AMPHIBIAN LIMB REGENERATION
223
of the apical cap in attracting mesenchymatous cells is unknown, but
recent experiments (Thornton and Sprague, unpublished) in which whole
limb skin, or epidermis alone, is removed from an axolotl limb, treated
with actinomycin D, and returned to the limb, are of interest. The limb
stumps, completely covered with the actinomycin D treated whole skin
or epidermis, but the mesodermal tissues of which were untreated, failed
to regenerate or regenerated atypically after considerable delay, indicating the possibility that DNA-dependent RNA synthesis is in some way
involved with the regeneration-promoting action of skin.
At first it was thought that penetration of the wound epithelium by
nerves was prerequisite to apical cap formation (Thornton, 1954; see also
Trampusch and Harrebomee, 1965). Subsequently, however, apical cap
formation was seen in regenerating asensory limbs in which nerves were
present in the limb stump but not in the epidermis (Sidman and Singer,
1960; Thornton, 1960b). Also apical cap formation is present in limb
stumps possessing subthreshold numbers of nerve fibers (Singer and
Inoue, 1964). In these cases blastema formation does not occur but, as
the authors suggest, these results do not necessarily negate the possibility
of a morphogenetic influence of the apical cap but, rather, indicate that
the cooperation of other factors operative in regeneration is probably
needed also. This suggestion is given emphasis by experiments in this
laboratory in which superinnervation of limbs, by deviation of the right
sciatic nerve to the left hind limb, does not stimulate regeneration of
these limbs in hypophysectomized newts (Shuraleff and Tassava, unpublished). Also, growth hormone injections, reported to stimulate regeneration in hypophysectomized newts by Wilkerson (1963), will not bring
about regeneration in denervated urodele limbs (DeFazio and Thornton,
unpublished). The combined activities of nerves, hormones, and apical
cap are apparently essential for limb regeneration and one agent is
unable to substitute for another.
Other investigators have also noted a relationship between wound
epidermis and blastemal outgrowth. Thus Skowron and Walknowska
(1963) implanted young blastemata of axolotl limbs into unamputated
hind limbs. When the mesodermal core became separated from the
blastemal epidermis, further development of the implants ceased. However, if the mesodermal core became associated with the wound epidermis
of the insertion wound, then regenerative outgrowth occurred. Jordan
(1965) grafted limb blastemata to the brain in Xenopus. If these blastemata were deprived of epithelium they formed only cartilage nodules but
gave rise to limb regenerates when grafted with epithelium. Michael and
223
of the apical cap in attracting mesenchymatous cells is unknown, but
recent experiments (Thornton and Sprague, unpublished) in which whole
limb skin, or epidermis alone, is removed from an axolotl limb, treated
with actinomycin D, and returned to the limb, are of interest. The limb
stumps, completely covered with the actinomycin D treated whole skin
or epidermis, but the mesodermal tissues of which were untreated, failed
to regenerate or regenerated atypically after considerable delay, indicating the possibility that DNA-dependent RNA synthesis is in some way
involved with the regeneration-promoting action of skin.
At first it was thought that penetration of the wound epithelium by
nerves was prerequisite to apical cap formation (Thornton, 1954; see also
Trampusch and Harrebomee, 1965). Subsequently, however, apical cap
formation was seen in regenerating asensory limbs in which nerves were
present in the limb stump but not in the epidermis (Sidman and Singer,
1960; Thornton, 1960b). Also apical cap formation is present in limb
stumps possessing subthreshold numbers of nerve fibers (Singer and
Inoue, 1964). In these cases blastema formation does not occur but, as
the authors suggest, these results do not necessarily negate the possibility
of a morphogenetic influence of the apical cap but, rather, indicate that
the cooperation of other factors operative in regeneration is probably
needed also. This suggestion is given emphasis by experiments in this
laboratory in which superinnervation of limbs, by deviation of the right
sciatic nerve to the left hind limb, does not stimulate regeneration of
these limbs in hypophysectomized newts (Shuraleff and Tassava, unpublished). Also, growth hormone injections, reported to stimulate regeneration in hypophysectomized newts by Wilkerson (1963), will not bring
about regeneration in denervated urodele limbs (DeFazio and Thornton,
unpublished). The combined activities of nerves, hormones, and apical
cap are apparently essential for limb regeneration and one agent is
unable to substitute for another.
Other investigators have also noted a relationship between wound
epidermis and blastemal outgrowth. Thus Skowron and Walknowska
(1963) implanted young blastemata of axolotl limbs into unamputated
hind limbs. When the mesodermal core became separated from the
blastemal epidermis, further development of the implants ceased. However, if the mesodermal core became associated with the wound epidermis
of the insertion wound, then regenerative outgrowth occurred. Jordan
(1965) grafted limb blastemata to the brain in Xenopus. If these blastemata were deprived of epithelium they formed only cartilage nodules but
gave rise to limb regenerates when grafted with epithelium. Michael and