AMPHIBIAN LIMB REGENERATION
229
areas of the blastema have the highest cell density, but this is no longer
true at the paddle stage (Fig. 3). Of particular interest is the effect on
blastemal mitotic patterns of moving the prominent apical cap to an
acentric position. When this is done at the early bud stage (10 days), the
mitotic index 4 days later (at 14 days) is highest in the quadrant underlying the new location of the apical cap but next highest in the region
where the apical cap was previously located (Fig. 4). At 16 and 18 days,
however, the mitotic index of this latter area drops, while the section of
blastema beneath the apical cap in its new location continues to exhibit
the highest mitotic index of the blastema. These results indicate that the
apical cap, in addition to an effect on cell aggregation, may influence
mitotic proliferation in the blastemal cells, but further evidence is needed
before definite conclusions can be drawn.
3. Morphogenesis
of the
Blastema
Morphogenesis of the limb blastema resembles, superficially at least,
morphogenesis of the embryonic limb bud. There is the important difference, however, of the association of the blastema proximally with the
formed tissues of the limb stump. Since blastemal morphogenesis is concerned only with structures distal to those of the limb stump, a dependent
morphogenetic relationship of blastema to stump has long been thought
to hold (see review by Goss, 1961). This view was strengthened by the
results of de Giorgi (1924) who grafted young blastemata to neutral sites
on the back of the newt and found that these failed to develop unless a
segment of limb stump was included in the graft. However, Mettetal
(1939), Faber (1960), Pietsch (1961a), and others have shown that with
care isolated blastemal grafts will develop into regenerates in the larval
urodele. Faber, extending the earlier experiments of Mettetal, grafted
axolotl blastemata of early bud, mound, cone, and paddle stages to
neutral sites on the back and found that only 1 or 2 digits with
phalanges and metacarpals developed from early bud blastemata; 1-4
digits with occasional carpals formed from mound stage blastemata;
digits, carpals, and up to 2 forearm elements developed from cone blastemata ; and more or less normal limbs, minus the humerus, developed from
paddle stage blastemata. The skeleton was used as an index of distalization because of its clear distoproximal anatomical differences. There was,
thus, a marked distalization evident in these blastemal grafts; areas of
the blastema which would be expected normally to differentiate into more
proximal limb parts formed digital structures. Carbon marking experiments demonstrated that proximal blastemal mesenchyme did indeed
229
areas of the blastema have the highest cell density, but this is no longer
true at the paddle stage (Fig. 3). Of particular interest is the effect on
blastemal mitotic patterns of moving the prominent apical cap to an
acentric position. When this is done at the early bud stage (10 days), the
mitotic index 4 days later (at 14 days) is highest in the quadrant underlying the new location of the apical cap but next highest in the region
where the apical cap was previously located (Fig. 4). At 16 and 18 days,
however, the mitotic index of this latter area drops, while the section of
blastema beneath the apical cap in its new location continues to exhibit
the highest mitotic index of the blastema. These results indicate that the
apical cap, in addition to an effect on cell aggregation, may influence
mitotic proliferation in the blastemal cells, but further evidence is needed
before definite conclusions can be drawn.
3. Morphogenesis
of the
Blastema
Morphogenesis of the limb blastema resembles, superficially at least,
morphogenesis of the embryonic limb bud. There is the important difference, however, of the association of the blastema proximally with the
formed tissues of the limb stump. Since blastemal morphogenesis is concerned only with structures distal to those of the limb stump, a dependent
morphogenetic relationship of blastema to stump has long been thought
to hold (see review by Goss, 1961). This view was strengthened by the
results of de Giorgi (1924) who grafted young blastemata to neutral sites
on the back of the newt and found that these failed to develop unless a
segment of limb stump was included in the graft. However, Mettetal
(1939), Faber (1960), Pietsch (1961a), and others have shown that with
care isolated blastemal grafts will develop into regenerates in the larval
urodele. Faber, extending the earlier experiments of Mettetal, grafted
axolotl blastemata of early bud, mound, cone, and paddle stages to
neutral sites on the back and found that only 1 or 2 digits with
phalanges and metacarpals developed from early bud blastemata; 1-4
digits with occasional carpals formed from mound stage blastemata;
digits, carpals, and up to 2 forearm elements developed from cone blastemata ; and more or less normal limbs, minus the humerus, developed from
paddle stage blastemata. The skeleton was used as an index of distalization because of its clear distoproximal anatomical differences. There was,
thus, a marked distalization evident in these blastemal grafts; areas of
the blastema which would be expected normally to differentiate into more
proximal limb parts formed digital structures. Carbon marking experiments demonstrated that proximal blastemal mesenchyme did indeed