AMPHIBIAN LIMB REGENERATION
217
uniform irradiation of limbs had not been achieved. In experiments in
which nucleic acids and protein were injected into X-irradiated axolotl
limbs (Polejaieff et al, 1963), regenerative responses were obtained in
from 56% (with DNA) to 90% [with ribonucleic acid (RNA) ] of the
cases after repeated amputations. Repeated amputations of irradiated
control limbs, however, induced regenerative responses in well over half
the cases. The authors conclude that trauma alone, therefore, can alleviate to a considerable degree the X-ray inhibition of regeneration in the
axolotl limb. These experiments have been extended by Tuchkova (1966)
who traumatized irradiated axolotl limb tissues with forceps, or removed
the irradiated limb tissues, minced them, and replaced them in the limb
stump. The axolotls were grouped into eight experiments. Positive regeneration of 50, 53, and 71% of cases in three groups were obtained.
The remaining five groups showed no significant increase in cases of regeneration as compared with controls (0-11.5%). She concludes that
trauma restores regeneration in X-irradiated limbs by bringing about a
liberation of tissue products which induce tissue dedifferentiation and
blastemal cell accumulation.
Results which are so contradictory as are those of Stinson and
Polejaieff and his group, require careful analysis. Certainly the consistent
regeneration of irradiated controls after repeated amputation, in the
experiments of Polejaieff, are unusual, particularly in light of the work
of Brunst and Scheremetjewa (1937) in which X-rayed limbs were
kept for years without ever recovering the capacity to regenerate. Rose
(1964) has suggested that trauma may well have induced migration of
regeneration-competent fibroblasts into the irradiated limb, coming from
shielded regions of the shoulder or pelvic area. The fact that Polejaieff
did use only local irradiation is particularly significant. He irradiated the
entire limb as far as its junction with the body. Thus the shoulder or
pelvic area, which is limb-regeneration competent, was shielded by lead
and so protected from irradiation. Experiments of Verwoerd (1963) may
help in understanding the results obtained by Polejaieff. Verwoerd found
that when axolotl hind limbs were X-irradiated as far as the junction
with the body and amputation was performed soon thereafter through
the irradiated region, no regeneration occurred. After 11 weeks, during
which no signs of regenerative activity were seen, three successive
amputations through the previously irradiated region produced a high
percentage of regenerates (33 of 36). When the X-irradiation included
the pelvic and spinal cord region as well as the entire hind limb, no
regeneration was ever induced later, even when the limb underwent re-
217
uniform irradiation of limbs had not been achieved. In experiments in
which nucleic acids and protein were injected into X-irradiated axolotl
limbs (Polejaieff et al, 1963), regenerative responses were obtained in
from 56% (with DNA) to 90% [with ribonucleic acid (RNA) ] of the
cases after repeated amputations. Repeated amputations of irradiated
control limbs, however, induced regenerative responses in well over half
the cases. The authors conclude that trauma alone, therefore, can alleviate to a considerable degree the X-ray inhibition of regeneration in the
axolotl limb. These experiments have been extended by Tuchkova (1966)
who traumatized irradiated axolotl limb tissues with forceps, or removed
the irradiated limb tissues, minced them, and replaced them in the limb
stump. The axolotls were grouped into eight experiments. Positive regeneration of 50, 53, and 71% of cases in three groups were obtained.
The remaining five groups showed no significant increase in cases of regeneration as compared with controls (0-11.5%). She concludes that
trauma restores regeneration in X-irradiated limbs by bringing about a
liberation of tissue products which induce tissue dedifferentiation and
blastemal cell accumulation.
Results which are so contradictory as are those of Stinson and
Polejaieff and his group, require careful analysis. Certainly the consistent
regeneration of irradiated controls after repeated amputation, in the
experiments of Polejaieff, are unusual, particularly in light of the work
of Brunst and Scheremetjewa (1937) in which X-rayed limbs were
kept for years without ever recovering the capacity to regenerate. Rose
(1964) has suggested that trauma may well have induced migration of
regeneration-competent fibroblasts into the irradiated limb, coming from
shielded regions of the shoulder or pelvic area. The fact that Polejaieff
did use only local irradiation is particularly significant. He irradiated the
entire limb as far as its junction with the body. Thus the shoulder or
pelvic area, which is limb-regeneration competent, was shielded by lead
and so protected from irradiation. Experiments of Verwoerd (1963) may
help in understanding the results obtained by Polejaieff. Verwoerd found
that when axolotl hind limbs were X-irradiated as far as the junction
with the body and amputation was performed soon thereafter through
the irradiated region, no regeneration occurred. After 11 weeks, during
which no signs of regenerative activity were seen, three successive
amputations through the previously irradiated region produced a high
percentage of regenerates (33 of 36). When the X-irradiation included
the pelvic and spinal cord region as well as the entire hind limb, no
regeneration was ever induced later, even when the limb underwent re-