Keratocytes pull with similar forces on their dorsal and ventral surfaces

As cells move forward, they pull rearward against extracellular matrices (E CMs), exerting traction forces. However, no rearward forces have been seen in the fish keratocyte. To address this discrepancy, we have measured the p ropulsive forces generated by the keratocyte lamella on both the ventral an d the dorsal surfaces. On the ventral surface, a micromachined device revea led that traction forces were small and rearward directed under the lamella , changed direction in front of the nucleus, and became larger under the ce ll body. On the dorsal surface of the lamella, an optical gradient trap mea sured rearward forces generated against fibronectin-coated beads. The retro grade force exerted by the cell on the bead increased in the thickened regi on of the lamella where myosin condensation has been observed (Svitkina,T.M ., A,B. Verkhovsky, K.M, McQuade, and G.G. Borisy. 1997. J. Cell Biol. 139: 397-415). Similar forces were generated on both the ventral:(0.2 nN/mu m(2 )) and the dorsal (0.4 nN/mu m(2)) surfaces of the lamella, suggesting that dorsal matrix contacts are as effectively linked to the force-generating c ytoskeleton as ventral contacts. The correlation between the level of tract ion force and the density of myosin suggests a model for keratocyte movemen t in which myosin condensation in the perinuclear region generates rearward forces in the lamella and forward forces in the cell rear.