Ultrastructural observations were made on the initial adhesion process at the adherent region of Hydra endodermal cell pairs brought into contact (following dissociation) using a three-dimensional laser manipulator. Total contact length across the diameter of the adherent region decreased during the period 10–60 min after initial adhesion. However, the mean numbers of closest (<4 nm) and medium (5–25 nm) separation distances between membranes (thought to be important in total cell adhesion) were not significantly different. These data indicate that adherent cell pairs maintain a constant adhesiveness during the first 60 min of the adhesion process, despite membrane rearrangements. The relative length of each separation distance in adherent cell pairs approached that reported previously for intact Hydra. The sums of lengths in both the closest and medium categories (as a proportion of total contact length) increased because the length of cleavages (distances >25 nm) decreased significantly during the same time period. These results suggest that adherent cell pairs undergo rapid, active membrane changes in the adherent region, which might be associated with cell sorting. The possible significance of these changes for active rearrangement are discussed.

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