Ultrastructural observations of adherent cell pairs in hydra vulgaris

Abstract

Previous morphological studies of cell sorting in Hydra vulgaris have considered only clusters of cells. Here, we present ultrastructural observations on the adherent region of cell pairs brought into contact (following dissociation) using a three-dimensional laser manipulator. There was a much larger area of close membrane contact between endodermal cell pairs in comparison with ectodermal cell pairs. Separation distances between membranes were categorized into three classes: closest distance (<4 nm); medium distance (5-25 nm); and cleavage (>25 nm). The sum of distances in the closest and medium categories as a proportion of total contact length was significantly greater (P<0.01) for endodermal cells (49.0+/-6.5 %) than for ectodermal cells (26.7+/-4. 4 %). In intact Hydra, this sum of distances was also significantly greater for endodermal cells, indicating that newly adherent cells, even after adhesion for only 10 min, display similar morphological characteristics to cells in intact Hydra. This suggests that close membrane contacts contribute to differential cell adhesion, which may form the basis of the cell sorting process.