Characterization of an extremely motile cellular network in the rotifer Asplanchna spp. - Structure, kinetics, and the cytoskeleton

The pseudocoelomic body cavity of the rotifer Asplanchna spp. contains free cells that form a highly dynamic, three-dimensional polygonal network of f ilopodia. Using video-enhanced differential interference contrast microscop y, we have qualitatively and quantitatively characterized the motion types involved with network motility: (1) filopodial junctions are displaced late rally at 10.52+/-0.46 mu m/s; (2) free-ending filopodia form and extend at rates of 8.77+/-0.40 mu m/s, until they retract again at 7.23+/-0.87 mu m/s ; (3) filopodial strands fuse either laterally or tip to the lateral side. The combination of these motion types results in enlargements, diminutions, and extinctions of filopodial polygons, and in the formation of new polygo ns. Moreover, there is intense and fast (5.11+/-0.28 mu m/s) particle trans port within the filopodial strands. The organization of the cytoskeleton in filopodia was examined by electron microscopy and by labeling with fluores cent-tagged phalloidin. Filopodia contain several microtubules that are oft en organized in a bundle. Moreover, F-actin is present within the filopodia . To characterize which of these cytoskeletal systems is involved with cell and organelle motility, we have examined cell dynamics after incubations w ith colchicine or cytochalasin D. The results of these pharmacological expe riments provide evidence that microtubules are required for both cell and o rganelle motility, but that actin filaments contribute to these phenomena a nd are required for the structural maintenance of slender filopodia.