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. 1997 Mar 24;136(6):1287–1305. doi: 10.1083/jcb.136.6.1287

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Model for the generation of motile force for forward motility of the cell body during heart fibroblast locomotion. (a) Observed dynamic behavior and polarity of actin filaments in locomoting fibroblasts. Actin filaments (rows of consecutive solid parallelograms and triangles) are marked with zones of fluorescence (open parallelograms and triangles). Actin filament polarity (in a and b) is simplified. Barbed ends are the broad ends of triangles (e.g. in a this is to the right). Polarity is not represented for parallelograms. In the cell body in graded polarity actin bundles (parallelograms) we do not know the spatial location of the forward moving and stationary actin filament populations. We suspect that on the ventral surface of the cell body, a proportion of or all filaments in graded polarity actin bundles are stationary, because this is the behavior of all the filaments in the lamella, and graded polarity actin bundles in the lamella (triangles) are continuous with those in the cell body (e.g., Fig. 3, a and b, and Fig. 5, a and b). In the lamellipodium, actin filaments have uniform polarity and flow rearward with respect to substrate. (b) We suggest that (1) in graded polarity actin bundles myosin (two balls and one stick) moving in the direction of (lower arrow) the barbed end of a population of stationary actin filaments (row of triangles attached to substrate by a vertical black bar) pulls a second population of actin filaments (row of parallelograms) forward, and this drives the cell body forward (upper arrow). (2) Myosin (one ball and stick) may also directly move the nucleus forward. Both mechanisms 1 and 2 may exist in cells.