Functional significance of the variations in the geometrical organization of tight junction networks.

Abstract

Using freeze-fracture techniques, the morphology of tight junction networks found along the length of the alimentary tract of Xenopus laevis before and after metamorphosis were examined. The geometrical organization of the network is determined by the stress-induced shape changes normally experienced by the cells linked by the network. Consistent with this theory, tight junctions can be classified into 2 distinct types of network organization which differ in their response to normal and experimentally induced stress conditions: loosely interconnected networks which can stretch or compress extensively under tension, thereby adapting to stress changes in the tissue; and evenly cross-linked networks which retain their basic morphology under normal stress conditions. The absorptive cells of the large intestine and the mucous cells of the gastrointestine or stomach are sealed by the 1st, flexible type of tight junction. The 2nd type of junctional organization, the evenly cross-connected network, is found between absorptive cells of the small intestine and ciliated cells of the esophagus, and reflects in its constant morphology the relative stability of the apical region of both of these cell types. Networks intermediate between these 2 types arise when a cell which would normally form a loosely interconnected network borders a cell which tends to form a more evenly cross-linked network, as is found in the esophagus where ciliated and goblet cells adjoin. Despite the change in the animal''s diet during metamorphosis from herbivorous to carnivorous, the basic geometrical organization of the networks associated with each tissue of the alimentary tract remains the same.