III.
REGENERATION OF VERTEBRATE APPENDAGES
115
IV. Morphogenesis of the Blastema
Morphogenesis of a regenerate is the expression of developmental
potentialities in the blastema. Although the blastema owes its origin to
the subjacent stump tissues, the genesis of its ultimate morphology may
conceivably reside either in the blastema itself or in the older tissues of
the stump. If the blastema cells come endowed with genetic information
dictating the directions of their future differentiation, then the blastema,
from the time of its inception, would be a predetermined structure. At
the other extreme, it is conceivable that the undifferentiated blastema
cells are pluripotent, their eventual fates being determined by influences
emanating from the fully differentiated tissues in the stump. An
alternative possibility lies between these two hypotheses, namely, that
the potentialities of blastema cells may be partially predetermined
(e.g., destined to partake in the formation of a leg or a tail only,
depending on their origin), but still labile, for example, as far as their
specific histodifferentiation is concerned. Fortunately, these hypotheses
are amenable to experimental testing simply by separating the blastema
at the appropriate stage of development from the stump and observing
its subsequent growth under segregated conditions. However, the technical difficulties encountered in such experiments have repeatedly cast
doubt on the validity of the results. It is especially important in any
such study to take into account the age of the blastema as well as the
situation in which it is to grow. The latter includes in vitro and in vivo
conditions. In vivo transplantation of blastemas may be either to internal
locations in the body, or to the surface of the host. In the latter cases it
is important whether the blastema is grafted to a normally nonE
A.M. I
changes in the latter, much has been learned about the roles of individual
tissues in morphogenesis. Similarly, factors affecting the wholeness of
regenerates have been analysed by quantitative, rather than qualitative,
changes in the stumps. Experiments of these kinds, plus many other
variations to be described in detail below, have not only led to some
very valuable conclusions, but have also emphasized the remarkable
regulatory powers of developing regenerates such that normal structures
often develop despite defects of considerable magnitude in their stumps
of origin. This rather perplexing situation has led to the utilization of
such general terms as 'morphogenetic field' to refer to the sum of the
influences emanating from the stump and directing the morphological
development of the regenerate. Though it is universally recognized that
such a term is merely a name for what we do not understand, it is of
value in assisting the formulation of a conceptual framework within
which experimental tests can be designed.
REGENERATION OF VERTEBRATE APPENDAGES
115
IV. Morphogenesis of the Blastema
Morphogenesis of a regenerate is the expression of developmental
potentialities in the blastema. Although the blastema owes its origin to
the subjacent stump tissues, the genesis of its ultimate morphology may
conceivably reside either in the blastema itself or in the older tissues of
the stump. If the blastema cells come endowed with genetic information
dictating the directions of their future differentiation, then the blastema,
from the time of its inception, would be a predetermined structure. At
the other extreme, it is conceivable that the undifferentiated blastema
cells are pluripotent, their eventual fates being determined by influences
emanating from the fully differentiated tissues in the stump. An
alternative possibility lies between these two hypotheses, namely, that
the potentialities of blastema cells may be partially predetermined
(e.g., destined to partake in the formation of a leg or a tail only,
depending on their origin), but still labile, for example, as far as their
specific histodifferentiation is concerned. Fortunately, these hypotheses
are amenable to experimental testing simply by separating the blastema
at the appropriate stage of development from the stump and observing
its subsequent growth under segregated conditions. However, the technical difficulties encountered in such experiments have repeatedly cast
doubt on the validity of the results. It is especially important in any
such study to take into account the age of the blastema as well as the
situation in which it is to grow. The latter includes in vitro and in vivo
conditions. In vivo transplantation of blastemas may be either to internal
locations in the body, or to the surface of the host. In the latter cases it
is important whether the blastema is grafted to a normally nonE
A.M. I
changes in the latter, much has been learned about the roles of individual
tissues in morphogenesis. Similarly, factors affecting the wholeness of
regenerates have been analysed by quantitative, rather than qualitative,
changes in the stumps. Experiments of these kinds, plus many other
variations to be described in detail below, have not only led to some
very valuable conclusions, but have also emphasized the remarkable
regulatory powers of developing regenerates such that normal structures
often develop despite defects of considerable magnitude in their stumps
of origin. This rather perplexing situation has led to the utilization of
such general terms as 'morphogenetic field' to refer to the sum of the
influences emanating from the stump and directing the morphological
development of the regenerate. Though it is universally recognized that
such a term is merely a name for what we do not understand, it is of
value in assisting the formulation of a conceptual framework within
which experimental tests can be designed.