Stromal Collagen Arrangement Correlates with Stiffness of the Canine Cornea

Abstract

The cornea is the most external layer of the eye and serves two important roles in (1) the refraction of light and (2) protection from the outside environment, both of which are highly dependent on the collagen assembly of the corneal stroma. This study sought to determine the collagen fiber arrangement of the canine corneal stroma and correlate the stromal organization with tissue stiffness in the anterior and posterior cornea. Collagen organization of the canine cornea was visualized through second-harmonic generation (SHG) imaging, and tissue stiffness of the anterior and posterior corneal stroma was determined by atomic force microscopy. Analysis of the canine anterior corneal stroma using SHG imaging documented intertwining of the collagen fibers with a high degree of fiber branching, with a more lamellar and non-branching posterior stroma. The anterior stroma had significantly higher tissue stiffness in both dogs and humans, when compared with the posterior corneal stroma (canine median: 1.3 kPa vs. 0.3 kPa; human median: 14.6 kPa vs. 2.1 kPa, respectively). There was a direct correlation between corneal collagen stromal organization and tissue stiffness in the dog, which was consistent with other mammalian species previously examined and likely reflects the need for maintenance of rigidity and corneal curvature.

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“…It must be considered that in a mammal’s cornea, collagen organization of the anterior stroma has a very different arrangement when compared with the posterior cornea. As specified by Leonard and colleagues, there is a direct correlation between corneal collagen stromal organization and tissue stiffness in the mammalian species, and this likely reflects the need for maintenance of rigidity and corneal curvature [ 50 ].…”

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“…16.2-33.1 71,72 1.3 71 1.1 73 25-40 74 6 1.7-3.0 84,85 1.1-1.8 24,85 0.0053 11,24 0.013-0.054 11,24 Abbreviation: The morphology of the canine lacrimal drainage system is remarkably similar to that of humans, except for a longer nasolacrimal duct (notably in long-nosed dogs), and the presence of accessory duct openings into the nasal cavity. 92 In both species, tear drainage begins with the lower and upper nasolacrimal puncta and canaliculi in the medial canthus, joining into a lacrimal sac in the bony lacrimal fossa, and extending into the nasolacrimal duct that runs through an osseous channel towards the nasal cavity.…”

“…71,73 Differences in collagen intertwining, along with the absence of Bowman's membrane in laboratory species, explain the vast disparity in stiffness of the anterior stroma (16.2, 1.3, and 1.1 kPa) and posterior stroma (2.5, 0.5, and 0.4 kPa) in humans, dogs, and rabbits, respectively. 71,73 The elastic modulus of the cornea is reportedly higher in rodents, although the methodology used was different. 74,75 Corneal rigidity should be considered in comparative studies in which the biophysical attributes of the cornea are important (e.g., wound healing, keratoprosthesis).…”

“…The corneal stroma, comprising nearly 90% of the total corneal thickness in most mammals, is primarily composed of collagen fibrils arranged in lamellae. While extensive collagen intertwining is noted in the majority of the corneal stroma in humans, it is only present in the anterior most‐aspect of the cornea in dogs and rabbits 71,73 . Differences in collagen intertwining, along with the absence of Bowman's membrane in laboratory species, explain the vast disparity in stiffness of the anterior stroma (16.2, 1.3, and 1.1 kPa) and posterior stroma (2.5, 0.5, and 0.4 kPa) in humans, dogs, and rabbits, respectively 71,73 .…”

“…While extensive collagen intertwining is noted in the majority of the corneal stroma in humans, it is only present in the anterior most‐aspect of the cornea in dogs and rabbits 71,73 . Differences in collagen intertwining, along with the absence of Bowman's membrane in laboratory species, explain the vast disparity in stiffness of the anterior stroma (16.2, 1.3, and 1.1 kPa) and posterior stroma (2.5, 0.5, and 0.4 kPa) in humans, dogs, and rabbits, respectively 71,73 . The elastic modulus of the cornea is reportedly higher in rodents, although the methodology used was different 74,75 .…”