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and also when moving away on the inside. Either these cells prefer to migrate towards the
anode, or the field (if at all relevant to gastrulation movements) is actually used to limit
the rate of their migration. There is evidence that the electric field is used for some develop-
mental purpose in these embryos; if it is opposed by an artificially applied field, about 75
percent of embryos develop abnormally. 29 Unfortunately, the abnormalities are variable,
possibly because precise placing of electrodes is difficult, which precludes the identification
of a single developmental event that requires the electric field.
Endogenous electric fields and currents have also been detected in chick embryos, partic-
ularly in the region of the posterior intestinal portal, where the developing alimentary canal
is open to the ventral surface of the embryo ( Figure 10.8 ). Again, conventional current is
directed primarily out of the pole and back in through other regions of the embryo, and again
at least half is carried by sodium ions. 30 If these currents are shorted out, by bridging the
embryonic epithelium with fine glass needles filled with conductive agarose gel, over 80
percent of embryos develop abnormally, especially in the region of the tail. Control glass nee-
dles that are non-conducting cause only 11 percent of embryos to become abnormal, confirm-
ing that abnormalities are caused by the shorting of the electric fields, rather than by simple
surgical damage. 31 Again, the spectrum of abnormalities observed is too broad to pin down
one single mechanism, and the effect may have nothing to do with control of cell movement.
There is evidence for an electric field in one of developmental biology's most studied
systems, the vertebrate limb bud. The embryos of both mice and chicks show an inwardly
directed conventional current along the flank of the embryo, far from the limb buds, and
an outward current along the limb buds. 32 The cause of the current is not clear, but it may
be due to the 'normal' inward flux of Na รพ ions and their leakage from imperfectly sealed
epithelia in the limb bud; alternatively, the bud may possess active mechanisms to drive these
FIGURE 10.8 Flows of current (arrows) out of the posterior intestinal portal of a chick embryo.
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