AMPHIBIAN LIMB REGENERATION
209
damage produced by ligaturing the limb, introducing foreign tissues
beneath the skin of the limb, irradiating a segment of the limb with
ultraviolet light, or the presence of the cut end of a deviated nerve. The
nature of the regeneration-releasing stimulus is unknown, but Needham
(1941) has postulated the production of a "wound factor" by the tissues
directly injured by the amputation. This factor supposedly initiates the
processes of tissue dedifferentiation, or regression, from which the
mesenchymatous blastema cells are formed. In the frog tadpole tail,
treatment of the amputation surface alone with beryllium nitrate inhibits
subsequent regeneration if the treatment occurs within 1 hour of the
injury. In Ambystoma
larvae, however, inhibition by beryllium treatment
is correlated with extent of wound healing. Thus beryllium nitrate treatment of the limb stump inhibits regeneration at 3 hours after amputation
but not at 6 hours (Thornton, 1949, 1950, 1951). Wound healing is
incomplete at 3 hours but at 6 hours a complete wound epithelium covers
the amputation surface and this membrane apparently limits the entrance
of the beryllium into the limb stump. It is interesting that the beryllium
nitrate, in equivalent doses, has no apparent deleterious effect on uninjured limb tissues but causes considerable destruction to stump tissues
injured by amputation. Scheuing and Singer (1957) also found this to
be true and in addition found that infusion of beryllium nitrate into the
already formed, limb blastema of the adult newt stopped further regeneration and set in motion progressive destruction of the stump tissues.
They point out, therefore, that an open wound is not prerequisite to the
effect of beryllium, which seems to be in the nature of a general tissue
poison rather than a specific inhibitor. There are, however, a number of
experiments which indicate that regeneration promoters may be active
in later stages of regeneration. Thus Issekutz-Wolsky and Fogarty (1962)
increased regeneration rates in newts by implantation of regeneration
blastemata beneath the skin of the lateral surface of the body. Wiecek
(1964) implanted homogenized blastemata beneath the skin of the body
of axolotls and observed increased regeneration rates if the implantation
was made immediately after amputation but not during differentiating
blastemal stages, indicating an effect on proliferation. The promoter
substance was found to be temperature-sensitive (Wiecek, 1966). Weber
and Maron (1965) reported increased regeneration rates after implanting
stump muscle beneath abdominal skin in axolotls where it dedifferentiated
in situ to produce blastemalike cells. They concluded that the growth
factor was somehow associated with muscle dedifferentiation. They also
found that the soluble protein fraction of blastema homogenates was
209
damage produced by ligaturing the limb, introducing foreign tissues
beneath the skin of the limb, irradiating a segment of the limb with
ultraviolet light, or the presence of the cut end of a deviated nerve. The
nature of the regeneration-releasing stimulus is unknown, but Needham
(1941) has postulated the production of a "wound factor" by the tissues
directly injured by the amputation. This factor supposedly initiates the
processes of tissue dedifferentiation, or regression, from which the
mesenchymatous blastema cells are formed. In the frog tadpole tail,
treatment of the amputation surface alone with beryllium nitrate inhibits
subsequent regeneration if the treatment occurs within 1 hour of the
injury. In Ambystoma
larvae, however, inhibition by beryllium treatment
is correlated with extent of wound healing. Thus beryllium nitrate treatment of the limb stump inhibits regeneration at 3 hours after amputation
but not at 6 hours (Thornton, 1949, 1950, 1951). Wound healing is
incomplete at 3 hours but at 6 hours a complete wound epithelium covers
the amputation surface and this membrane apparently limits the entrance
of the beryllium into the limb stump. It is interesting that the beryllium
nitrate, in equivalent doses, has no apparent deleterious effect on uninjured limb tissues but causes considerable destruction to stump tissues
injured by amputation. Scheuing and Singer (1957) also found this to
be true and in addition found that infusion of beryllium nitrate into the
already formed, limb blastema of the adult newt stopped further regeneration and set in motion progressive destruction of the stump tissues.
They point out, therefore, that an open wound is not prerequisite to the
effect of beryllium, which seems to be in the nature of a general tissue
poison rather than a specific inhibitor. There are, however, a number of
experiments which indicate that regeneration promoters may be active
in later stages of regeneration. Thus Issekutz-Wolsky and Fogarty (1962)
increased regeneration rates in newts by implantation of regeneration
blastemata beneath the skin of the lateral surface of the body. Wiecek
(1964) implanted homogenized blastemata beneath the skin of the body
of axolotls and observed increased regeneration rates if the implantation
was made immediately after amputation but not during differentiating
blastemal stages, indicating an effect on proliferation. The promoter
substance was found to be temperature-sensitive (Wiecek, 1966). Weber
and Maron (1965) reported increased regeneration rates after implanting
stump muscle beneath abdominal skin in axolotls where it dedifferentiated
in situ to produce blastemalike cells. They concluded that the growth
factor was somehow associated with muscle dedifferentiation. They also
found that the soluble protein fraction of blastema homogenates was