2. Pushing and Pulling
All crawling cells move by first protruding a cell front and subsequently retracting the cell rear. Some examples of cells moving on planar substrates coated with extracellular matrix proteins are shown in Fig. 2-1. The rate of motility is highly variable for different animal cell types, with the keratocyte from a fish scale leading at a speed of around 15µm per minute.
Figure 2-1: Video sequences of different cells moving in vitro. Top left: mouse fibroblasts moving into an artificial wound created in a petri dish (total video time, 3h). Bottom left: a chick fibroblasts, moving alone (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.
(Movie 13Mb) Click on picture to watch movie
To give a rough guide, the fish epidermal keratocyte can move one cell diameter in about 2 mins, as compared to about 1h required by the fibroblast. Rather than reflecting differences in the rate of cytoskeleton turnover, this difference stems 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 (Anderson et al. 1996)
Figure 2-2: 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 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.
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The processes of protrusion and retraction are both driven by the turnover and reorganisation of the actin cytoskeleton. A schematic illustration of the actin cytoskeleton of a polarised fibroblastic cell is shown in Fig. 3-1 (section 3).
