Biology Reference
In-Depth Information
wild-type mice, shows little correlation between field strength and healing speed. 21 In the
cornea, therefore, electric fields seem not to be critical in maintaining directional migration
during wound healing (the timescale of these experiments leaves the question of whether
fields are critical in initiating healing responses unanswered).
Regardless of whether or not electric fields are an important guidance mechanism in
normal wound healing, they have been used to promote healing in experimental systems.
The spinal cord of the larva of the sea lamprey Petromyzon marinus is an unusually accessible
experimental system, because it can be maintained for many days in culture and it contains
axons large enough to distinguish in whole-mount observation. If a cultured P. marinus spinal
cord is severed, a strong current flows from the medium into the cut end of the cord; the
current falls to about 1/20th of its initial value after a few hours, and is then maintained at
that level. It leaves the sides of the cord, starting some distance away from the cut site. 22
This flow of current into the transacted spinal cord implies an electric field orientated the
wrong way for regrowth of the cut axon stumps into the wound site, and indeed the axons
die back. Experimental application of a counter-current so that the local field reverses and the
wound is cathodic relative to the axon strongly promotes regeneration by growth of the axon
stumps into the wound. 23 The growing axons have large active growth cones, absent in
controls, suggesting that galvanotactic encouragement of migration may be one mechanism
by which healing is promoted, but it remains possible that healing is promoted by entirely
different mechanisms instead. For example, the imposed current strongly reduces how far
the axons die back following injury, probably by controlling Ca 2 รพ build-up, rather than
any galvanotactic response. 24 There is growing evidence that imposed electric currents of
various sorts may also promote healing from spinal injuries in mammals, 25 e 27 although
the research is still at a fairly early stage.
The electric fields of embryos can be studied by using a vibrating probe to compare the
potential difference between two closely spaced points just above a tissue of interest.
A modern vibrating probe apparatus 28 typically consists of a conventional reference elec-
trode immersed in the bulk medium in which the embryo is placed, and a very fine insulated
probe electrode that terminates in a small uninsulated platinum sphere ( Figure 10.6 ). The
probe electrode is supported by two piezoelectric crystals arranged at 90 degrees to each
other, one of which can impart a tiny movement in the x axis and the other in the y axis.
The crystals are supplied with sine wave currents 90 degrees out of phase with each other
at the resonant frequency of the probe; the result is that the probe tip describes a tiny circular
movement. The voltage sensed by the probe tip as it orbits, and the sine waves that drive it,
are monitored by a computer; from the places at which the tip senses the maximum and
minimum voltages, and from the differences between these voltages, the computer can calcu-
late the direction and magnitude of the local electric field vector and plot it on a video image
of the embryo. The currents that must be flowing locally can be inferred from the fields and
the conductivity of the medium.
An example of an embryonic electric field that has much in common with wound fields is
seen at the blastopore of embryos of the frog Xenopus laevis . The blastopore is a pit in the side
of the embryo, through which cells fated to be endodermal flow so that they leave the outer
surface of the embryo and can create a new inner surface; it persists from the beginning of
gastrulation to late neurulation ( Figure 10.7 ). There is a strong outward flow of conventional
current through the blastopore throughout its existence, which peaks at over 1 A/m 2 and at
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