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CHARLES S. THORNTON
surface. In the reconstitution of a vertebrate limb, although Child's
regeneration is primarily involved, there is also considerable reorganization of stump tissues so that here no clear-cut distinction between Child's
redifferentiation and regeneration phenomena can be made. Since "regenesis" does not imply specific developmental or physiological processes
operating exclusively to restore lost limb parts it would be the term of
choice here. However, the term "regeneration" has become so firmly
imbedded in the literature that any attempt to discard it would probably
be doomed to failure. In the present review, therefore, the term "regeneration" is equated with Needham's "regenesis" and is meant to imply no
specific process of restoration of lost parts.
Limb regeneration occurs most readily in urodele amphibians where it
is found in both larval and adult stages. The anuran amphibians generally
exhibit good limb regeneration in larval stages but, except for certain
primarily aquatic forms (i.e., Xenopus, Hymenochirus)
this ability is
lost during metamorphosis. Reptiles have very limited powers of limb
regeneration; the few positive cases reported being limited primarily to
growth without morphogenesis. Birds and mammals do not normally
regenerate lost limb parts, but interesting recent experiments (Mizell,
personal communication) indicate that limbs of the newborn opossum
can be induced to regenerate after implantation of brain tissue to them.
Much of our knowledge of limb regeneration has, therefore, come from
intensive investigation of the phenomenon in urodele amphibians. There
are close similarities between limb regeneration and limb ontogenesis;
there are also important differences. Regeneration of a salamander limb
proceeds by means of an increasingly numerous population of mesenchymatous cells which form a bud (the blastema) at the stump tip similar
in morphological appearance, and in its gradual proximodistal histogenesis, to the embryonic limb bud. Unlike the latter, however, the formed
tissues of the limb stump constitute an ever present outside source of
morphogenetic influence on the developing regeneration blastema. There
is some controversy as to how extensive stump influence on blastemal
morphogenesis is, and how independent the regeneration blastema may
be in its development. Evidence, however, points to an interaction
between blastema and stump tissues, both at the time of first appearance
of the blastema and at later stages of its development when morphogenesis takes place. Furthermore, the origin and state of differentiation
of the mesenchymatous cells which make up the cellular population of
the regeneration blastema contrast sharply with the situation in the
embryonic limb bud. The possibility that embryonic reserve cells, stimu-
CHARLES S. THORNTON
surface. In the reconstitution of a vertebrate limb, although Child's
regeneration is primarily involved, there is also considerable reorganization of stump tissues so that here no clear-cut distinction between Child's
redifferentiation and regeneration phenomena can be made. Since "regenesis" does not imply specific developmental or physiological processes
operating exclusively to restore lost limb parts it would be the term of
choice here. However, the term "regeneration" has become so firmly
imbedded in the literature that any attempt to discard it would probably
be doomed to failure. In the present review, therefore, the term "regeneration" is equated with Needham's "regenesis" and is meant to imply no
specific process of restoration of lost parts.
Limb regeneration occurs most readily in urodele amphibians where it
is found in both larval and adult stages. The anuran amphibians generally
exhibit good limb regeneration in larval stages but, except for certain
primarily aquatic forms (i.e., Xenopus, Hymenochirus)
this ability is
lost during metamorphosis. Reptiles have very limited powers of limb
regeneration; the few positive cases reported being limited primarily to
growth without morphogenesis. Birds and mammals do not normally
regenerate lost limb parts, but interesting recent experiments (Mizell,
personal communication) indicate that limbs of the newborn opossum
can be induced to regenerate after implantation of brain tissue to them.
Much of our knowledge of limb regeneration has, therefore, come from
intensive investigation of the phenomenon in urodele amphibians. There
are close similarities between limb regeneration and limb ontogenesis;
there are also important differences. Regeneration of a salamander limb
proceeds by means of an increasingly numerous population of mesenchymatous cells which form a bud (the blastema) at the stump tip similar
in morphological appearance, and in its gradual proximodistal histogenesis, to the embryonic limb bud. Unlike the latter, however, the formed
tissues of the limb stump constitute an ever present outside source of
morphogenetic influence on the developing regeneration blastema. There
is some controversy as to how extensive stump influence on blastemal
morphogenesis is, and how independent the regeneration blastema may
be in its development. Evidence, however, points to an interaction
between blastema and stump tissues, both at the time of first appearance
of the blastema and at later stages of its development when morphogenesis takes place. Furthermore, the origin and state of differentiation
of the mesenchymatous cells which make up the cellular population of
the regeneration blastema contrast sharply with the situation in the
embryonic limb bud. The possibility that embryonic reserve cells, stimu-