The Embryo at the Wound 55
DEDIFFERENTIATE: PARTIAL OR COMPLETE?
the major, probably the only, way a blastema forms in complex animals
like a salamander.
Nerve Connections
The other major question about the blastema's origin is: What triggers
it? The best candidates for a "carrier" of the stimulus are the nerves. In
complex multicellular animals, there's no regeneration without nerve
tissue. Back in 1823, the English amateur Tweedy John Todd found
that if the nerves into a salamander's leg were cut when the amputation
was made, the limb wouldn't regrow. In fact, the stump itself shriveled
up and disappeared. However, Todd got normal regeneration when he
gave the nerves time to reconnect before severing the leg. Science wasn't
ready to make anything of his observation, but many experiments since
have confirmed it. Over a century later, Italian biologist Piera Locatelli
showed that an extra leg would grow if a nerve was rerouted so that it
ended near an intact leg. She cut the large sciatic nerve partway down
the salamander's hind leg, leaving it attached to the spinal column and
fully threading it up untder the skin so that its end touched the skin