Cell Movements in Hydra

Abstract

All cells in hydra undergo continuous and systematic locomotion across the polyp body. Different patterns and mechanisms of movement are exhibited by the various cell types. Passive displacement is one of the most conspicuous forms of cell movement: The epithelial cells of the body column and tentacles move centrifugally due to the expansive growth of the tissues. Another factor in the passive displacement of epithelial cells involves the propagation of morphological characters across the tissue, rather than actual movement of cells or tissues per se. True cell locomotion, where cells move relative to neighboring tissue by means of self-propulsion, is also apparently exhibited by epithelial cells in certain situations, notably at sites where morphogenetic changes are taking place (e.g., bud and tentacle bases). Epithelial cells always migrate as sheets of cells, never individually. In contrast, nematocytes and interstitial cells undergo active migration individually. Nematocyte migration is strongly polarized, very possibly by the mat of epithelial cell muscular processes along which they move. Interstitial cell migration appears to occur under many conditions, but not nearly as commonly as was classically supposed. Finally it appears that interstitial cells and nematoblasts move in bulk through the intercellular spaces in some situations. This movement is probably due to epithelial cell activities and is used to redistribute cells located in the interstitial spaces.