226
CHARLES S. THORNTON
2. Blastemal Cell
Proliferation
Once stump tissue dedifferentiation has stopped, the continued growth
of the blastema is dependent on the mitotic proliferation of its individual
cells. Deoxyribonucleic acid synthesis has been observed to occur in
dedifferentiating newt limb stump tissues as early as 4 days after
amputation (Hay and Fischman, 1961) so that mitotic proliferation
begins in early phases of limb regeneration. The greatest burst of mitotic
activity, however, begins at the mound blastema stage and continues to
the paddle stage. In a definitive study, Chalkley (1954) followed the
proliferation of tissues in the newt limb stump. Thus, for both epidermal
and mesodermal tissues, he has described a gradual distal shift from
the limb stump to the blastema in both cell number and mitotic index.
Tissue origins were no longer discernible, of course, in the cells of the
blastema, but by the nineteenth day after amputation the peak of mitotic
activity was totally resident within the blastema proper. At this time the
mesenchymal mitotic index was highest in roughly the distal half of the
blastema. Since only the eighth section (10 jx thick) of each block of
sixteen sections was counted, with no count in the distalmost block if it
contained less than sixteen sections, the mitotic pattern in these blastemata was skewed somewhat proximally—a factor of little importance to
the main purpose of Chalkley's study, however. Chalkley (1959) notes
that "the distribution of the mitotic index . . . suggests an 'apical growth
cone' such as that described by Holtzer and Avery (1953) for the tail"
(p. 44). Litwiller (1939) had earlier described a similar distribution of
mitoses in limb regeneration in Triturus pyrrhogaster. Faber (1960) also
obtained evidence of a distal predominance of mitoses in the axolotl limb
blastema by means of carbon marking. The most detailed analysis of
mitotic proliferation patterns in the limb blastema, however, is that of
Hearson (1966) who divided the developing blastema into zones and
determined mitotic indices for these zones during four main phases of
development—early bud (10 days), mound (13 days), cone (16 days),
and paddle (19 days). These studies, therefore, are focused on blastemal
stages of proliferation. In the early bud stage, the mitotic index in the
distal quadrant (I) of the blastema is 3.7; of the quadrant (II) below,
1.9; of the third quadrant (III), 1.28; and of the basal quadrant (IV),
0.945 (Fig. 3). The cell density, of the early bud, however, varies little
from proximal to distal quadrants at this stage. Distal predominance
continues, although with decreasing intensity, through mound and cone
stages until it is lost in the differentiating paddle stage. Cell densities
increase, during these stages, so that in mound and cone stages the distal
CHARLES S. THORNTON
2. Blastemal Cell
Proliferation
Once stump tissue dedifferentiation has stopped, the continued growth
of the blastema is dependent on the mitotic proliferation of its individual
cells. Deoxyribonucleic acid synthesis has been observed to occur in
dedifferentiating newt limb stump tissues as early as 4 days after
amputation (Hay and Fischman, 1961) so that mitotic proliferation
begins in early phases of limb regeneration. The greatest burst of mitotic
activity, however, begins at the mound blastema stage and continues to
the paddle stage. In a definitive study, Chalkley (1954) followed the
proliferation of tissues in the newt limb stump. Thus, for both epidermal
and mesodermal tissues, he has described a gradual distal shift from
the limb stump to the blastema in both cell number and mitotic index.
Tissue origins were no longer discernible, of course, in the cells of the
blastema, but by the nineteenth day after amputation the peak of mitotic
activity was totally resident within the blastema proper. At this time the
mesenchymal mitotic index was highest in roughly the distal half of the
blastema. Since only the eighth section (10 jx thick) of each block of
sixteen sections was counted, with no count in the distalmost block if it
contained less than sixteen sections, the mitotic pattern in these blastemata was skewed somewhat proximally—a factor of little importance to
the main purpose of Chalkley's study, however. Chalkley (1959) notes
that "the distribution of the mitotic index . . . suggests an 'apical growth
cone' such as that described by Holtzer and Avery (1953) for the tail"
(p. 44). Litwiller (1939) had earlier described a similar distribution of
mitoses in limb regeneration in Triturus pyrrhogaster. Faber (1960) also
obtained evidence of a distal predominance of mitoses in the axolotl limb
blastema by means of carbon marking. The most detailed analysis of
mitotic proliferation patterns in the limb blastema, however, is that of
Hearson (1966) who divided the developing blastema into zones and
determined mitotic indices for these zones during four main phases of
development—early bud (10 days), mound (13 days), cone (16 days),
and paddle (19 days). These studies, therefore, are focused on blastemal
stages of proliferation. In the early bud stage, the mitotic index in the
distal quadrant (I) of the blastema is 3.7; of the quadrant (II) below,
1.9; of the third quadrant (III), 1.28; and of the basal quadrant (IV),
0.945 (Fig. 3). The cell density, of the early bud, however, varies little
from proximal to distal quadrants at this stage. Distal predominance
continues, although with decreasing intensity, through mound and cone
stages until it is lost in the differentiating paddle stage. Cell densities
increase, during these stages, so that in mound and cone stages the distal