Epithelial cells probably constitute the most diverse group of cells found in the body. In addition to serving as interfaces between external and internal environments, their functions include ion and fluid secretion and reabsorp tion, protein exocytosis, hormone secretion, recognition, surface protection and the control of ciliary movement. By their very exposure on the surfaces of the body, epithelial cells are subjected to wide-ranging assault, by micro organisms and by chemical and physical forces. They are the targets for abrasion, infection and malignant transformation. Some epithelial...