Corneal Hydration Comparative Physiology of Fish and Mammals

The Proctor Award Lecture

¹ Department of Ophthalmology, College of Physicians and Surgeons, Columbia University.

Corneas of all species studied were basically similar in structure, consisting of an orderly arrangement of collagen fibers bounded by epithelia. All were metachromatic, indicating the common possession of mucopolysaccharides. The corneas of teleost fish were found to swell in various physiologic salt solutions as did those of mammals. The degree of corneal swelling (in distilled water) was greater in the scup which is adapted to life in sea water and less in the carp which lives in a hypotonic medium. The elasmobranch cornea differs from those of all other species so far studied. It is not hydrophilic, and was not found to swell in any of the media studied. No evidence of anaerobic metabolic control of corneal hydration, such as is characteristic of mammals, was demonstrable in the teleost fish (scup). Corneal swelling ivas inhibited in the presence of salts, less by monovalent than by bivalent cations, and was enhanced when bivalent unions were present. Scup corneas were found to swell readily in aqueous solutions of polyvinyl-pyrrolidone (PVP) in the absence of salts. In the presence of salts, scup corneas could be maintained deturgesced in solutions of these large polymers (PVP). It is postulated that salts affect the macromolecular structure of the mucopolysaccharides so that that of the mucoid ground substance is more compact, stable, or rigid, and that this is a major factor in the maintenance of normal corneal hydration generally. It is suggested that the xoater balance of teleost corneas and, perhaps, of those of other animals is maintained in the presence of salts by mucopolysaccharides in the aqueous humor which counteract the swelling pressure of the cornea. Descemet's endothelium serves in this system as a membrane permeable to salts and xoater but not to the colloids of the aqueous humor.

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