Fig. 8. A schematic model of how the cell exerts the traction force against the substrate during migration. The flexible silicone substrate is shown as straight or wavy lines. The wavy lines indicate the distortion of the silicone surface. The gray large arrows (a,f) show the direction of the cell movement. The circles and crosses indicate the present and the original position of beads, respectively. The displacement of beads is shown by thin black arrows parallel to the substrate. The shaded part of the cell represents the putative region generating motive force for migration. The cell transmits the traction force or the motive force (thick black arrows) to the substrate through putative feet (black vertical bars). (a-e) In the wild-type cell, during the extension phase (a-c), the extension force of the anterior region drags the posterior cell body and causes the pulling traction force at the anterior feet, and the posterior feet are passively dragged and pull the beads forward. During this phase, formation of new feet may occur in the newly extended anterior region (c,d). During the retraction phase (d,e), the posterior region of the cell contracts (two small arrows at the rear) mediated by the myosin II, which localizes at this region. It causes the detachment of the adhesion sites in the posterior region, the substrate is relaxed, and the beads return to their original positions (the circle overlaps with the cross) in d. (e) Then the posterior contraction generates a pushing force of the anterior cell body and also causes anterior extension. At this time, the posterior feet exert the traction force backward to the substrate and the anterior feet push the beads forward. (f-i) In the case of MHC-null cells, the beads always move toward the cell body since the contraction of posterior regions is deficient. The posterior edge moves at a constant rate whereas the extension progresses cyclically at the anterior region. Figs j and k explain the difference between the `pulling force' and the `pushing force' at the anterior region. (j) The pulling force is caused by the reaction to the anterior motive force generated by actin polymerization or myosin Is. (k) The pushing force is generated by the posterior contraction mediated by myosin II.