Biology Reference
In-Depth Information
FIGURE
13.1
Collective
decision-making
by groups of
migrating cells
allows sampling of
a much broader
environment than
any cell on its own
could explore.
durotactic influences) than one cell alone could. This increases the probability of the group
finding a narrow path or an isolated waypoint (
Figure 13.1
a) Even where the target is broad,
there is the opportunity to improve accuracy by averaging the cells' individual estimates of the
direction of their correct path; effectively the standard error of their estimate would be
expected to reduce by the half-power of the number of cells contributing to that estimate
(
Figure 13.1
b).
The other theory is that collective cell migration is a means of avoiding a cellular traffic
jam. To understand this idea, it helps to gain some appreciation of the phenomena that arise
when many independently moving objects share limited space. One striking discovery that
has emerged from research into human-scale movement (stadium crowds, road traffic, and
so on) is the existence of phase transitions in behaviour, analogous to those between a gas,
a liquid and a solid.
3
Consider a large number of identical objects that either show completely
random movement in a two-dimensional plane, or directed movement with an additional
random component (it does not matter for now). Where the density of objects is low, each
object has plenty of space. Interactions between objects will be infrequent and of short dur-
ation, and there will be almost no correlation between the movement of one object and
another (
Figure 13.2
a). In other words, they will move rather as do molecules of a gas. Where
the density is high, on the other hand, each object will be hemmed-in by its neighbours and
its motion will be constrained. When an object is trying to travel against the flow of its imme-
diate neighbours, its progress will quickly be arrested by collisions whereas, when it is
