Biology Reference
In-Depth Information
CHAPTER
13
Cooperative Migration
of
Mesenchymal Ce
lls
So far in this topic, cell migration has been considered mainly as an activity of individual
cells, each moving by its own power and each navigating according to its own reading of the
environment and its own internal decision-making. Some cells do move that way. Others
move in groups and interact to such an extent that the existence of the group becomes impor-
tant to the way that migration takes place. During gastrulation in Xenopus laevis, for example,
mesenchymal cells migrate across the roof of the blastocoel as a group. If cells of this mesen-
chyme are placed on a blastocoel roof on their own, their migration is far less efficient.
1
The generic term for movements that use the fact that cells are in a group is 'collective cell
migration'. It is important to note that this means more than the simple fact that many cells
are moving in the same way at the same time: for cell migration to be truly collective there
must be some cooperative mechanism at work that makes the behaviour of the whole collec-
tive something more than simply the sum of the behaviours that the cells would display were
they each migrating alone. Evidence for migration as a group is easy to obtain by simple
observation: evidence for migration as a cooperating collective is more elusive
2
but it has
been demonstrated convincingly in a few systems.
The most dramatic examples of collective cell migrations are provided by continuous
sheets and tubes of epithelial and endothelial tissue. Already touched on in the section on
wound healing in Chapter 10, they are so common and so important to development that
they have
Section IV
(Epithelial Morphogenesis) of this topic to themselves. This chapter
will focus on cells with a more mesenchymal nature, which show both individual motility
and cooperation and migrate as streams or chains.
WHY MIGRATE AS A COLLECTIVE?
Before examining specific examples of cell streaming, it is worth considering why collective
migration should have evolved: that is, what advantage it might confer on an embryo. There
are two main theories, which are not mutually exclusive. The first theory is that cooperativity
evolved as a way to make migration more accurate and reliable than it could be if every cell
were independent. When cells migrate as a cooperative phalanx rather than as individuals,
the group of cells can sample more space (for chemotactic, galvanotactic, haptotactic or
