The Organ Tree
185
ions in wet animals. In the last two decades nearly all tissues have been
proven to produce or carry various kinds of electrical charge. Skin may
play an as-yet-unknown role in regeneration besides its part in the NEJ,
or it may merely be producing unrelated electrical effects. In any case,
there are far too many data about the role of nerves to call skin the major
source of the regeneration current. In fact, even the Purdue group has
measured a stump current that's independent of sodium concentration.
A recent experiment by Meryl Rose gave further evidence of neural
DC, without clearing up all aspects of the question. Rose removed the
nerves From larval salamander legs before amputation. Normally such
denervated larval stumps die right back to the body wall, but when Rose
artificially supplied direct currents like those I measured in my first ex-
periment, they regrew normally. This is pretty conclusive proof that the
nerves are the electrical source in phase one. However, since it looks as
though nerves also organize regeneration's second phase (see below), it's
hard to understand how the new legs could have been completely normal
when they were disconnected from the rest of the nervous system. Per-
haps salamanders can pinch-hit for nerves at this stage through a tissue
other than nerve. On the other hand, new nerves may simply have re-
grown into the limbs unbeknownst to Rose by the later stages of the
experiment.
Phase two begins as the embryonal cells pile up and the blastema
elongates. Early in this stage a sort of spatial memory becomes fixed in the
blastema cells so the limb-to-be will have its proper orientation to the rest
of the body. At the same time or shortly afterward, the cells at the inner
edge of the blastema receive their new marching orders and platoon
assignments. Then they redifferentiate and take their places in the new
structure.
We can infer two things about the control for this part of the process.
Since the blastema forms the right structure in relation to the whole organ-
ism, the guidance can't be purely local, but must come from a system
that likewise pervades the whole body. Furthermore, there are no de-
differentiated cells left over when the work is done; there are just enough
and no more. Thus there must be a feedback mechanism between the
redifferentiation controls at the body side of the blastema and the NEJ's
dedifferentiation stimulus at its outer edge.
A large body of earlier work has shown that the redifferentiation in-
structions are passed along a tissue arc whose main element is the circuit
already established between nerves and epidermis in the first phase. The
electrical component persuasively explains how this arc, an update of the
morphogenetic field, may work. The direction (polarity) plus the magni-