Biology Reference
In-Depth Information
fibroblasts, for example, they migrate towards the cathode.
2
This field strength is very low
compared with the fields across a typical plasma membrane (60 mV across 7 nm
z
8,600,000 V/m), and, to give a human-scaled example, the field strength capable of steering
fibroblasts is approximately equal to the field between the conductor rails in the tracks of the
London Underground. Other cells, such as avian neural crest,
3
mammalian keratinocytes,
4
corneal epithelium cells
5
and neuronal growth cones
6
exhibit similar behaviour, albeit with
minor differences in timing and minimum effective field strength (often down to 10 V/m).
7
The response of cells to electric fields depends on the substratum on which they are growing;
some substrates cause cells to migrate towards the cathode but others cause the same cells to
migrate towards the anode.
8
Not all cells show galvanotaxis; human skin-derived skin melanocytes
9
and fibroblasts
5
fail to respond to fields that are capable of steering keratinocytes from the same kind of
skin. This variation in galvanotactic ability supports the assertion that galvanotaxis is a 'delib-
erate' facility possessed by just some cell types, rather than an 'accidental' property that
emerges from the general features of cell biology. This suggests, but does not prove, that gal-
vanotaxis is really used
in vivo
.
Cells do not appear to have any structures that are obviously dedicated to detection of
large-scale DC electric fields and the work on chemotaxis discussed in Chapter 9 has shown
that cells' sense of direction is encoded by internal chemical gradients. Together, these facts
imply that the most likely mechanism for galvanotaxis is one by which electric fields can
influence the orientation of chemical gradients inside the cell. Several experimental findings
support this idea. It might be expected that, if electric fields steer growth cones by biasing the
direction of chemical signalling, they will be more influential when there is a strong chemical
signal on which to work. The turning of neuronal growth cones by electric fields will take
place at lower field strengths if neurotrophins are also supplied homogenously rather than
in a gradient.
10
Corneal epithelial cells are unresponsive to electric fields in serum-free media
but become responsive when serum or EGF is added.
11
Chemical activity caused by applica-
tion of neurotransmitters has a similar effect. Using turbocurarine to block the acetylcholine
signalling that arises from small amounts of acetylcholine released by the neurons them-
selves inhibits cathode-directed turning by growth cones,
12
as does blocking the rise in cyto-
plasmic calcium elicited by acetylcholine signalling.
13
Similarly, blocking the activity of EGF
receptors on human keratinocytes with the specific pharmacological inhibitor PD158780
renders the cells insensitive to electric fields although they still migrate.
14
The most obvious way in which DC electric fields might influence a chemical system is by
electrophoresis, the attraction of negatively charged molecules towards the anode and posi-
tively charged ones towards the cathode. This could operate either at the level of biasing the
distribution of extracellular chemoattractants or at the level of directly biasing components of
intracellular signalling. Direct injection of fluorescently labelled charged proteins into early
amphibian limb buds, where high electric fields naturally exist (see below), results in a diffu-
sion pattern that is indeed biased along the electric field, rather like the patterns seen on
rocket electrophoresis (
Figure 10.2
). In principle, electric fields could therefore act indirectly
by giving rise to chemotactic gradients through concentrating a chemoattractant to one end
of the tissue.
There is also some evidence for electrophoresis of cellular components; cellular glycopro-
teins capable of binding the lectin, concanavalin A (Con A), accumulate on the cathodal side
