microm). Thickness ratios of the corneal substantia propria to the entire cornea were approximately 86% in both portions. In addition, number of collagen lamellae, collagen fibril diameter, and collagen fibril index of the central substantia propria are different from those of the periphery (253 vs 236 lamellae, 29.1 vs 32.0 nm, and 39.0 vs 41.6%, respectively). These differences are thought to be due to site-dependent accumulation of proteoglycans (decorin and lumican) which are responsible for production of thin fibrils. The central portion with higher proteoglycans would have abundant thin fibrils with less slippage but better elasticity to buffer against the direct impact of intraocular pressure on the cornea. In contrast, thick fibrils in the peripheral substantia propria would contribute to the maintenance of tensile strength acting on the transition zone between the cornea and sclera.