III.
REGENERATION OF VERTEBRATE APPENDAGES
121
involving the addition of extra cartilaginous rods to a barbel, followed
by amputation, does not appreciably alter the eventual outcome of the
regenerate. As many as four cartilaginous rods can be fitted into a single
barbel stump, yet the resulting regenerate contains only one cartilaginous
rod. The extra rods of cartilage all contribute to the regenerate, but
their cartilaginous extensions fuse into a single structure. Nor does
reversal of the proximodistal polarity of the cartilaginous rod in the
barbel alter the normal shape of the regenerate produced. The results of
these, plus other experiments, tend to implicate tissues other than the
cartilaginous rod (perhaps the connective tissue proper) as origins of
morphogenetic influences in the taste barbel.
In contrast, the skeletal parts of the teleost fin, though not essential
for fin regeneration, are necessary for the formation of the bony parts
of the fin regenerate (Goss and Stagg, 1957). When a fin regenerates,
osteoblasts derived from the stumps of the fin rays migrate into the
blastema where they mediate the development of new rays in the
regenerate. Since each fin ray consists of two components, one on either
side of the fin, two strands of osteoblasts are produced in the blastema.
These cells become intimately associated with the overlying epidermis
where the earliest indications of the new ray become visible in the
basement membrane. As more ossified matrix is laid down, the osteoblasts become situated on the outer and inner surfaces of the developing
ray regenerate. The new fin rays, like their counterparts in the stump,
are segmentally jointed and have a propensity to branch dichotomously.
In view of the manner in which fin ray regenerates are formed, it is not
surprising that prior extirpation of rays from a fin precludes their
regeneration. Under these circumstances, the regenerating fin lacks
the component rays corresponding to the missing parts of the stump.
Fin ray regeneration, therefore, requires the presence of pre-existing
osteoblasts and cannot occur de novo from cells of other origins. Indeed,
accessory fin rays can be induced to form in fin regenerates by the
transplantation of extra rays (Birnie, 1947; Goss and Stagg, 1957).
The amphibian limb, more than any other vertebrate appendage, has
been subjected to intensive experimentation to elucidate the morphogenetic effects of skeletal components. The skeleton of amphibian
extremities is cartilaginous in young larval forms, and gradually becomes
ossified with metamorphosis and maturity. Adult bones are unable to
regenerate following excision (Wendelstadt, 1904), but larval cartilaginous elements, as well as similar skeletal parts in limb regenerates of
adult animals, can generally be replaced after total extirpation (Goss,
1958). Because of the facility with which bones can be experimentally
manipulated, more is known about the morphogenetic roles of skeletal
tissues than any other histological component of the limb.
REGENERATION OF VERTEBRATE APPENDAGES
121
involving the addition of extra cartilaginous rods to a barbel, followed
by amputation, does not appreciably alter the eventual outcome of the
regenerate. As many as four cartilaginous rods can be fitted into a single
barbel stump, yet the resulting regenerate contains only one cartilaginous
rod. The extra rods of cartilage all contribute to the regenerate, but
their cartilaginous extensions fuse into a single structure. Nor does
reversal of the proximodistal polarity of the cartilaginous rod in the
barbel alter the normal shape of the regenerate produced. The results of
these, plus other experiments, tend to implicate tissues other than the
cartilaginous rod (perhaps the connective tissue proper) as origins of
morphogenetic influences in the taste barbel.
In contrast, the skeletal parts of the teleost fin, though not essential
for fin regeneration, are necessary for the formation of the bony parts
of the fin regenerate (Goss and Stagg, 1957). When a fin regenerates,
osteoblasts derived from the stumps of the fin rays migrate into the
blastema where they mediate the development of new rays in the
regenerate. Since each fin ray consists of two components, one on either
side of the fin, two strands of osteoblasts are produced in the blastema.
These cells become intimately associated with the overlying epidermis
where the earliest indications of the new ray become visible in the
basement membrane. As more ossified matrix is laid down, the osteoblasts become situated on the outer and inner surfaces of the developing
ray regenerate. The new fin rays, like their counterparts in the stump,
are segmentally jointed and have a propensity to branch dichotomously.
In view of the manner in which fin ray regenerates are formed, it is not
surprising that prior extirpation of rays from a fin precludes their
regeneration. Under these circumstances, the regenerating fin lacks
the component rays corresponding to the missing parts of the stump.
Fin ray regeneration, therefore, requires the presence of pre-existing
osteoblasts and cannot occur de novo from cells of other origins. Indeed,
accessory fin rays can be induced to form in fin regenerates by the
transplantation of extra rays (Birnie, 1947; Goss and Stagg, 1957).
The amphibian limb, more than any other vertebrate appendage, has
been subjected to intensive experimentation to elucidate the morphogenetic effects of skeletal components. The skeleton of amphibian
extremities is cartilaginous in young larval forms, and gradually becomes
ossified with metamorphosis and maturity. Adult bones are unable to
regenerate following excision (Wendelstadt, 1904), but larval cartilaginous elements, as well as similar skeletal parts in limb regenerates of
adult animals, can generally be replaced after total extirpation (Goss,
1958). Because of the facility with which bones can be experimentally
manipulated, more is known about the morphogenetic roles of skeletal
tissues than any other histological component of the limb.