III.
REGENERATION OF VERTEBRATE APPENDAGES
119
(3) substitution of one part by another, derived either from a different
level of the same structure, from another kind of appendage, or from a
nonregenerating part of the body; (4) rearrangement of tissues within
an appendage; and (5) selective irradiation of certain component tissues
in a stump. These techniques have been applied mostly to regenerating
amphibian limbs, and to a lesser extent to other types of regenerating
vertebrate appendages. The results of such experiments will be considered below with reference to specific tissue types, and, wherever
possible, the results of comparable experiments on different kinds of
appendages will be contrasted.
1. Nerves
It has been abundantly demonstrated that regeneration cannot occur
in the absence of adequate innervation (Singer, 1946, 1952). Nearly all
structures technically amenable to denervation (fins, taste barbels,
limbs, tails) have been shown incapable of regeneration after interruption of their nerve supplies. Apparent exceptions to this general rule
include the regeneration of aneurogenic limbs in larval urodeles
(Yntema, 1949), the regeneration of denervated regenerates of postmetamorphic Xenopus laevis (Skowron and Komala, 1957), and the
renewal, albeit retarded, of denervated antlers in the deer (Wislocki and
Singer, 1946). Taste buds and lateral line organs however are dependent
upon innervation, for when separated from their nerve supplies they
undergo degeneration. Reinnervation is followed by their regeneration.
Other kinds of reparative growth do not require the trophic influence
of nerves (e.g., lens regeneration, epidermal wound healing, fracture
healing).
Notwithstanding the very important role played by nerve fibres in
promoting the regeneration of many vertebrate structures, there is no
convincing evidence that the nervous influence is anything more than
stimulatory. The effect of nerves in regenerating structures is not
directly related to morphogenesis. The chief exception to this is the
abortive regeneration of amphibian limbs following partial denervation.
Under these conditions, where only a near threshold number of nerve
fibres is available to the limb, abnormal regenerates may be produced
(Singer, 1946), but their abnormalities are almost certainly attributable to
other factors, and only indirectly to nerve influences or the lack of same.
More convincing evidence that nerves do not affect the morphogenesis
of the regenerates they stimulate is that derived from experiments
involving delayed denervation of amputated appendages. These investigations (Singer and Craven, 1948; Butler and Schotte, 1949) have
shown that nerves are necessary for successful regeneration only during
the earlier phases of the process, namely, dedifferentiation and blastema
REGENERATION OF VERTEBRATE APPENDAGES
119
(3) substitution of one part by another, derived either from a different
level of the same structure, from another kind of appendage, or from a
nonregenerating part of the body; (4) rearrangement of tissues within
an appendage; and (5) selective irradiation of certain component tissues
in a stump. These techniques have been applied mostly to regenerating
amphibian limbs, and to a lesser extent to other types of regenerating
vertebrate appendages. The results of such experiments will be considered below with reference to specific tissue types, and, wherever
possible, the results of comparable experiments on different kinds of
appendages will be contrasted.
1. Nerves
It has been abundantly demonstrated that regeneration cannot occur
in the absence of adequate innervation (Singer, 1946, 1952). Nearly all
structures technically amenable to denervation (fins, taste barbels,
limbs, tails) have been shown incapable of regeneration after interruption of their nerve supplies. Apparent exceptions to this general rule
include the regeneration of aneurogenic limbs in larval urodeles
(Yntema, 1949), the regeneration of denervated regenerates of postmetamorphic Xenopus laevis (Skowron and Komala, 1957), and the
renewal, albeit retarded, of denervated antlers in the deer (Wislocki and
Singer, 1946). Taste buds and lateral line organs however are dependent
upon innervation, for when separated from their nerve supplies they
undergo degeneration. Reinnervation is followed by their regeneration.
Other kinds of reparative growth do not require the trophic influence
of nerves (e.g., lens regeneration, epidermal wound healing, fracture
healing).
Notwithstanding the very important role played by nerve fibres in
promoting the regeneration of many vertebrate structures, there is no
convincing evidence that the nervous influence is anything more than
stimulatory. The effect of nerves in regenerating structures is not
directly related to morphogenesis. The chief exception to this is the
abortive regeneration of amphibian limbs following partial denervation.
Under these conditions, where only a near threshold number of nerve
fibres is available to the limb, abnormal regenerates may be produced
(Singer, 1946), but their abnormalities are almost certainly attributable to
other factors, and only indirectly to nerve influences or the lack of same.
More convincing evidence that nerves do not affect the morphogenesis
of the regenerates they stimulate is that derived from experiments
involving delayed denervation of amputated appendages. These investigations (Singer and Craven, 1948; Butler and Schotte, 1949) have
shown that nerves are necessary for successful regeneration only during
the earlier phases of the process, namely, dedifferentiation and blastema