The Body Electric - Micro-ondes

56 The Body Electric
near one of the forelegs. An extra foreleg sprouted there. When she
placed the nerve end near a hind leg, an extra hind leg grew. It didn't
matter where the nerve was supposed to be; the kind of extra structure
depended on the target area. This indicated that some sort of energy
from the nerves was adapted by local conditions that determined the
pattern of what grew back.
Soon afterward, other researchers found that when they sewed full-
thickness skin grafts over the stumps of amputated salamander legs, the
dermis, or inner layer of the skin, acted as a barrier between the apical
cap and an essential something in the leg, thereby preventing regenera-
tion. Even a tiny gap in the barrier, however, was enough to allow
regrowth.
In the early 1940s this discovery led S. Meryl Rose, then a young
anatomy instructor at Smith College, to surmise that the rapid forma-
tion of full-thickness skin over the stumps of adult frogs' legs might be
what prevented them from regenerating. Rose tried dipping the wounds
in saturated salt solution several times a day to prevent the dermis from
growing over the stump. It worked! Most of the frogs, whose forelimbs
he'd amputated between the elbow and wrist, replaced some of what
they'd lost. Several regrew well-formed wrist joints, and a few even be-
gan to produce one new finger. Even though the replacements were in-
complete, this was a tremendously important breakthrough, the first
time any regeneration had been artificially induced in an animal nor-
mally lacking the ability. However, the dermis did grow over the
stump, so the experiment worked by some means Rose hadn't expected.
Later, other investigators showed that in normal regeneration the api-
cal cap, minus the dermis, was important because regrowing nerve fibers
made unique connections with the epidermal cells in the first stage of
the process, before the blastema appeared. These connections are collec-
tively called the neuroepidermal junction (NEJ). In a series of detailed
experiments, Charles Thornton of Michigan State University cut the
nerves to salamander legs at various times before amputating the legs,
then followed the progress of the regrowing nerves. Regeneration began
only after the nerves had reached the epidermis, and it could be pre-
vented by any barrier separating the two, or started by any breach in the
barrier. By 1954 Thornton had proved that the neuroepidermal junction
was the one pivotal step that must occur before a blastema could form
and regeneration begin.
Shortly thereafter, Elizabeth D. Hay, an anatomist then working at
Cornell University Medical College in New York, studied the neu-
roepidermal junction with an electron microscope. She found that as