ABSTRACT. The fine structure in the center and periphery of the cornea of 16 beagle dogs were characterized and compared. The central cornea (about 540 m) was apparently thinner than the peripheral cornea (about 720 m). 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.KEY WORDS: canine, collagen fibrils, collagen lamellae, cornea, corneal substantia propria.J. Vet. Med. Sci. 71(9): 1229-1231, 2009 The general structure of the cornea has been described in many textbooks [1,6,17,18]. With the recent development of technology in regenerative medicine, biomedical studies on the cornea of dogs and other animals have been increasing [11,19,21]. An understanding of the fine structure of the normal canine cornea is essential for the development of corneal tissue engineering for treatment of corneal diseases. The fine structure of the corneal substantia propria, which occupies approximately 90% of the entire thickness of the cornea, has not yet been completely determined. The objective of this study was to characterize and compare the fine structures in the central portion and peripheral portion of the normal corneal substantia propria of beagle dogs.The use of experimental animals was approved by the Ethics Committee of Rakuno Gakuen University, Japan, and compliance with the NIH Guide for the Care and Sixteen male beagles at the age of 16.8  7.0 months with body weight of 10.7  1.1 kg were used. After anesthesia with 25 mg/kg of pentobartital sodium, IV (Kyoritsu Seiyaku Corporation, Tokyo, Japan), these animals were euthanized by exsanguination. Then, the left eyeball was surgically removed from each dog. Twelve eyeballs were perfused with 10% formalin, and bisected in the median plane by using a razor blade for measurements of central and peripheral corneal thicknesses at ten sites for each sample. Four eyeballs were fixed in 3.0% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4) for 24 hr at room temperature. Then, 2-mm 2 corneal samples were excised from the center and periphery of the eyeballs and post-fixed in 1.0% osmium tetroxide in 0.1 M phosphate buffer for 1 hr at room temperature. Thereafter, the samples were washed with 0.1 M phosphate buffer (pH 7.4), dehydrated in a grad...