The Organ Tree
183
specific electrical signal in regenerating animals: a few hundred nanoam-
peres of direct current, initially positive, then changing in the course of
a few days to negative.
The pituitary hormone prolactin, the same substance that stimulates
milk flow in nursing mothers, seems to sensitize cells to electric current.
Then the signal causes nearby cells to dedifferentiate and form a
blastema, apparently by changing the way cell membranes pass calcium
ions. After confirming our frog blood-cell work, Art Pilla went on to
produce the same changes by using pulsed DC to make a wave of cal-
cium ions flowing across the culture dish. Steve Smith then confirmed
the importance of calcium by preventing dedifferentiation with a cal-
cium-blocking compound, and restarting it with another substance that
enhanced passage of calcium ions. Working together, Smith and Pilla
next used the same PEMF wave form now in clinical use to nearly dou-
ble the rate of salamander limb regeneration, while completely prevent-
ing it with a different pulse pattern. Widespread recent work on
calcium-binding proteins, such as calmodulin, has made it fairly certain
that electrical control of calcium movement through cell membranes di-
rects the give-and-take among these proteins, which in turn supervises
the cell's entire genetic and metabolic industry.
Although not conclusive, the available evidence suggests that the cur-
rent flows through the perineural cells rather than the neurons them-
selves (see Chapter 13). These are several types of cells that completely
surround every nerve cell, enclosing all the peripheral fibers in a sheath
and composing 90 percent of the brain. Lizards can replace their tails
without the spinal cord, as long as the ependyma, or perineural cells
surrounding the cord, remains intact, and ependymal tissue transplanted
to leg stumps gives lizards some artificial regeneration there. However,
the circuit may shift tissues near the wound, for Elizabeth Hay's electron
microscope studies clearly show that the peripheral nerve's Schwann cell
sheaths stop just short of the epidermis, and only the naked neuron tips
participate in the NEJ. The exact current pathway in this microscopic
area remains to be charted.
Not all cells can respond, however, as Jim Cullen and I found in one
part of our fortuitous rat-regeneration experiments of 1979. Dedifferen-
tiation occurred only when we passed the deviated sciatic nerve to the
epidermis through the bone marrow. When we led it through the muscle
yet sutured it to the skin in exactly the same way, an NEJ producing the
right current appeared, but no blastema and no regrowth.
Muscle cells
apparently weren't
competent to differentiate in the adult rat. The
cellular target proved to be just as important as the electrical arrow.