AMPHIBIAN LIMB REGENERATION
221
no indication of similarity of function, but nonregenerating amputated
limbs of adult frogs and higher vertebrates do not form an apical cap and
mesenchymatous cells take part only in local tissue repair and do not
aggregate to form a blastema. Can it be that, in addition to a histolytic
activity, the apical cap may be involved in the aggregation of mesenchymatous cells to form the blastema? This possibility was systematically investigated when it was found that denervated, injured, but
unamputated limbs of larval Ambystoma
would undergo excessive regression but would then regenerate when nerves were later allowed to re-enter
the limb remnant. If, during the induced regression phase, all distal
organs had disappeared so that the limb remnant consisted only of a
moundlike mass of dissociated cells, re-entering nerve fibers penetrated
the apical epidermis and this tissue transformed into an apical cap
similar to one formed on an amputation surface. In these cases, however,
no wound surface had ever been present since these limbs had not been
amputated! Beneath the newly formed apical epidermal cap the mesenchymatous cells, derived from the previous dedifferentiation of the stump
tissue, crowded together, proliferated, and developed a typical regeneration blastema (Thornton, 1954).
If the epidermal apical cap is somehow essential for the aggregation of
blastemal cells and is not simply itself a response to "wound factors/'
then removal of the cap should prevent blastema formation. When
surgical removal of the apical cap from limb stumps of
Ambystoma
larvae was attempted, it was found that due to its regeneration each day,
the operation of cap removal had to be repeated daily. Control experiments included limb stumps from which skin patches were removed equal
in area to the apical caps and from regions at the stump tip adjacent
to the undisturbed apical cap. Typical regeneration occurred in all control limbs. However, daily removal of apical caps in Ambystoma
maculatum larvae slowed, but did not stop, blastema formation and limb
regeneration. In this species, a new apical cap regenerated in 12 hours
and since cap removal occurred once every 24 hours, it was possible for
each new apical cap to be active for 12 hours each day. In
Ambystoma
tigrinum larvae, on the other hand, apical cap regeneration took more
than 24 hours so that daily removal of apical wound epithelium, in this
species, did prevent blastema formation and regeneration (Thornton,
1957).
Irradiation of the apical cap with ultraviolet light (2537 A) also
inhibits apical cap formation yet offers the advantage that no open
wound is produced. Phosphors applied to the inner surface of the apical
cap were used to determine the ultraviolet exposure which inhibited the
221
no indication of similarity of function, but nonregenerating amputated
limbs of adult frogs and higher vertebrates do not form an apical cap and
mesenchymatous cells take part only in local tissue repair and do not
aggregate to form a blastema. Can it be that, in addition to a histolytic
activity, the apical cap may be involved in the aggregation of mesenchymatous cells to form the blastema? This possibility was systematically investigated when it was found that denervated, injured, but
unamputated limbs of larval Ambystoma
would undergo excessive regression but would then regenerate when nerves were later allowed to re-enter
the limb remnant. If, during the induced regression phase, all distal
organs had disappeared so that the limb remnant consisted only of a
moundlike mass of dissociated cells, re-entering nerve fibers penetrated
the apical epidermis and this tissue transformed into an apical cap
similar to one formed on an amputation surface. In these cases, however,
no wound surface had ever been present since these limbs had not been
amputated! Beneath the newly formed apical epidermal cap the mesenchymatous cells, derived from the previous dedifferentiation of the stump
tissue, crowded together, proliferated, and developed a typical regeneration blastema (Thornton, 1954).
If the epidermal apical cap is somehow essential for the aggregation of
blastemal cells and is not simply itself a response to "wound factors/'
then removal of the cap should prevent blastema formation. When
surgical removal of the apical cap from limb stumps of
Ambystoma
larvae was attempted, it was found that due to its regeneration each day,
the operation of cap removal had to be repeated daily. Control experiments included limb stumps from which skin patches were removed equal
in area to the apical caps and from regions at the stump tip adjacent
to the undisturbed apical cap. Typical regeneration occurred in all control limbs. However, daily removal of apical caps in Ambystoma
maculatum larvae slowed, but did not stop, blastema formation and limb
regeneration. In this species, a new apical cap regenerated in 12 hours
and since cap removal occurred once every 24 hours, it was possible for
each new apical cap to be active for 12 hours each day. In
Ambystoma
tigrinum larvae, on the other hand, apical cap regeneration took more
than 24 hours so that daily removal of apical wound epithelium, in this
species, did prevent blastema formation and regeneration (Thornton,
1957).
Irradiation of the apical cap with ultraviolet light (2537 A) also
inhibits apical cap formation yet offers the advantage that no open
wound is produced. Phosphors applied to the inner surface of the apical
cap were used to determine the ultraviolet exposure which inhibited the