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CHARLES S. THORNTON
zymes may be involved in the proteolysis of regressing limb tissues as
they are known to be in regressing tadpole tail tissues (Weber, 1965).
3.
Dedifferentiation
Evidence of proteolytic activity in the limb stump tissues continues
for several more days, but phagocytes disappear and so do pycnotic
nuclei. Dissolution of the intercellular matrix of cartilage (in the larval
limb) and sarcolysis of muscle continues in the distal third of the limb
stump liberating muscle and cartilage mononucleate cells into the limb
tip (Thornton, 1938a). Ultrastructural studies by Hay (1958, 1959) and
radioautographic studies by Hay and Fischman (1961), Bodemer and
Everett (1959), and Anton (1965) clearly indicate that these mononucleate cells are viable since they begin to synthesize deoxyribonucleic
acid (DNA) and proteins at 4 days postamputation. In the larval salamander limb at least, the mononucleate cells derived from muscle, cartilage, and connective tissue show a change in the endoplasmic reticulum
which becomes discontinuous and vesicular. Even under the electron
microscope these mesenchymatous cells, as they may be called, cannot be
distinguished from each other morphologically. This morphological similarity, however, in no way implies a genetic or developmental similarity.
As Hay (1966) points out, the mesenchymatous cell, stripped of its
encumbering apparatus of differentiated structure, is in a particularly
favorable state for mitotic proliferation—a function particularly important for reconstituting the tissue mass lost at amputation. The mesenchymatous cells possess a high ratio of free ribosomes-to-membranebound ribosomes. The nuclei are large with prominent nucleoli and, as
demonstrated by Hay and Revel (1963), are actively synthesizing nucleic acid and protein (see also Bodemer, 1962a,b). Immunobiological
investigations (DeHann, 1956; Laufer, 1959) indicate that specific
muscle proteins gradually disappear from muscle-derived mesenchymatous cells following the dedifferentiation period so that both biochemical
and morphological specialization is lost. Thus the formed tissues at the
stump tip are gradually replaced by a population of mesenchymatous
cells which form a regeneration bud, or blastema.
a. Origin of the Blastemal Cells. Since the new tissues of the regenerating limb will be reconstituted largely by the differentiation of the
blastemal cells and since the latter are indistinguishable from one
another, the question of the correspondence of their tissue of origin and
of redifferentiation is of fundamental importance. Before one can answer
the question: Is there metaplasia in blastemal morphogenesis? one must
CHARLES S. THORNTON
zymes may be involved in the proteolysis of regressing limb tissues as
they are known to be in regressing tadpole tail tissues (Weber, 1965).
3.
Dedifferentiation
Evidence of proteolytic activity in the limb stump tissues continues
for several more days, but phagocytes disappear and so do pycnotic
nuclei. Dissolution of the intercellular matrix of cartilage (in the larval
limb) and sarcolysis of muscle continues in the distal third of the limb
stump liberating muscle and cartilage mononucleate cells into the limb
tip (Thornton, 1938a). Ultrastructural studies by Hay (1958, 1959) and
radioautographic studies by Hay and Fischman (1961), Bodemer and
Everett (1959), and Anton (1965) clearly indicate that these mononucleate cells are viable since they begin to synthesize deoxyribonucleic
acid (DNA) and proteins at 4 days postamputation. In the larval salamander limb at least, the mononucleate cells derived from muscle, cartilage, and connective tissue show a change in the endoplasmic reticulum
which becomes discontinuous and vesicular. Even under the electron
microscope these mesenchymatous cells, as they may be called, cannot be
distinguished from each other morphologically. This morphological similarity, however, in no way implies a genetic or developmental similarity.
As Hay (1966) points out, the mesenchymatous cell, stripped of its
encumbering apparatus of differentiated structure, is in a particularly
favorable state for mitotic proliferation—a function particularly important for reconstituting the tissue mass lost at amputation. The mesenchymatous cells possess a high ratio of free ribosomes-to-membranebound ribosomes. The nuclei are large with prominent nucleoli and, as
demonstrated by Hay and Revel (1963), are actively synthesizing nucleic acid and protein (see also Bodemer, 1962a,b). Immunobiological
investigations (DeHann, 1956; Laufer, 1959) indicate that specific
muscle proteins gradually disappear from muscle-derived mesenchymatous cells following the dedifferentiation period so that both biochemical
and morphological specialization is lost. Thus the formed tissues at the
stump tip are gradually replaced by a population of mesenchymatous
cells which form a regeneration bud, or blastema.
a. Origin of the Blastemal Cells. Since the new tissues of the regenerating limb will be reconstituted largely by the differentiation of the
blastemal cells and since the latter are indistinguishable from one
another, the question of the correspondence of their tissue of origin and
of redifferentiation is of fundamental importance. Before one can answer
the question: Is there metaplasia in blastemal morphogenesis? one must