III.
REGENERATION OF VERTEBRATE APPENDAGES
109
attachment to the pedicle on the skull. This annual cycle is influenced
by environmental changes mediated by hormones.
The annual renewal of antlers is a true instance of the regeneration of
histologically complex appendages in mammals. It occurs following the
spontaneous autotomy of the previous antlers. Unlike other examples
of regeneration in the vertebrates, however, antler growth can be
initiated only at the base, for removal of part of an antler is not always
followed by its replacement. The form of an antler does not necessarily
duplicate that of its immediate predecessor. In young animals, antlers
may be unbranched but with maturity they generally become larger and
are equipped with increasing numbers of tines. Thus, up to a point, each
succeeding set of antlers is more elaborate than before. Senile animals,
however, commonly possess reduced or abnormal antlers.
The shape assumed by a mature antler is the result of the dichotomous
branching characteristic of its developmental pattern. At each bifurcation, however, one of the branches is usually dominant in the sense that
it is larger and may express the capability of branching again. The other
branch develops into one of the tines. This relatively simple morphological pattern of antlers should be amenable to experimental analysis.
Accordingly, it would be of interest to learn to what extent young
antler buds can regulate for such defects as the removal of various
amounts of tissue. Indeed, it remains to be demonstrated conclusively
what tissues of the pedicle constitute the source of antler forming
materials. The answers to these and other questions concerning the morphogenesis of regenerating antlers must await further experimentation.
III. Process of Regeneration
The histological sequence of events characterizing the process of
regeneration is essentially the same for all morphologically complex
vertebrate appendages. There are variations in rates, and differences in
the tissues involved which are accounted for by the array of histological
entities represented in the many different vertebrate appendages capable
of regeneration, but aside from these minor discrepancies there is but
one fundamental modus operandi of regeneration. This commences as a
result of an initial stimulus (usually amputation), and is followed by
wound healing. Many of the tissues in the stump then undergo dedifferentiation which is manifested by the loss of many cellular characteristics by which tissues are normally distinguished from one another.
Following this, there accumulates at the end of the stump a mass of
undifferentiated cells derived from the immediately adjacent tissues.
This terminal aggregate of cells is called a blastema. It subsequently
undergoes proliferation, and still later, differentiation into the cell types
REGENERATION OF VERTEBRATE APPENDAGES
109
attachment to the pedicle on the skull. This annual cycle is influenced
by environmental changes mediated by hormones.
The annual renewal of antlers is a true instance of the regeneration of
histologically complex appendages in mammals. It occurs following the
spontaneous autotomy of the previous antlers. Unlike other examples
of regeneration in the vertebrates, however, antler growth can be
initiated only at the base, for removal of part of an antler is not always
followed by its replacement. The form of an antler does not necessarily
duplicate that of its immediate predecessor. In young animals, antlers
may be unbranched but with maturity they generally become larger and
are equipped with increasing numbers of tines. Thus, up to a point, each
succeeding set of antlers is more elaborate than before. Senile animals,
however, commonly possess reduced or abnormal antlers.
The shape assumed by a mature antler is the result of the dichotomous
branching characteristic of its developmental pattern. At each bifurcation, however, one of the branches is usually dominant in the sense that
it is larger and may express the capability of branching again. The other
branch develops into one of the tines. This relatively simple morphological pattern of antlers should be amenable to experimental analysis.
Accordingly, it would be of interest to learn to what extent young
antler buds can regulate for such defects as the removal of various
amounts of tissue. Indeed, it remains to be demonstrated conclusively
what tissues of the pedicle constitute the source of antler forming
materials. The answers to these and other questions concerning the morphogenesis of regenerating antlers must await further experimentation.
III. Process of Regeneration
The histological sequence of events characterizing the process of
regeneration is essentially the same for all morphologically complex
vertebrate appendages. There are variations in rates, and differences in
the tissues involved which are accounted for by the array of histological
entities represented in the many different vertebrate appendages capable
of regeneration, but aside from these minor discrepancies there is but
one fundamental modus operandi of regeneration. This commences as a
result of an initial stimulus (usually amputation), and is followed by
wound healing. Many of the tissues in the stump then undergo dedifferentiation which is manifested by the loss of many cellular characteristics by which tissues are normally distinguished from one another.
Following this, there accumulates at the end of the stump a mass of
undifferentiated cells derived from the immediately adjacent tissues.
This terminal aggregate of cells is called a blastema. It subsequently
undergoes proliferation, and still later, differentiation into the cell types