AMPHIBIAN LIMB REGENERATION
235
dermal tissues are not stimulated to form a regeneration blastema. These
data would allow the speculation that aneurogenic limb skin possesses
regeneration-promoting capacities which are lacking in corresponding
mesodermal tissues. These experiments involve trauma to limb tissues,
and, ever since the demonstration by Rose (1944) of the importance of
trauma for regeneration, additional evidence of its regeneration-stimulating effect has accumulated. First, Bodemer (1960) reported that accessory limbs in the adult newt could be evoked by a subthreshold nerve
supply if nerve deviation to the upper arm was accompanied by implantation of newt liver. He postulated that the excessive trauma associated
with the host reaction to the implant might have reduced the neural
threshold for regenerative response of the limb tissues. Later, Singer and
Mutterperl (1963) grafted sections of the newt forelimb to the back, and,
although ingrowth of local nerve fibers occurred, these were found to be
well below the threshold number—yet regeneration occurred in some of
these limb segments. The authors interpreted these results to mean that
the trauma of transplantation had lowered the neural threshold. More
recently Singer (1965) has proposed the possibility that "the chemical
agent which the nerve contributes to the growth process is not unique
for the neuron but is also manufactured by all other cells; that the
neuron manufactures much more and that other cells depend quantitatively upon the extra contribution; and, finally, that under certain circumstances nonnervous cells can be caused to produce more and therefore
require less from the nerve" (p. 24). This hypothesis might also explain
the regeneration of aneurogenic limbs, since in the absence of nerves the
nonneural production of trophic substance might be maintained at higher
levels, particularly in the skin, than occurs in these tissues in normally
innervated limbs.
Trauma may be even more significant for the production of trophic
substance in nonneural tissues than the above investigations indicate.
Although the evidence is somewhat circumstantial, Polejaieff (1939)
grafted the developing limbs of tadpoles to the ventral body wall, and
Pietsch and Webber (1965) grafted limbs of Ambystoma
larvae to the
orbit and in each case regeneration, after amputation, was reported as
occurring before nerves could have entered the limb grafts. Regeneration
also occurred (50% of cases) in denervated limb stumps of
Ambystoma
larvae which had been homoplastically grafted in place of forelimbs of
host larvae, with repeated section of host brachial nerves to prevent
nerve growth into the graft (Thornton and Tassava, unpublished). We
have, in addition, obtained supernumerary limb induction in aneurogenic
235
dermal tissues are not stimulated to form a regeneration blastema. These
data would allow the speculation that aneurogenic limb skin possesses
regeneration-promoting capacities which are lacking in corresponding
mesodermal tissues. These experiments involve trauma to limb tissues,
and, ever since the demonstration by Rose (1944) of the importance of
trauma for regeneration, additional evidence of its regeneration-stimulating effect has accumulated. First, Bodemer (1960) reported that accessory limbs in the adult newt could be evoked by a subthreshold nerve
supply if nerve deviation to the upper arm was accompanied by implantation of newt liver. He postulated that the excessive trauma associated
with the host reaction to the implant might have reduced the neural
threshold for regenerative response of the limb tissues. Later, Singer and
Mutterperl (1963) grafted sections of the newt forelimb to the back, and,
although ingrowth of local nerve fibers occurred, these were found to be
well below the threshold number—yet regeneration occurred in some of
these limb segments. The authors interpreted these results to mean that
the trauma of transplantation had lowered the neural threshold. More
recently Singer (1965) has proposed the possibility that "the chemical
agent which the nerve contributes to the growth process is not unique
for the neuron but is also manufactured by all other cells; that the
neuron manufactures much more and that other cells depend quantitatively upon the extra contribution; and, finally, that under certain circumstances nonnervous cells can be caused to produce more and therefore
require less from the nerve" (p. 24). This hypothesis might also explain
the regeneration of aneurogenic limbs, since in the absence of nerves the
nonneural production of trophic substance might be maintained at higher
levels, particularly in the skin, than occurs in these tissues in normally
innervated limbs.
Trauma may be even more significant for the production of trophic
substance in nonneural tissues than the above investigations indicate.
Although the evidence is somewhat circumstantial, Polejaieff (1939)
grafted the developing limbs of tadpoles to the ventral body wall, and
Pietsch and Webber (1965) grafted limbs of Ambystoma
larvae to the
orbit and in each case regeneration, after amputation, was reported as
occurring before nerves could have entered the limb grafts. Regeneration
also occurred (50% of cases) in denervated limb stumps of
Ambystoma
larvae which had been homoplastically grafted in place of forelimbs of
host larvae, with repeated section of host brachial nerves to prevent
nerve growth into the graft (Thornton and Tassava, unpublished). We
have, in addition, obtained supernumerary limb induction in aneurogenic