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Abstract
The corneal epithelial surfaces of the rabbit, rat, cat, dog, and monkey are covered by myriads of fine microvilli. This study is in agreement with transmission electron reports showing only microvilli and no microplicae of surface corneal epithelium. Surface corneal epithelial cells have been characterized by scanning electron microscopy (SEM) as light, medium, and dark, depending on the density of secondary electrons. This SEM study shows that the dark-cell surface has twice as many microvilli per square micron than the light cell (dark cells 26 microvilli per square micron, light cells 13 microvilli per square micron) but the microvilli of the dark cell are less than one-half the length of light-cell microvilli (dark cell, 0.25 µm; light cell, 0.6 µm). Random groups of dark cells were found to contain depressions and full-thickness holes varying from 1 to 10 µ in diameter. The microvilli of the second-layer cell seen at the base of the hole had the appearance, length, and density consistent with a light cell. The findings are interpreted as the sequential development of a hole in an aging, dark-surface cell which expands in a controlled manner to expose its succeeding second-layer cell. This unique process of surface cellular destruction could impart considerable stability to the normal tear film.