146
R.
J.
GOSS
IX. Discussion and Conclusions
Pre-eminent among the major problems of development is that
concerned with the genesis of form. In the regenerating vertebrate
appendage, as in the developing embryo, morphogenesis can be resolved
into (1) factors of a genetic nature which determine what is to be formed
and which communicate this information to the reacting system, and (2)
the subsidiary mechanisms by which these instructions are translated
into specific effects. In early embryogenesis, the reacting system itself is
usually the source of morphogenetic information. With greater complexity, however, the development of parts becomes increasingly
dependent upon influences from other regions or tissues, as illustrated
by the establishment of embryonic induction systems, hormonal relationships and nervous integration. Thus, the blastema of a regenerating
limb is not a self-differentiating system as is the embryonic limb
bud. Unlike the latter, it is composed largely of dedifferentiated cells,
depends upon sufficient innervation for its formation, is influenced by
various hormonal factors, and develops according to morphogenetic
orders dictated by the stump. Although its development into a regenerate
superficially, at least, recapitulates embryonic limb growth, the blastema
is subjected to many modulating influences and morphogenetic constraints, from which the embryonic limb bud is free. Nevertheless, the
limb bud and the blastema are both undoubtedly similar in the detailed
mechanisms by which they achieve specific cellular differentiations
(e.g., myogenesis, chondrogenesis). It is the nature of the fundamental
governing influences that control these reactions, which distinguishes
the embryonic limb bud from the regenerating blastema.
When a blastema first forms, there is no conclusive evidence that it is
in any way determined. Young transplanted blastemas either fail to
grow altogether or develop according to their site of transplantation,
probably as a result of regression and replacement by host cells when
the latter are capable of regeneration. Moreover, if an early blastema
is scrambled with a fine needle, its cells become reorganized and
develop normally. In older blastemas, however, the cells are determined, with the result that their transplantation elsewhere fails to alter
their course of development. These determining factors reside in the
tissues of the stump and communicate their influences to the blastema
cells.
Accordingly, investigations of morphogenesis in regenerating vertebrate appendages have been concentrated on the effects of experimental
interventions in stump tissues. Such studies have demonstrated that
despite the infliction of many kinds of defects, stump tissues can
nevertheless give rise to typical regenerates. The striking regulatory
R.
J.
GOSS
IX. Discussion and Conclusions
Pre-eminent among the major problems of development is that
concerned with the genesis of form. In the regenerating vertebrate
appendage, as in the developing embryo, morphogenesis can be resolved
into (1) factors of a genetic nature which determine what is to be formed
and which communicate this information to the reacting system, and (2)
the subsidiary mechanisms by which these instructions are translated
into specific effects. In early embryogenesis, the reacting system itself is
usually the source of morphogenetic information. With greater complexity, however, the development of parts becomes increasingly
dependent upon influences from other regions or tissues, as illustrated
by the establishment of embryonic induction systems, hormonal relationships and nervous integration. Thus, the blastema of a regenerating
limb is not a self-differentiating system as is the embryonic limb
bud. Unlike the latter, it is composed largely of dedifferentiated cells,
depends upon sufficient innervation for its formation, is influenced by
various hormonal factors, and develops according to morphogenetic
orders dictated by the stump. Although its development into a regenerate
superficially, at least, recapitulates embryonic limb growth, the blastema
is subjected to many modulating influences and morphogenetic constraints, from which the embryonic limb bud is free. Nevertheless, the
limb bud and the blastema are both undoubtedly similar in the detailed
mechanisms by which they achieve specific cellular differentiations
(e.g., myogenesis, chondrogenesis). It is the nature of the fundamental
governing influences that control these reactions, which distinguishes
the embryonic limb bud from the regenerating blastema.
When a blastema first forms, there is no conclusive evidence that it is
in any way determined. Young transplanted blastemas either fail to
grow altogether or develop according to their site of transplantation,
probably as a result of regression and replacement by host cells when
the latter are capable of regeneration. Moreover, if an early blastema
is scrambled with a fine needle, its cells become reorganized and
develop normally. In older blastemas, however, the cells are determined, with the result that their transplantation elsewhere fails to alter
their course of development. These determining factors reside in the
tissues of the stump and communicate their influences to the blastema
cells.
Accordingly, investigations of morphogenesis in regenerating vertebrate appendages have been concentrated on the effects of experimental
interventions in stump tissues. Such studies have demonstrated that
despite the infliction of many kinds of defects, stump tissues can
nevertheless give rise to typical regenerates. The striking regulatory