Control of cell shape and locomotion by external calcium.

Dependence of locomotion of Xenopus laevis epidermal cells on calcium influx from the external medium was investigated. Inhibition of Ca2+ influx by 2 mM La3+ or 4 mM Tb3+ in the culture medium causes an immediate stop to locomotion and a loss of motion at the outer margin of the lamella; microcolliculi disappear and the entire lamella becomes flat and very thin. The cell body region enlarges by spreading into the lamella to an extent approximately coincident with the distribution of myosin. The increase in thickness of this area is the result. The cytoskeletal elements actin, alpha-actinin and myosin become homogeneously distributed throughout the cell and a great number of straight microtubules extend to the margin after 20 min in La3+-containing media. Prekeratin distribution does not change. Reduction of calcium concentration in the external medium by EGTA leads to cessation of cell locomotion. Sr2+ (1-4 mM) is also able to replace calcium for triggering locomotion. These findings point to a control of Ca2+-activated contractions of actomyosin by influx of external Ca2+. According to our model of cell locomotion [14] the contractions generate a hydrostatic pressure extending the lamella by flow of hyaloplasm towards the margin. Small swellings (microcolliculi) appearing thereby will be dislocated by a calcium-dependent sol-gel transformation in this area, which contains actin but not myosin.