Fig. 4. Actin foci were incorporated into retraction fibers and were eventually shed on the substratum during cell migration. (A-L) Some of the actin foci, especially in the posterior region, were incorporated into retraction fibers, ripped off from the cell and finally left on the substratum (arrowheads). The sites where actin foci had appeared and then disappeared were also incorporated into retraction fibers (circles). (M) Superimposed images of the posterior region of the cell shown in A-L. The black dots represent the positions where the actin foci had been located. Note that all these dots were located within retraction fibers. Scale bar: 5 µm.