  Eukaryotic cells are large enough to detect signals and then orient to them
by differentiating the signal strength across the length and breadth of the
cell. Amoebae, fibroblasts, neutrophils and growth cones all behave in this
way. Little is known however about cell motion and searching behavior in the
absence of a signal. Is individual cell motion best characterized as a random
walk? Do individual cells have a search strategy when they are beyond the range
of the signal they would otherwise move toward? Here we ask if single,
isolated, Dictyostelium and Polysphondylium amoebae bias their motion in the
absence of external cues. We placed single well-isolated Dictyostelium and
Polysphondylium cells on a nutrient-free agar surface and followed them at 10
sec intervals for ~10 hr, then analyzed their motion with respect to velocity,
turning angle, persistence length, and persistence time, comparing the results
to the expectation for a variety of different types of random motion. We find
that amoeboid behavior is well described by a special kind of random motion:
Amoebae show a long persistence time (~10 min) beyond which they start to lose
their direction; they move forward in a zig-zag manner; and they make turns
every 1-2 min on average. They bias their motion by remembering the last turn
and turning away from it. Interpreting the motion as consisting of runs and
turns, the duration of a run and the amplitude of a turn are both found to be
exponentially distributed. We show that this behavior greatly improves their
chances of finding a target relative to performing a random walk. We believe
that other eukaryotic cells may employ a strategy similar to Dictyostelium when
seeking conditions or signal sources not yet within range of their detection
system.
