232
CHARLES S. THORNTON
The quantitative neural requirement for regeneration may be lower in
larval amphibians than it is in adults. The first indication that fewer
nerve fibers may be required for limb regeneration in larvae came from
the work of Karczmar (1946) who observed that 20% of limbs of
Ambystoma
larvae regenerated with spinal nerve 5 alone in the limb
stump. Since nerve 5 contributes only about 10% of the nerve supply of
the stump, these limbs were apparently regenerating at a much lower
neural threshold than pertains for adult newts (about 30 to 50% of total
innervation). Deck (1961a) obtained limb regeneration in
Ambystoma
larvae in 73% of the cases when spinal nerve 5 constituted the sole innervation of the limb stump. Peadon and Singer (1965), however, found
that the seemingly lower neural threshold for regeneration was directly
correlated with the fact that the larval limb is more richly innervated
than is the adult limb. The newt, Notophthalmus,
provides an excellent
form for analysis of neural thresholds since there are two metamorphoses
and thus three stages in the life cycle: larval, land phase, and aquatic
adult. Peadon and Singer found values of 75.5, 47.6, and 27.7 fibers per
(100//,)
2 of amputation surface in limb stumps of larvae, small land
phase, and adult newts, respectively. They also found, assuming that
threshold fiber requirement for regeneration of larvae and small land
phase newts is similar to that of adults [8.3-13.3 fibers per (100 /A)
2
evokes regeneration in adult forelimbs in 50% of cases], that the threshold
is attained in the small land forms by only 17.4 to 27.9% of the total
number of nerve fibers and in the larvae by only 10.9 to 17.6%. Thus, in
larval newts at least, the assumption that the larva has a lower neural
threshold for limb regeneration is not sustained by these data. Experiments of Van Stone, however, provide evidence that young tadpole hind
limbs do have a lower neural threshold than older limbs. Thus, in young
tadpoles of Rana sylvatica during developing hind limb stages VII to
XI, limb regeneration capacity is lost without a corresponding
significant
drop in fibers per unit area of amputation surface (Van Stone, 1964). It
would seem, therefore, that a qualitative change of some kind has occurred in the limb tissues which has rendered the existing nerve number
insufficient to support regeneration. Singer (1954) found that limb regeneration may, however, be evoked again in young adult frogs when the
total number of nerve fibers in the limb is increased by deviating the
sciatic nerve to the forelimb stump. Surprisingly, the forelimb of adult
Xenopus, which can regenerate a long spike, shows just opposite reactions
to "superinnervation"—it regenerates much more poorly (KoniecznaMarczynska and Skowron-Cendrzak, 1958). That this may constitute a
CHARLES S. THORNTON
The quantitative neural requirement for regeneration may be lower in
larval amphibians than it is in adults. The first indication that fewer
nerve fibers may be required for limb regeneration in larvae came from
the work of Karczmar (1946) who observed that 20% of limbs of
Ambystoma
larvae regenerated with spinal nerve 5 alone in the limb
stump. Since nerve 5 contributes only about 10% of the nerve supply of
the stump, these limbs were apparently regenerating at a much lower
neural threshold than pertains for adult newts (about 30 to 50% of total
innervation). Deck (1961a) obtained limb regeneration in
Ambystoma
larvae in 73% of the cases when spinal nerve 5 constituted the sole innervation of the limb stump. Peadon and Singer (1965), however, found
that the seemingly lower neural threshold for regeneration was directly
correlated with the fact that the larval limb is more richly innervated
than is the adult limb. The newt, Notophthalmus,
provides an excellent
form for analysis of neural thresholds since there are two metamorphoses
and thus three stages in the life cycle: larval, land phase, and aquatic
adult. Peadon and Singer found values of 75.5, 47.6, and 27.7 fibers per
(100//,)
2 of amputation surface in limb stumps of larvae, small land
phase, and adult newts, respectively. They also found, assuming that
threshold fiber requirement for regeneration of larvae and small land
phase newts is similar to that of adults [8.3-13.3 fibers per (100 /A)
2
evokes regeneration in adult forelimbs in 50% of cases], that the threshold
is attained in the small land forms by only 17.4 to 27.9% of the total
number of nerve fibers and in the larvae by only 10.9 to 17.6%. Thus, in
larval newts at least, the assumption that the larva has a lower neural
threshold for limb regeneration is not sustained by these data. Experiments of Van Stone, however, provide evidence that young tadpole hind
limbs do have a lower neural threshold than older limbs. Thus, in young
tadpoles of Rana sylvatica during developing hind limb stages VII to
XI, limb regeneration capacity is lost without a corresponding
significant
drop in fibers per unit area of amputation surface (Van Stone, 1964). It
would seem, therefore, that a qualitative change of some kind has occurred in the limb tissues which has rendered the existing nerve number
insufficient to support regeneration. Singer (1954) found that limb regeneration may, however, be evoked again in young adult frogs when the
total number of nerve fibers in the limb is increased by deviating the
sciatic nerve to the forelimb stump. Surprisingly, the forelimb of adult
Xenopus, which can regenerate a long spike, shows just opposite reactions
to "superinnervation"—it regenerates much more poorly (KoniecznaMarczynska and Skowron-Cendrzak, 1958). That this may constitute a