A comparative SEM study of the vertebrate corneal epithelium

Purpose: The anterior surface of the cornea of mammals, including humans, h as numerous folds in the anterior epithelial cell membranes in the form of microvilli and microplicae. The role of these surface irregularities may be to increase cell-surface area and therefore aid in intra- and extracellula r movement of nutritional and waste products across the cell membranes in a ddition to stabilizing the corneal tear film. The aim of this study was to investigate and compare the nature of these corneal-surface features in var ious vertebrate classes residing in different environments. Methods. The an terior corneal surfaces of various vertebrates were investigated by using f ield emission scanning electron microscopy. Cell areas were analyzed by usi ng image-analysis software. Results. Representative species were examined f rom all the vertebrate classes, with the exception of the Cephalaspidomorph i. The mean epithelial cell density of aquatic vertebrates (17,602 +/- 9,60 4 cells/mm(2)) is greater (p = 0.000018) than that of aerial and terrestria l vertebrate species, including amphibians (3,755 +/- 2,067 cells/mm(2)). S imilarly, the mean epithelial cell density for the marine vertebrates (22,5 53 +/- 8,878 cells/mm(2)) is greater (p = 0.0015) than that of the freshwat er and estuarine species (10,529 +/- 5,341 cells/mm(2)). The anterior corne al surfaces of all species examined were found to show a variety of cell-su rface structures. Microvilli are predominant in reptiles, birds, and mammal s; microridges appear to be characteristic of the Osteichthyes; and microho les were observed only in the Chondrichthyes. Conclusion. The function of t hese morphologic variations in surface structure appear to be correlated wi th the range of ecologic environments (marine. aerial. and terrestrial) occ upied by each species, corneal phylogeny, and the demands placed on the cor nea to ensure clear vision.