Motility of endodermal epithelial cells plays a major role in reorganizing the two epithelial layers in Hydra.

Cell-cell interactions and cell rearrangements play important roles during development. Aggregates of Hydra cells reorganize into the two epithelial layers and subsequently form a normal animal. Examination of the formation of the two layers under various situations, indicates that the motility of endodermal epithelial cells, but not the differential adhesive forces of the two types of epithelial cells, plays the critical role in setting up the two epithelial layers. (1) When aggregates of ectodermal cells and of endodermal cells were placed in direct contact, the endodermal cells migrated into the interior of the ectodermal aggregate. This process was completely inhibited by cytochalasin B although initial firm attachment between the two aggregates was not blocked. (2) A single endodermal epithelial cell placed in contact with an ectodermal aggregate, actively extended pseudopod-like structures and migrated toward the center of the ectodermal aggregate. In contrast, an ectodermal epithelial cell remained in contact with an endodermal aggregate and never exhibited migratory behavior. Cytochalasin treatment of only endodermal epithelial cells abolished the migration. (3) One to 4 endodermal epithelial cells and/or ectodermal epithelial cells were placed in contact with one another forming up to 4-cell aggregates. Endodermal epithelial cells exhibited high motility that can be attributed to the migratory movement described above. Finally, formation of actin bundles, as visualized with rhodamine-phalloidin, was always correlated with pseudopod formation in endodermal epithelial cells during early and mid stages of aggregate formation.