III.
REGENERATION OF VERTEBRATE
APPENDAGES
111
bears no relation to the nature of the stimulatory influence. The latter
evidently triggers the regenerative process, but the reacting system
determines what is to be the outcome.
B. Dedifferentiation
Once wound healing has been completed, which occurs promptly
after injury, the underlying mesodermal tissues undergo a gradual
change in appearance. During the several days following amputation
the soft tissues become disorganized and their individual cells lose many
of their specific cytological characteristics. This typical alteration of
cells situated in close proximity to sites of injury is referred to as
dedifferentiation. Unquestionably, there is a morphological dedifferentiation of cells. Whether or not there is also a dedifferentiation of their
future potencies remains unknown.
Each different kind of cell undergoes its own kind of dedifferentiation.
Striated muscle fibres, for example, seem to be especially susceptible to
dedifferentiation if they have been directly severed by amputation.
Thus, those which are oriented at right angles to the plane of amputation will dedifferentiate, while those arranged parallel to the amputation surface may remain unaffected. Muscle dedifferentiation is
characterized by the disappearance of myofibrils and their attending
cross striations. The sarcoplasm becomes increasingly homogenous and
eventually reorganizes around the residual nuclei. Each muscle fibre
therefore breaks up into a number of smaller uninucleate cells.
Fibroblasts, being relatively undifferentiated anyway, do not exhibit
any profound cytological changes: at most, they may change to rounded
configurations from original spindle shapes. Where large numbers of
fibroblasts are involved in regeneration, as in the teleost fin, there takes
place a dramatic change in their orientation. Ordinarily arranged transversely in the fin, those fibroblasts subjacent to the level of amputation
become distorted in preparation for their distally directed migration.
Cells associated with the skeletal system may also show morphological alterations. Chondroblasts, usually elongate cells in the perichondrium, tend to assume a more rounded configuration. This is
particularly evident in the regeneration of the catfish taste barbel where
the excessively thickened perichondrium of the central cartilaginous rod
constitutes the nearly exclusive source of blastema cells. Here the
terminally located chondroblasts, originally flattened cells arranged in
layers in the perichondrium, approach spherical configurations as they
migrate distally to contribute to the blastema. Osteoblasts also leave
their original locations in the periosteum and move into the blastema,
changing shape somewhat as they do so. Even the matrices of cartilage
REGENERATION OF VERTEBRATE
APPENDAGES
111
bears no relation to the nature of the stimulatory influence. The latter
evidently triggers the regenerative process, but the reacting system
determines what is to be the outcome.
B. Dedifferentiation
Once wound healing has been completed, which occurs promptly
after injury, the underlying mesodermal tissues undergo a gradual
change in appearance. During the several days following amputation
the soft tissues become disorganized and their individual cells lose many
of their specific cytological characteristics. This typical alteration of
cells situated in close proximity to sites of injury is referred to as
dedifferentiation. Unquestionably, there is a morphological dedifferentiation of cells. Whether or not there is also a dedifferentiation of their
future potencies remains unknown.
Each different kind of cell undergoes its own kind of dedifferentiation.
Striated muscle fibres, for example, seem to be especially susceptible to
dedifferentiation if they have been directly severed by amputation.
Thus, those which are oriented at right angles to the plane of amputation will dedifferentiate, while those arranged parallel to the amputation surface may remain unaffected. Muscle dedifferentiation is
characterized by the disappearance of myofibrils and their attending
cross striations. The sarcoplasm becomes increasingly homogenous and
eventually reorganizes around the residual nuclei. Each muscle fibre
therefore breaks up into a number of smaller uninucleate cells.
Fibroblasts, being relatively undifferentiated anyway, do not exhibit
any profound cytological changes: at most, they may change to rounded
configurations from original spindle shapes. Where large numbers of
fibroblasts are involved in regeneration, as in the teleost fin, there takes
place a dramatic change in their orientation. Ordinarily arranged transversely in the fin, those fibroblasts subjacent to the level of amputation
become distorted in preparation for their distally directed migration.
Cells associated with the skeletal system may also show morphological alterations. Chondroblasts, usually elongate cells in the perichondrium, tend to assume a more rounded configuration. This is
particularly evident in the regeneration of the catfish taste barbel where
the excessively thickened perichondrium of the central cartilaginous rod
constitutes the nearly exclusive source of blastema cells. Here the
terminally located chondroblasts, originally flattened cells arranged in
layers in the perichondrium, approach spherical configurations as they
migrate distally to contribute to the blastema. Osteoblasts also leave
their original locations in the periosteum and move into the blastema,
changing shape somewhat as they do so. Even the matrices of cartilage