148
R. J.
GOSS
limited histogenetic fields, arranged not in mosaic fashion but as a
number of overlapping spheres of influence.
Although these fields have been mapped out only indistinctly, on the
basis of limited experimental evidence (Sections II, Β, 1 and 2) certain
generalizations can be made about them. Each major tissue component
in the limb is able to induce not only the differentiation of its own
histological counterpart in the regenerate, but also those of other tissues
should the latter be removed or inactivated. The morphogenetic field of
the dermis includes the underlying muscle and bone, that of muscle
embraces its associated skeletal elements, while the influence of bone is
most clearly recognized on cartilaginous regenerates. The possibility
remains, however, that under appropriate experimental conditions the
extension of the morphogenetic potentialities beyond these limits could
be demonstrated. Indeed, there is reason to believe that in ideal circumstances the existence of only one of these participating components in
a limb is sufficient to assure the production of a completely organized
regenerate, provided, however, that such a tissue is present in full
complement.
The expression of such morphogenetic influences depends upon the
transference of inductive messages from stump tissues to blastema cells.
The mechanism of this has been most thoroughly studied with reference
to the inductive capacities of bones in regenerating systems. Whereas
normal bones added to stumps can bring about the development of
cartilage in the overlying blastema, bones that have been devitalized
(by heat or desiccation) fail to cause chondrification. Similarly, bones
exposed to growth-inhibiting doses of X-rays are unable to induce
cartilage formation. Indeed, normal bones can do so only when injured.
In the case of skeletal structures, therefore, the communication of
morphogenetic information appears to depend upon live cells capable of
proliferation and stimulated by trauma to participate in reparative
processes. From this it can be inferred that morphogenetic influences
are mediated by cells derived from stump tissues and incorporated into
the blastema. These cells may, by assimilative induction, effect the
differentiation of tissues homologous with their own histological origins,
or they may, perhaps by indirect means, cause the development of
other kinds of associated tissues. Although it is doubtful that the cells
of a regenerate pursue a course of differentiation in strict accordance
with their origins, the degree of versatility which blastema cells are
capable of expressing remains one of the principal enigmas of vertebrate
appendage regeneration.
R. J.
GOSS
limited histogenetic fields, arranged not in mosaic fashion but as a
number of overlapping spheres of influence.
Although these fields have been mapped out only indistinctly, on the
basis of limited experimental evidence (Sections II, Β, 1 and 2) certain
generalizations can be made about them. Each major tissue component
in the limb is able to induce not only the differentiation of its own
histological counterpart in the regenerate, but also those of other tissues
should the latter be removed or inactivated. The morphogenetic field of
the dermis includes the underlying muscle and bone, that of muscle
embraces its associated skeletal elements, while the influence of bone is
most clearly recognized on cartilaginous regenerates. The possibility
remains, however, that under appropriate experimental conditions the
extension of the morphogenetic potentialities beyond these limits could
be demonstrated. Indeed, there is reason to believe that in ideal circumstances the existence of only one of these participating components in
a limb is sufficient to assure the production of a completely organized
regenerate, provided, however, that such a tissue is present in full
complement.
The expression of such morphogenetic influences depends upon the
transference of inductive messages from stump tissues to blastema cells.
The mechanism of this has been most thoroughly studied with reference
to the inductive capacities of bones in regenerating systems. Whereas
normal bones added to stumps can bring about the development of
cartilage in the overlying blastema, bones that have been devitalized
(by heat or desiccation) fail to cause chondrification. Similarly, bones
exposed to growth-inhibiting doses of X-rays are unable to induce
cartilage formation. Indeed, normal bones can do so only when injured.
In the case of skeletal structures, therefore, the communication of
morphogenetic information appears to depend upon live cells capable of
proliferation and stimulated by trauma to participate in reparative
processes. From this it can be inferred that morphogenetic influences
are mediated by cells derived from stump tissues and incorporated into
the blastema. These cells may, by assimilative induction, effect the
differentiation of tissues homologous with their own histological origins,
or they may, perhaps by indirect means, cause the development of
other kinds of associated tissues. Although it is doubtful that the cells
of a regenerate pursue a course of differentiation in strict accordance
with their origins, the degree of versatility which blastema cells are
capable of expressing remains one of the principal enigmas of vertebrate
appendage regeneration.