114
R. J. GOSS
E. Morphogenesis
Differentiation, sensu stricto, is a cellular phenomenon, whilst
morphogenesis expresses itself at the tissue and organ levels of
organization. Any attempt to segregate these two processes, however,
is entirely artificial, for neither one is conceivable without the other.
Cells cannot differentiate without acquiring, in the process of doing so,
a certain degree of morphological organization, nor can morphogenesis
occur unless there are differentiating cells available. In general, morphogenesis constitutes a constellation of processes, including the determination of what kind of regenerate is to be formed, what specific
tissues are to be differentiated, and what spatial relationship shall be
expressed between these tissues. In addition, such attributes as polarity,
axes of orientation, the eventual cessation of development and the
wholeness of the regenerate are all governed by morphogenetic processes.
In brief, morphogenesis accounts for the production of a completely
normal structure, which is no small responsibility considering all the
subsidiary events which cannot be left to chance.
To investigate the nature of morphogenetic influences the method of
choice has been to attempt to bring about the predictable alteration of
developing regenerates by various experimental interventions in the
stump. This approach is predicated on the assumption that morphogenetic influences reside in the tissues of the stump and are transmitted
to the undifferentiated cells of the blastema. Thus, by studying the
relation between stump tissues and regenerating tissues, and noting
what kinds of alterations in the former are reflected in corresponding
The regenerating jaw of the newt even forms new teeth, and in intermandibular regeneration rudimentary salivary glands may sometimes
be encountered. In the regenerating lizard tail, scales are reconstituted
in the skin, and in the growing antlers of deer, hair follicles and sebaceous
glands are formed anew. In only one major instance does differentiation
in a regenerate markedly depart from embryonic ontogeny. This is the
regeneration of parts of the central nervous system, as exemplified in
the tails of amphibians and urodeles. Here the severed spinal cord
grows out along with the regenerating tail, not by the invagination of
ectoderm to give rise to a new neural tube but by the direct elongation
of the pre-existing neural canal. This is achieved largely by proliferation
of ependymal cells, and may be accompanied by the outgrowth of
regenerating new fibres. In all other cases—chondrogenesis, osteogenesis, myogenesis and various kinds of epidermal differentiation—
regenerative development faithfully recapitulates the original modes of
differentiation in the embryo.
R. J. GOSS
E. Morphogenesis
Differentiation, sensu stricto, is a cellular phenomenon, whilst
morphogenesis expresses itself at the tissue and organ levels of
organization. Any attempt to segregate these two processes, however,
is entirely artificial, for neither one is conceivable without the other.
Cells cannot differentiate without acquiring, in the process of doing so,
a certain degree of morphological organization, nor can morphogenesis
occur unless there are differentiating cells available. In general, morphogenesis constitutes a constellation of processes, including the determination of what kind of regenerate is to be formed, what specific
tissues are to be differentiated, and what spatial relationship shall be
expressed between these tissues. In addition, such attributes as polarity,
axes of orientation, the eventual cessation of development and the
wholeness of the regenerate are all governed by morphogenetic processes.
In brief, morphogenesis accounts for the production of a completely
normal structure, which is no small responsibility considering all the
subsidiary events which cannot be left to chance.
To investigate the nature of morphogenetic influences the method of
choice has been to attempt to bring about the predictable alteration of
developing regenerates by various experimental interventions in the
stump. This approach is predicated on the assumption that morphogenetic influences reside in the tissues of the stump and are transmitted
to the undifferentiated cells of the blastema. Thus, by studying the
relation between stump tissues and regenerating tissues, and noting
what kinds of alterations in the former are reflected in corresponding
The regenerating jaw of the newt even forms new teeth, and in intermandibular regeneration rudimentary salivary glands may sometimes
be encountered. In the regenerating lizard tail, scales are reconstituted
in the skin, and in the growing antlers of deer, hair follicles and sebaceous
glands are formed anew. In only one major instance does differentiation
in a regenerate markedly depart from embryonic ontogeny. This is the
regeneration of parts of the central nervous system, as exemplified in
the tails of amphibians and urodeles. Here the severed spinal cord
grows out along with the regenerating tail, not by the invagination of
ectoderm to give rise to a new neural tube but by the direct elongation
of the pre-existing neural canal. This is achieved largely by proliferation
of ependymal cells, and may be accompanied by the outgrowth of
regenerating new fibres. In all other cases—chondrogenesis, osteogenesis, myogenesis and various kinds of epidermal differentiation—
regenerative development faithfully recapitulates the original modes of
differentiation in the embryo.