AMPHIBIAN LIMB REGENERATION
233
special case, however, is likely since there is a stimulatory effect in the
regeneration-competent hind limb of the axolotl when the contralateral
sciatic nerve is deviated to the limb stump (Shuraleff and Thornton,
1965).
The nerve requirement in regeneration is more critical for early phases
of the process than for later ones. In larval urodeles, Schotte and Butler
(1941) found that denervation of the limb at amputation not only inhibited regeneration but the dedifferentiation phase became prolonged to
the extent that much of the limb stump regressed. Such excessive regression seems to be more characteristic of the larval limb since denervated
limb stumps of adult newts retain their structure. Schotte and Butler
(1944) found that denervation inhibited limb regeneration in the larva if
the operation followed amputation by 1 to 6 or 7 days, but regeneration
would proceed in limb stumps denervated 9 or more days after amputation. Similarly, Singer and Craven (1948) found that regeneration was
suppressed when denervation of the adult newt limb occurred as late as
13 days after amputation, but when denervation was performed at 17
days or later, differentiation proceeded and normal, but small, regenerates developed. They found that mitotic proliferation of the blastemal
cells was inhibited by the denervation. The neural influence on limb
regeneration would seem to be of particular importance, therefore, for the
early growth phases of regeneration (see also Hay, 1956, 1966).
The problem of how nerves exert their effect on growth was investigated
particularly by Singer and his associates (review in Singer, 1960). As a
number of chemical substances (i.e., acetylcholine, sympathin) were
implicated in the transmission of the nerve impulse, the idea that nerve
influence on regeneration—the "trophic" effect—was also mediated by a
chemical mechanism seemed logical. Schotte (1926) was the first to test
this hypothesis experimentally on the limb stump by applying various
drugs associated with sympathetic nerve metabolism but without success.
Taban (1955) found that acetylcholine and other neurodynamic substances failed to support regeneration in denervated urodele limb stumps.
Singer (1960) infused into newt limb stumps a variety of substances
known to block acetylcholine mechanism—atropine, procaine hydrochloride, tetraethylammonium hydroxide—and achieved delay and even
blockage of regeneration. However, regeneration-inhibiting concentrations were also destructive of limb tissues, particularly the epidermis;
therefore, a general toxic effect seems more likely than a specific antiacetylcholine action (Singer et al., 1960). Furthermore, a normal motor
nerve component alone possesses too few fibers to support limb regenera-
233
special case, however, is likely since there is a stimulatory effect in the
regeneration-competent hind limb of the axolotl when the contralateral
sciatic nerve is deviated to the limb stump (Shuraleff and Thornton,
1965).
The nerve requirement in regeneration is more critical for early phases
of the process than for later ones. In larval urodeles, Schotte and Butler
(1941) found that denervation of the limb at amputation not only inhibited regeneration but the dedifferentiation phase became prolonged to
the extent that much of the limb stump regressed. Such excessive regression seems to be more characteristic of the larval limb since denervated
limb stumps of adult newts retain their structure. Schotte and Butler
(1944) found that denervation inhibited limb regeneration in the larva if
the operation followed amputation by 1 to 6 or 7 days, but regeneration
would proceed in limb stumps denervated 9 or more days after amputation. Similarly, Singer and Craven (1948) found that regeneration was
suppressed when denervation of the adult newt limb occurred as late as
13 days after amputation, but when denervation was performed at 17
days or later, differentiation proceeded and normal, but small, regenerates developed. They found that mitotic proliferation of the blastemal
cells was inhibited by the denervation. The neural influence on limb
regeneration would seem to be of particular importance, therefore, for the
early growth phases of regeneration (see also Hay, 1956, 1966).
The problem of how nerves exert their effect on growth was investigated
particularly by Singer and his associates (review in Singer, 1960). As a
number of chemical substances (i.e., acetylcholine, sympathin) were
implicated in the transmission of the nerve impulse, the idea that nerve
influence on regeneration—the "trophic" effect—was also mediated by a
chemical mechanism seemed logical. Schotte (1926) was the first to test
this hypothesis experimentally on the limb stump by applying various
drugs associated with sympathetic nerve metabolism but without success.
Taban (1955) found that acetylcholine and other neurodynamic substances failed to support regeneration in denervated urodele limb stumps.
Singer (1960) infused into newt limb stumps a variety of substances
known to block acetylcholine mechanism—atropine, procaine hydrochloride, tetraethylammonium hydroxide—and achieved delay and even
blockage of regeneration. However, regeneration-inhibiting concentrations were also destructive of limb tissues, particularly the epidermis;
therefore, a general toxic effect seems more likely than a specific antiacetylcholine action (Singer et al., 1960). Furthermore, a normal motor
nerve component alone possesses too few fibers to support limb regenera-