The process of cell adhesion among dissociated single cells of Hydra: Morphological observations

Ultrastructural observations were made on the initial adhesion process at t he adherent region of Hydra endodermal cell pairs brought into contact (fol lowing dissociation) using a three-dimensional laser manipulator. Total con tact length across the diameter of the adherent region decreased during the period 10-60 min after initial adhesion. However, the mean numbers of clos est (<4 nm) and medium (5-25 nm) separation distances between membranes (th ought to be important in total cell adhesion) were not significantly differ ent. These data indicate that adherent cell pairs maintain a constant adhes iveness during the first 60 min of the adhesion process, despite membrane r earrangements. The relative length of each separation distance in adherent cell pairs approached that reported previously for intact Hydra. The sums o f lengths in both the closest and medium categories (as a proportion of tot al contact length) increased because the length of cleavages (distances > 2 5 nm) decreased significantly during the same time period. These results su ggest that adherent cell pairs undergo rapid, active membrane changes in th e adherent region, which might be associated with cell sorting. The possibl e significance of these changes for active rearrangement are discussed.