The corneal surface of aquatic vertebrates: microstructures with optical and nutritional function?

The anterior surface of the mammalian cornea plays an important role in mai ntaining a smooth optical interface and consequently a sharp retinal image. The smooth surface is produced by a tear film, which adheres to a variety of microprojections, which increase the cell surface area, improve the abso rbance of oxygen and nutrients and aid in the movement of metabolic product s across the outer cell membrane. However, little is known of the structura l adaptations and lear film support provided in other vertebrates from diff erent environments. Using field emission scanning electron microscopy, this stud); examines the density and surface structure of corneal epithelial ce lls in representative species of the classes Cephalaspidomorphi, Chondricht hyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia, including some M arsupialia. Variations in cell density and the structure and occurrence of microholes, microridges, microplicae and microvilli are described with resp ect to the demands placed upon the cornea in different aquatic environments such as marine and freshwater. A progressive decrease in epithelial cell d ensity occurs from marine (e.g. 29 348 cells mm(-2) in the Dover sole Micro stomius pacificus) to estuarine or freshwater (e.g. 5999 cells mm(-2) in th e black bream Acanthopagrus butcheri) to terrestrial (e.g. 2126 cells mm(-2 ) in the Australian koala Phascolarctos cinereus) vertebrates, indicating t he reduction in osmotic stress across the corneal surface. Tilt: microholes found ill the Southern Hemisphere lampreys, namely the pouched lamprey (Ge otria australis) and the shorthead lamprey (Mordacia mordax) represent open ings for the release of mucus, which may protect the cornea from abrasion d uring their burrowing phase. Characteristic of marine teleosts, fingerprint -like patterns of corneal microridges are a ubiquitous feature, covering ma ny types of sensory epithelia (including the olfactory epithelium and the o ral mucosa). Like microplicae and microvilli, microridges stabilize the tea r film to maintain a smooth optical surface and increase the surface area o f the epithelium, assisting in diffusion and active transport. The clear in terspecific differences in corneal surface structure suggest an adaptive pl asticity in the composition and stabilization of the corneal tear. film in various aquatic environments.