234
CHARLES S. THORNTON
tion in the adult newt, yet the acetylcholine content of such motor
innervated limb stumps is much greater than in limb stumps regenerating
with the large sensory component alone (Singer, 1960). Acetylcholine
content in the limb stump and regenerative capacity, therefore, does
not coincide and the nature of the neural trophic influence remains
unknown. Whatever its nature may be, Singer (1963, 1964, 1965),
proposes that the trophic substance is produced in great abundance
in the neuron primarily to maintain its great mass of active neuroplasm, but that significant amounts "spill over" onto other tissues
which thus depend on the nerve for their own regenerative activity. The
quantitative neural basis of regeneration in the adult newt limb would
depend, therefore, on the amount of trophic substance conveyed to the
stump tissues by nerve fibers. Since its utilization or inactivation by
enzymic action might be rapid, constant renewal may be of great importance. The transport of threshold amounts of trophic substance to
limb tissues could be by way of many nerve fibers of small diameter or
fewer nerves of large diameter. Indeed, Rzehak and Singer (1966) have
recently discovered that, although the number of nerve fibers in the
regenerating forelimb of adult Xenopus is well below the threshold level
characteristic of the newt forelimb, the diameter of the individual fibers
is very large so that total amount of neuroplasm at the amputation
surface in Xenopus is equivalent to that of the newt.
Lately, it has been necessary to modify the neurotrophic theory of
regeneration (Singer, 1965) to accommodate recent evidence that nerves
are unnecessary for limb regeneration under certain conditions. Thus,
Yntema (1959a,b, 1962) developed aneurogenic limbs in
Ambystoma
larvae by excising the neural tube of tail-bud embryos. Amputation of
the nerveless limbs was followed by typical regeneration. These results
were extended by Thornton and Steen (1962) and Steen and Thornton
(1963) who have reported that it is the nature of the skin, particularly
the wound epithelium, which determines whether or not regeneration of
aneurogenic limbs will occur. For example, if the mesodermal tissues of
an aneurogenic limb are replaced by mesodermal tissues taken from an
innervated limb (larval Ambystoma),
regeneration of this limb complex
will subsequently occur quite typically. Thus, the aneurogenic skin of
the stump directs the nonaneurogenic mesodermal tissues (which are in
a denervated situation) to form a blastema. However, if the skin of an
aneurogenic limb is replaced by skin taken from an innervated limb,
subsequent regeneration of this particular limb complex fails. In this
latter situation, therefore, the regeneration-competent aneurogenic meso-
CHARLES S. THORNTON
tion in the adult newt, yet the acetylcholine content of such motor
innervated limb stumps is much greater than in limb stumps regenerating
with the large sensory component alone (Singer, 1960). Acetylcholine
content in the limb stump and regenerative capacity, therefore, does
not coincide and the nature of the neural trophic influence remains
unknown. Whatever its nature may be, Singer (1963, 1964, 1965),
proposes that the trophic substance is produced in great abundance
in the neuron primarily to maintain its great mass of active neuroplasm, but that significant amounts "spill over" onto other tissues
which thus depend on the nerve for their own regenerative activity. The
quantitative neural basis of regeneration in the adult newt limb would
depend, therefore, on the amount of trophic substance conveyed to the
stump tissues by nerve fibers. Since its utilization or inactivation by
enzymic action might be rapid, constant renewal may be of great importance. The transport of threshold amounts of trophic substance to
limb tissues could be by way of many nerve fibers of small diameter or
fewer nerves of large diameter. Indeed, Rzehak and Singer (1966) have
recently discovered that, although the number of nerve fibers in the
regenerating forelimb of adult Xenopus is well below the threshold level
characteristic of the newt forelimb, the diameter of the individual fibers
is very large so that total amount of neuroplasm at the amputation
surface in Xenopus is equivalent to that of the newt.
Lately, it has been necessary to modify the neurotrophic theory of
regeneration (Singer, 1965) to accommodate recent evidence that nerves
are unnecessary for limb regeneration under certain conditions. Thus,
Yntema (1959a,b, 1962) developed aneurogenic limbs in
Ambystoma
larvae by excising the neural tube of tail-bud embryos. Amputation of
the nerveless limbs was followed by typical regeneration. These results
were extended by Thornton and Steen (1962) and Steen and Thornton
(1963) who have reported that it is the nature of the skin, particularly
the wound epithelium, which determines whether or not regeneration of
aneurogenic limbs will occur. For example, if the mesodermal tissues of
an aneurogenic limb are replaced by mesodermal tissues taken from an
innervated limb (larval Ambystoma),
regeneration of this limb complex
will subsequently occur quite typically. Thus, the aneurogenic skin of
the stump directs the nonaneurogenic mesodermal tissues (which are in
a denervated situation) to form a blastema. However, if the skin of an
aneurogenic limb is replaced by skin taken from an innervated limb,
subsequent regeneration of this particular limb complex fails. In this
latter situation, therefore, the regeneration-competent aneurogenic meso-