Biology Reference
In-Depth Information
CHAPTER
9
Gui
dance by Chemot
axis
Chemotaxis is the directional migration of cells, or parts of cells, in response to
a gradient of concentration of a diffusible molecule. It is the diffusibility of the guidance
molecule(s) that distinguishes chemotaxis from haptotaxis, in which cells are guided by
molecules that are anchored on a substrate (Chapter 11). Chemotaxis is an important
mechanism for cell guidance during development and continues to be used in some
aspects of adult life, such as the recruitment of neutrophils in the vertebrate inflammatory
response.
For chemotaxis to work, a developing system must possess the following features:
A mechanism for creating the gradient of the chemoattractant (/chemorepellent).
A mechanism that allows parts of migrating cells to sense local chemoattractant
concentrations.
A mechanism that translates typically shallow external gradients in external
chemoattractant into typically steep gradients in protrusive activity within a cell.
This chapter will consider each of these elements in turn, and will go on to address how
chemotaxis is identified experimentally. Finally, some examples of chemotaxis in develop-
ment will be reviewed.
THE CHEMOTACTIC GRADIENT
There are many possible ways of producing a chemotactic gradient, but most biological
examples studied to date seem to rely on producing a chemoattractant from a localized
source and then allowing it to diffuse away. The rate of diffusion is determined by the chemo-
attractant's diffusion constant and by the local concentration gradient. The change in concen-
tration over time at any point in the field is given by Fick's law, which states for an essentially
one-dimensional system such as a long, hollow pipe, that:
J ¼D
vf
vx
where J is the net flow of substance per cross sectional area of pipe per unit second (in mol
m
2
sec
1
),
F
is the concentration (in mol m
3
)and
is the distance along the pipe (in m),
and D is the diffusion constant (in m
2
sec
1
).
