III. REGENERATION
OF VERTEBRATE
APPENDAGES
123
In contrast to the above results, Bischler and Guyénot (1925b)
obtained typical regenerates from forelimbs containing ribs, radii or
ulnae in lieu of the humeri. In all of these cases, limb bones were
replaced by those from heterologous locations. Hence, the combination
of tissues exerting different morphogenetic effects usually resulted in
rather disorganized outgrowths. The morphogenesis of regenerates is
even more drastically altered when vertebrae from the tail, substituted
for limb bones, give rise to tail-like regenerates (Okada, 1936).
Homologous substitutions, however, such as the exchange of humerus
and femur (Weiss, 1923a, 1925a; Bischler and Guyénot, 1925b;
Ichikawa and Okada, 1954) do not seriously affect the regeneration of
normal limbs. Replacement of bones from the same level of another,
albeit different limb, does not produce a sufficiently obvious effect on
the regenerate to distinguish it from a normal one. Attempts to
distinguish forelimbs from hind limbs on the basis of the numbers
of digits are far from reliable in view of the discrepancies in such
criteria.
Some insight into the mechanism, whereby bones influence the
morphogenesis of regenerates has been provided by the results of
experiments involving the addition of extra skeletal elements to limbs
(Goss, 1956a). In these investigations, it has been demonstrated that if
an ulna is transplanted to either the upper or lower forelimb of Triturus,
a corresponding supernumerary cartilaginous element will develop in
the regenerate formed after amputation through the treated region.
Other bones, e.g., the mandible, or part of the branchial arch, likewise
induce chondrification in the adjacent regenerate. Indeed, ulnae transplanted to the tail can bring about skeletal regeneration following amputation at the level of operation. It is apparent, therefore, that accessory
bones exert a nonspecific effect upon the differentiation of blastema
cells.
Further experiments have indicated that this type of chondrogenic
induction does not occur if the transplanted bones are previously
devitalized (by heat or desiccation) or exposed to X-rays in doses of
1000 r or more. This evidence, coupled with the fact that such bones
must be injured, suggests that cartilage induction in the blastema is
mediated by osteoblasts. These cells presumably become incorporated
into the blastema where they differentiate into cartilage, and, by
assimilative induction, initiate chondrification in other blastema cells
from different sources. This contention is substantiated by the fact that
limbs X-rayed with doses sufficient to inhibit their regeneration, when
provided with unirradiated bones can produce outgrowths derived from
the healthy skeletal tissues (Umanski, 1937, 1939; Trampusch, 1951).
Indeed, the types of regenerates produced are determined by the kinds
OF VERTEBRATE
APPENDAGES
123
In contrast to the above results, Bischler and Guyénot (1925b)
obtained typical regenerates from forelimbs containing ribs, radii or
ulnae in lieu of the humeri. In all of these cases, limb bones were
replaced by those from heterologous locations. Hence, the combination
of tissues exerting different morphogenetic effects usually resulted in
rather disorganized outgrowths. The morphogenesis of regenerates is
even more drastically altered when vertebrae from the tail, substituted
for limb bones, give rise to tail-like regenerates (Okada, 1936).
Homologous substitutions, however, such as the exchange of humerus
and femur (Weiss, 1923a, 1925a; Bischler and Guyénot, 1925b;
Ichikawa and Okada, 1954) do not seriously affect the regeneration of
normal limbs. Replacement of bones from the same level of another,
albeit different limb, does not produce a sufficiently obvious effect on
the regenerate to distinguish it from a normal one. Attempts to
distinguish forelimbs from hind limbs on the basis of the numbers
of digits are far from reliable in view of the discrepancies in such
criteria.
Some insight into the mechanism, whereby bones influence the
morphogenesis of regenerates has been provided by the results of
experiments involving the addition of extra skeletal elements to limbs
(Goss, 1956a). In these investigations, it has been demonstrated that if
an ulna is transplanted to either the upper or lower forelimb of Triturus,
a corresponding supernumerary cartilaginous element will develop in
the regenerate formed after amputation through the treated region.
Other bones, e.g., the mandible, or part of the branchial arch, likewise
induce chondrification in the adjacent regenerate. Indeed, ulnae transplanted to the tail can bring about skeletal regeneration following amputation at the level of operation. It is apparent, therefore, that accessory
bones exert a nonspecific effect upon the differentiation of blastema
cells.
Further experiments have indicated that this type of chondrogenic
induction does not occur if the transplanted bones are previously
devitalized (by heat or desiccation) or exposed to X-rays in doses of
1000 r or more. This evidence, coupled with the fact that such bones
must be injured, suggests that cartilage induction in the blastema is
mediated by osteoblasts. These cells presumably become incorporated
into the blastema where they differentiate into cartilage, and, by
assimilative induction, initiate chondrification in other blastema cells
from different sources. This contention is substantiated by the fact that
limbs X-rayed with doses sufficient to inhibit their regeneration, when
provided with unirradiated bones can produce outgrowths derived from
the healthy skeletal tissues (Umanski, 1937, 1939; Trampusch, 1951).
Indeed, the types of regenerates produced are determined by the kinds