Biology Reference
In-Depth Information
CHAPTER
11
G
uidance by Conta
ct
Most migrating cells migrate across and through solid tissues, and these tissues generally
show numerous fine-scale variations, consisting as they do of different types of cells and
matrices. These variations can be used as a navigation system by migrating cells, either in
conjunction with chemotactic and galvanotactic signals or as a complete guidance system
in their own right.
HAPTOTAXIS
Haptotaxis is the guidance by means of a gradient of adhesion. It is a phenomenon that can
affect purely physical as well as biological systems, and relies on another version of a Brow-
nian Ratchet (see Chapters 6 and 9). Adhesions between surfaces occur when the free energy
of the system (the surfaces plus solvent) is lower when the surfaces are in contact than when
they are separated. If this difference in free energy is not much greater than thermal energy in
the system, adhesive bonds will not be permanent and will be continuously made and
broken. The stronger an adhesive contact (the greater the difference in free energy), the
longer, on average, it will resist disruption. Consider an object able to roll, or to deform
like a membrane vesicle or cell, on a gradient of an adhesive molecule. The object will be buf-
feted by random thermal agitation in both up-gradient and down-gradient directions. Buffet-
ing in the up-gradient direction will strain the slightly weaker adhesions while buffeting in
the down-gradient direction will strain the slightly stronger ones. The slightly stronger adhe-
sions will resist disruptive forces marginally better and last longer. On average, therefore, the
down-gradient bonds will break and allow the object to be moved up-gradient more often
than
vice versa
, and the object will be translocated up the gradient (
Figure 11.1
). Haptotaxis
)
can be modelled in simple physical vesicles
1
and it can be demonstrated in a practical class
using everyday materials.
♥
)
Greek:
haptein
fasten;
t
´
xis
,
arrangement.
♥
Wrap a piece of paper at least a quarter of the way round a curved surface such as a gas cylinder, and spray
it lightly with spray adhesive from a direction that is perpendicular to the surface of one end of the paper.
The result will be a piece of paper with an adhesive gradient. Place the paper in a box so that it covers the
bottom of the box, scatter small spherical cake decorations (known as 'hundreds and thousands' in the UK)
on to the paper, and agitate the box by allowing it to make contact with a vortex machine or some other
gentle vibrator. The decorations move up the adhesive gradient.
¼
¼
