The in vitro organ culture system is useful in the study of the mechanics of gastric reepithelialization, cell migration and exfoliation. Reepithelialization is apparently effected by actively motile cells of all levels of differentiation, and at the completion of the process a new zone of cell exfoliation is initiated at points of contact when cells meet after growing towards the center of the specimen from its opposite margins. Mitotic activity is suppressed during the period of migration. (Am J Pathol 78:23-32, 1975)