Figure 12.

Mechanical and structural model of keratocyte locomotion. (A) Schematic diagram of a proposed two-phase mechanism of keratocyte locomotion. In the first phase, rearward-directed forces generated by the actin and myosin filament meshwork in the anterior lamellipodium pull the front of the cell body forward. In the second phase, higher lateral forces generated by actin-myosin fibers oriented across the cell body cause adhesions to detach, pulling the flanks and posterior cell body inward and forward. The midline represents a line of morphological, structural, and lateral force symmetry. A second line of symmetry is defined by smaller forces about the boundary of the lamellipodium and the cell body. These two lines cross to define a “point” of force symmetry. The large arrows in front of the cell refer to stronger lateral forces generated by transverse fibers, and the small arrows refer to weaker forces generated by network contraction, the net direction of which is parallel to the direction of motion. (B) Frontal view showing alternative consequences of transverse fiber contraction. (Top) Contraction of a cage of fibers tends to cause the cell body to round up on a compliant substratum that is easily pulled in by the flanks and depressed by the cell body; the cell body can also round up if it loses adhesion to the substratum. (Bottom) Internal shortening of curved transverse fibers causes them to straighten if they remain attached to stiff substrata at their ends. The fibers press inward on the cell body (down in the drawing), squeezing cytoplasm out toward the flanks.