AMPHIBIAN LIMB REGENERATION
207
lated to proliferate by the trauma of amputation, are the source of the
blastemal cells has not been rigorously excluded but neither is there
any reliable evidence in favor of this possibility. At present, the evidence
points to a contribution to the cellular population of the regeneration
blastema by all the stump tissues injured at amputation. An important
question remains: Can cells derived from one type of stump tissue redifferentiate, during blastemal histogenesis, into cells of another tissue type?
Present evidence indicates that connective tissue cells may be traced into
skeletal tissues redifferentiating in the late stages of blastemal development. Evidence of other kinds of metaplasia is not conclusive. Finally,
unlike the situation in limb ontogenesis, systemic factors—hormones and
nerves—play an important role in the phenomenon of limb regeneration.
Under special circumstances neither hormones or nerves are absolutely
needed for typical limb regeneration to proceed; but in normal ontogeny
the limb apparently develops dependence on both nerves and hormones
for setting in motion its regenerative mechanisms.
II. Phases of Limb Regeneration
Limb regeneration will be discussed according to the successive phases
of the phenomenon as seen in a typical urodele amphibian. In general,
limb regeneration may conveniently be divided into two main phases,
regressive and progressive, each of which is characterized by a number
of events which dominate at successive times following amputation.
Although this classification seems to divide limb regeneration into
discrete compartments, this is done for convenience of description. The
ranges of the various phases are imprecise and may vary from one
individual limb to another, from larva to adult, and according to individual and environmental conditions. The phases of regeneration thus
represent a rough statistical average and not a precise staging. The main
events of regeneration, as seen in the forelimb of the axolotl are shown
in Fig. 1.
A. Regressive Phase
1. Trauma
Typically regeneration begins with the act of amputation whether this
occurs in nature or under controlled laboratory conditions. The presence
of an injury of some kind seems to be an essential prerequisite to the
onset of regeneration. This may be an actual amputation, or tissue
207
lated to proliferate by the trauma of amputation, are the source of the
blastemal cells has not been rigorously excluded but neither is there
any reliable evidence in favor of this possibility. At present, the evidence
points to a contribution to the cellular population of the regeneration
blastema by all the stump tissues injured at amputation. An important
question remains: Can cells derived from one type of stump tissue redifferentiate, during blastemal histogenesis, into cells of another tissue type?
Present evidence indicates that connective tissue cells may be traced into
skeletal tissues redifferentiating in the late stages of blastemal development. Evidence of other kinds of metaplasia is not conclusive. Finally,
unlike the situation in limb ontogenesis, systemic factors—hormones and
nerves—play an important role in the phenomenon of limb regeneration.
Under special circumstances neither hormones or nerves are absolutely
needed for typical limb regeneration to proceed; but in normal ontogeny
the limb apparently develops dependence on both nerves and hormones
for setting in motion its regenerative mechanisms.
II. Phases of Limb Regeneration
Limb regeneration will be discussed according to the successive phases
of the phenomenon as seen in a typical urodele amphibian. In general,
limb regeneration may conveniently be divided into two main phases,
regressive and progressive, each of which is characterized by a number
of events which dominate at successive times following amputation.
Although this classification seems to divide limb regeneration into
discrete compartments, this is done for convenience of description. The
ranges of the various phases are imprecise and may vary from one
individual limb to another, from larva to adult, and according to individual and environmental conditions. The phases of regeneration thus
represent a rough statistical average and not a precise staging. The main
events of regeneration, as seen in the forelimb of the axolotl are shown
in Fig. 1.
A. Regressive Phase
1. Trauma
Typically regeneration begins with the act of amputation whether this
occurs in nature or under controlled laboratory conditions. The presence
of an injury of some kind seems to be an essential prerequisite to the
onset of regeneration. This may be an actual amputation, or tissue