AMPHIBIAN LIMB REGENERATION
231
and chondrogenesis in the regenerating limb are independent of each
other; that myogenic and chondrogenic cells in the regeneration blastema
are independent entities throughout the entire regeneration process. He
rejects the "field" theory as applied to regeneration by Weiss (1939) and
believes his results "allow as a working hypothesis the view that basic
reactions in differentiation of the limb regenerate are inherent functions
of the particular cells involved in production of each tissue, that the
environment in which they develop—while important—serves in an ancillary and non-specific capacity" (Pietsch, 1962, p. 124). The most
recent evidence of a failure of genome dedifferentiation during blastema
formation comes from the interesting experiments of Burgess (1967) who
implanted nuclei of blastemal cells into enucleate Xenopus eggs and
found that although in a few cases development went as far as the
blastula stage, these blastemal nuclei were not significantly different in
promoting development than were nuclei of limb epidermis. These results,
therefore, do not encourage a belief in the pluripotence of blastemal cells.
III. The Influence of Nerves on Limb Regeneration
As has been mentioned previously, systemic influences provide important support for limb regeneration. The peripheral nerves were the
first of the systemic agents to be fully investigated (see review by Singer,
1952). Primarily as a result of a long series of carefully designed experiments by Singer (1942, 1943, 1945, 1946a,b, 1947a,b), we now know that
normally in the limb of the adult newt, typical regeneration is a function
not of a qualitative neural agent but of the quantity of nerve fibers
present at the wound surface of the limb stump. Thus, for example, an
average of 9.6 fibers for each (100 ji)
2
of amputation surface is needed if
regeneration at the level of the upper arm is to occur in the forelimb.
The neural influence is a local one and does not necessarily involve the
central nervous system (CNS) reflexly in regeneration. This was established by Kamrin and Singer (1959) who implanted isolated, brachial
sensory ganglia into denervated, newt, limb blastemata and regeneration
occurred when sufficient nerve fibers grew into the blastema from the
ganglionic graft. It is important to note, however, that even the intracentral fibers of the CNS have the neural quality to support limb regeneration. Thus when segments of spinal cord, or even hindbrain, are
implanted together with a forelimb to the dorsal fin of an
Ambystoma
larva the limb deplant will become innervated by intracentral fibers from
the spinal cord deplant and subsequent amputation of the limb is followed by typical regeneration (Thornton, 1956).
231
and chondrogenesis in the regenerating limb are independent of each
other; that myogenic and chondrogenic cells in the regeneration blastema
are independent entities throughout the entire regeneration process. He
rejects the "field" theory as applied to regeneration by Weiss (1939) and
believes his results "allow as a working hypothesis the view that basic
reactions in differentiation of the limb regenerate are inherent functions
of the particular cells involved in production of each tissue, that the
environment in which they develop—while important—serves in an ancillary and non-specific capacity" (Pietsch, 1962, p. 124). The most
recent evidence of a failure of genome dedifferentiation during blastema
formation comes from the interesting experiments of Burgess (1967) who
implanted nuclei of blastemal cells into enucleate Xenopus eggs and
found that although in a few cases development went as far as the
blastula stage, these blastemal nuclei were not significantly different in
promoting development than were nuclei of limb epidermis. These results,
therefore, do not encourage a belief in the pluripotence of blastemal cells.
III. The Influence of Nerves on Limb Regeneration
As has been mentioned previously, systemic influences provide important support for limb regeneration. The peripheral nerves were the
first of the systemic agents to be fully investigated (see review by Singer,
1952). Primarily as a result of a long series of carefully designed experiments by Singer (1942, 1943, 1945, 1946a,b, 1947a,b), we now know that
normally in the limb of the adult newt, typical regeneration is a function
not of a qualitative neural agent but of the quantity of nerve fibers
present at the wound surface of the limb stump. Thus, for example, an
average of 9.6 fibers for each (100 ji)
2
of amputation surface is needed if
regeneration at the level of the upper arm is to occur in the forelimb.
The neural influence is a local one and does not necessarily involve the
central nervous system (CNS) reflexly in regeneration. This was established by Kamrin and Singer (1959) who implanted isolated, brachial
sensory ganglia into denervated, newt, limb blastemata and regeneration
occurred when sufficient nerve fibers grew into the blastema from the
ganglionic graft. It is important to note, however, that even the intracentral fibers of the CNS have the neural quality to support limb regeneration. Thus when segments of spinal cord, or even hindbrain, are
implanted together with a forelimb to the dorsal fin of an
Ambystoma
larva the limb deplant will become innervated by intracentral fibers from
the spinal cord deplant and subsequent amputation of the limb is followed by typical regeneration (Thornton, 1956).