All crawling cells move by first protruding a cell front and subsequently retracting the cell rear. This section shows examples of different cells moving on planar substrates.
Video sequences of different cell types moving in vitro. (Top left) mouse fibroblasts moving into an artificial wound created in a petri dish (total video time, 3h). (Bottom left) embryonic chick fibroblasts (total video time, 2h). (Top right) mouse melanoma cell (total video time, 20min). (Bottom right) trout epidermal keratocyte (total video time, 4min). The images were recorded using either phase contrast optics (chick fibroblasts and mouse melanoma cell) or Nomarski interference optics (fish keratocyte). The differences in migration speed can be appreciated from the different durations of the movie sequences. The rate of motility is highly variable with the fish keratocyte leading at a speed of around 15µm per minute.
Video sequence of a fish fibroblast expressing mCherry-actin (red) and myosin light chain (green). Myosin serves with actin in retraction of the cell body and is mainly excluded from the protruding lamellipodia and filopodia. Bar, 10µm.
Rather than reflecting differences in the rate of cytoskeleton turnover, the different translocation speeds stem from differences in the coordination of the systems of motility and anchorage. In the keratocyte, protrusion and retraction are tightly coupled, whereby the body of the keratocyte rolls behind the broad lamellipodium front.
Another example of epidermal fish keratocytes derived from scales of the alpine trout. Total time for the movie was 3.5 min.
The unique rolling motion of the epidermal keratocyte is revealed by placing fluorescent beads on the substrate: as the cell moves it picks up the beads and they are incorporated in the cortex of the cell body. Note the movement of the two beads in the middle of the elliptical cell body. In this video, the cell has been made to “run-on-the-spot” so as to emphasise the rolling motion. (Anderson et al., 1996)
Cultured HL-60 cells mimic blood neutrophils during movement towards a chemo-attractant. The movie shows HL-60 cells attracted by a chemo-attractive peptide (fMLP) flowing from a micro needle. Note the phases of protrusion at the front and retraction at the rear.
The processes of protrusion and retraction are both driven by the turnover and reorganisation of the actin cytoskeleton.