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CHARLES S. THORNTON
and Walker (1961) independently reported failure to find labeled cells in
newt blastemata of which the epidermis was labeled with thymidine3
!!.
Other tissues have been investigated. Patrick and Briggs (1964)
grafted triploid cartilage into axolotl limbs and found triploid cells
restricted to cartilage in the regenerate. They concluded that cartilage
cells are stable and do not dedifferentiate to a pluripotent cell type.
Steen (1967), in an extensive investigation, used a double label to identify specific cells throughout limb regeneration in the axolotl. Thus segments of triploid limb cartilage devoid of perichondrium and extraneous
tissue were labeled with thymidine3
!!. Segments of cartilage were removed from the diploid host limb and were then replaced by implants of
the double labeled cartilage. After amputation, regenerates exhibited the
label only in the new cartilage. Steen, therefore, concluded that no
metaplasia had occurred. Eggert (1966) also came to similar conclusions
after studying cartilage grafts in X-rayed limb stumps. When, however,
Steen implanted doubly labeled muscle into diploid host limbs, subsequent regenerates displayed this double label in the newly differentiated
cartilage. As Steen points out, muscle is a complex tissue which also has
connective tissue and endothelium associated with it. Thus, until pure
axolotl muscle clones are available for implanting so that fibroblasts can
be excluded, no conclusions as to muscle metaplasia can be made (see
also Thornton, 1942).
B. Progressive Phase
1. Blastemal
Cell
Aggregation
As stump tissues undergo dedifferentiation at their distal, injured ends,
mononuclear mesenchymatous cells liberated into the limb stump by this
process accumulate at the stump tip. The continued accumulation and
proliferation of these mesenchymatous cells results in the formation of
a mass of cells, the regeneration blastema. The appearance of the
blastema marks the beginning of the progressive phase of limb regeneration and the end of regression. As has been mentioned previously, the
urodele limb regeneration blastema morphologically resembles the embryonic limb bud. Saunders (1948) and Zwilling (1955) have shown that
an ectodermal thickening, the apical ridge, is of central importance in
directing the outgrowth of the wing and limb bud of the chick embryo
(see also Cairns, 1965). It is particularly interesting that the wound
epidermis that forms over the amputation surface of the urodele limb
stump becomes thickened into an apical cap. Morphological similarity is
CHARLES S. THORNTON
and Walker (1961) independently reported failure to find labeled cells in
newt blastemata of which the epidermis was labeled with thymidine3
!!.
Other tissues have been investigated. Patrick and Briggs (1964)
grafted triploid cartilage into axolotl limbs and found triploid cells
restricted to cartilage in the regenerate. They concluded that cartilage
cells are stable and do not dedifferentiate to a pluripotent cell type.
Steen (1967), in an extensive investigation, used a double label to identify specific cells throughout limb regeneration in the axolotl. Thus segments of triploid limb cartilage devoid of perichondrium and extraneous
tissue were labeled with thymidine3
!!. Segments of cartilage were removed from the diploid host limb and were then replaced by implants of
the double labeled cartilage. After amputation, regenerates exhibited the
label only in the new cartilage. Steen, therefore, concluded that no
metaplasia had occurred. Eggert (1966) also came to similar conclusions
after studying cartilage grafts in X-rayed limb stumps. When, however,
Steen implanted doubly labeled muscle into diploid host limbs, subsequent regenerates displayed this double label in the newly differentiated
cartilage. As Steen points out, muscle is a complex tissue which also has
connective tissue and endothelium associated with it. Thus, until pure
axolotl muscle clones are available for implanting so that fibroblasts can
be excluded, no conclusions as to muscle metaplasia can be made (see
also Thornton, 1942).
B. Progressive Phase
1. Blastemal
Cell
Aggregation
As stump tissues undergo dedifferentiation at their distal, injured ends,
mononuclear mesenchymatous cells liberated into the limb stump by this
process accumulate at the stump tip. The continued accumulation and
proliferation of these mesenchymatous cells results in the formation of
a mass of cells, the regeneration blastema. The appearance of the
blastema marks the beginning of the progressive phase of limb regeneration and the end of regression. As has been mentioned previously, the
urodele limb regeneration blastema morphologically resembles the embryonic limb bud. Saunders (1948) and Zwilling (1955) have shown that
an ectodermal thickening, the apical ridge, is of central importance in
directing the outgrowth of the wing and limb bud of the chick embryo
(see also Cairns, 1965). It is particularly interesting that the wound
epidermis that forms over the amputation surface of the urodele limb
stump becomes thickened into an apical cap. Morphological similarity is