AMPHIBIAN LIMB REGENERATION
225
Faber (1961) also reported that axolotl limb blastemata, grafted beneath
the skin of the back, formed regenerates only if they developed a close
association with the wound epithelium of the insertion wound. Ruben
and Frothingham (1958) discovered that frog kidney, when implanted
into newt limbs, induced accessory limbs only if a close association of
thickened wound epithelium and inner mesodermal tissues was developed.
Once the blastemal cells have accumulated to form a cone-shaped bud
the dedifferentiative modulations of the stump tissues halt. Evidence
indicates that this inhibition of regression is brought about by an action
of the blastema. Thus Butler and Puckett (1940) irradiated amputated
limbs of Ambystoma
larvae with X-rays or ultraviolet light. If irradiation
occurred immediately after amputation, no regeneration blastema developed and stump tissue dedifferentiation was excessive. On the other
hand, irradiation of a limb stump possessing a blastema inhibited
further regeneration, but excessive regression did not occur. These results
were interpreted to signify that the blastema had inhibited dedifferentiation by the time of irradiation. This hypothesis was further tested by
transplanting blastemata to denervated limbs. As will become clear in
subsequent pages, denervation of larval urodele limb stumps, like Xirradiation, results in excessive regression. Schotte, Butler, and Hood
(1941) grafted cone-stage blastemata to freshly amputated and denervated limb stumps of Ambystoma
larvae and obtained inhibition of
regression. Schotte and Harland (1943) were able to inhibit dedifferentiation in normal limb stumps after grafting blastemata to them. Experiments of Schotte and Butler (1944) indicate that the regeneration
blastema must attain a certain stage of development (8-9 days in
Ambystoma
larvae) to be effective in preventing limb stump regression.
FIG. 2. Movement of the apical epidermal cap of the forelimb stumps of Ambystoma larvae to an asymmetrical position. (1) The skin wound is shown, with blood
cells exuding from it, immediately after the operation. The 5-day old apical cap is
shown in its normal position at the stump tip. Postaxial surface of the limb is to the
right. (2) Limb stump fixed 2 hours after skin excision. Healing is just completed
and the apical cap is pulled to the postaxial margin of the limb, which in this case
is to the left. (3) Limb fixed 3 hours after skin excision. Apical cap is at the
postaxial margin (to the right) of the limb tip. (4) Limb stump fixed 12 hours after
skin excision. Prominent apical cap at the postaxial margin (to the right). (5)
Limb stump fixed 1 day after skin excision. Apical cap is at the postaxial margin of
the limb tip (to the left) and the first accumulation of blastemal cells is seen
beneath it. (6) Limb stump fixed 3 days after skin excision. Apical cap, and
underlying blastema, is at the postaxial margin of the limb tip (to the right).
(From Thornton, 1960.)
225
Faber (1961) also reported that axolotl limb blastemata, grafted beneath
the skin of the back, formed regenerates only if they developed a close
association with the wound epithelium of the insertion wound. Ruben
and Frothingham (1958) discovered that frog kidney, when implanted
into newt limbs, induced accessory limbs only if a close association of
thickened wound epithelium and inner mesodermal tissues was developed.
Once the blastemal cells have accumulated to form a cone-shaped bud
the dedifferentiative modulations of the stump tissues halt. Evidence
indicates that this inhibition of regression is brought about by an action
of the blastema. Thus Butler and Puckett (1940) irradiated amputated
limbs of Ambystoma
larvae with X-rays or ultraviolet light. If irradiation
occurred immediately after amputation, no regeneration blastema developed and stump tissue dedifferentiation was excessive. On the other
hand, irradiation of a limb stump possessing a blastema inhibited
further regeneration, but excessive regression did not occur. These results
were interpreted to signify that the blastema had inhibited dedifferentiation by the time of irradiation. This hypothesis was further tested by
transplanting blastemata to denervated limbs. As will become clear in
subsequent pages, denervation of larval urodele limb stumps, like Xirradiation, results in excessive regression. Schotte, Butler, and Hood
(1941) grafted cone-stage blastemata to freshly amputated and denervated limb stumps of Ambystoma
larvae and obtained inhibition of
regression. Schotte and Harland (1943) were able to inhibit dedifferentiation in normal limb stumps after grafting blastemata to them. Experiments of Schotte and Butler (1944) indicate that the regeneration
blastema must attain a certain stage of development (8-9 days in
Ambystoma
larvae) to be effective in preventing limb stump regression.
FIG. 2. Movement of the apical epidermal cap of the forelimb stumps of Ambystoma larvae to an asymmetrical position. (1) The skin wound is shown, with blood
cells exuding from it, immediately after the operation. The 5-day old apical cap is
shown in its normal position at the stump tip. Postaxial surface of the limb is to the
right. (2) Limb stump fixed 2 hours after skin excision. Healing is just completed
and the apical cap is pulled to the postaxial margin of the limb, which in this case
is to the left. (3) Limb fixed 3 hours after skin excision. Apical cap is at the
postaxial margin (to the right) of the limb tip. (4) Limb stump fixed 12 hours after
skin excision. Prominent apical cap at the postaxial margin (to the right). (5)
Limb stump fixed 1 day after skin excision. Apical cap is at the postaxial margin of
the limb tip (to the left) and the first accumulation of blastemal cells is seen
beneath it. (6) Limb stump fixed 3 days after skin excision. Apical cap, and
underlying blastema, is at the postaxial margin of the limb tip (to the right).
(From Thornton, 1960.)