III. REGENERATION OF VERTEBRATE
APPENDAGES
113
D. Differentiation
Once a mass of undifferentiated blastema cells has accumulated,
differentiation commences adjacent to the old tissues in the stump. In
continuity with endochondral skeletal elements, cartilage differentiates
in the blastema. Dermal bone, e.g., the dentary bone in the urodele jaw,
or the bony fin rays of teleosts, regenerates by the direct deposition
of ossified matrix terminal to the corresponding bones in the stump.
New muscle bundles develop in association with the skeletal parts by
the differentiation of other blastema cells. These cells become spindle
shaped, and as elongation proceeds they acquire myofibrils and cross
striations. The skin likewise achieves the differentiation of certain
specialized structures. In fish and amphibian epidermis, cells are
frequently observed to differentiate into mucus secreting organs, either
unicellular goblet cells or multicellular glands located in the dermis.
In the catfish taste barbel, new taste buds are formed in the epidermis.
standing this lack of conclusive proof, the weight of existing evidence
tends to indicate nearly all mesodermal stump tissues as potential
sources of blastema cells. Regenerating structures seem to utilize
whatever cells may be available. In the amphibian limb, cells from a
variety of tissues (muscle, cartilage, bone, tendons, dermis) ordinarily
participate in blastema formation. In the amputated lower jaws of
urodeles, large masses of muscle undergo dedifferentiation and appear
to give rise to the majority of blastema cells. In the fins of teleost fishes,
the blastema owes its origin largely to the numerous fibroblasts comprising the bulk of the fin tissue, although osteoblasts derived from the
bony fin rays also contribute. Taste barbels in the catfish produce
blastemas made up of former perichondrial cells. From these cited
examples, it is evident that there is no single source of blastema cells in
regenerating vertebrate appendages. Indeed, in some kinds of appendages, such as the amphibian limb, if one kind of tissue which normally
provides some cells for blastema formation is absent or deficient, other
tissues can apparently make up the difference. Thus, if some of the
skeletal elements or part of the musculature of a limb is removed, a
perfectly normal regenerate can still be produced from the residual
component tissues of the stump. In contrast, however, removal of the
cartilaginous rod from a taste barbel is sufficient to inhibit regeneration,
for in this instance the principal source of blastema cells (perichondrium)
is no longer present. Nevertheless, in most vertebrate appendages there
are several different sources of blastema cells, each of which is capable
of compensating for the loss of others. In the process of blastema
formation, there is usually a rather wide margin of safety.
APPENDAGES
113
D. Differentiation
Once a mass of undifferentiated blastema cells has accumulated,
differentiation commences adjacent to the old tissues in the stump. In
continuity with endochondral skeletal elements, cartilage differentiates
in the blastema. Dermal bone, e.g., the dentary bone in the urodele jaw,
or the bony fin rays of teleosts, regenerates by the direct deposition
of ossified matrix terminal to the corresponding bones in the stump.
New muscle bundles develop in association with the skeletal parts by
the differentiation of other blastema cells. These cells become spindle
shaped, and as elongation proceeds they acquire myofibrils and cross
striations. The skin likewise achieves the differentiation of certain
specialized structures. In fish and amphibian epidermis, cells are
frequently observed to differentiate into mucus secreting organs, either
unicellular goblet cells or multicellular glands located in the dermis.
In the catfish taste barbel, new taste buds are formed in the epidermis.
standing this lack of conclusive proof, the weight of existing evidence
tends to indicate nearly all mesodermal stump tissues as potential
sources of blastema cells. Regenerating structures seem to utilize
whatever cells may be available. In the amphibian limb, cells from a
variety of tissues (muscle, cartilage, bone, tendons, dermis) ordinarily
participate in blastema formation. In the amputated lower jaws of
urodeles, large masses of muscle undergo dedifferentiation and appear
to give rise to the majority of blastema cells. In the fins of teleost fishes,
the blastema owes its origin largely to the numerous fibroblasts comprising the bulk of the fin tissue, although osteoblasts derived from the
bony fin rays also contribute. Taste barbels in the catfish produce
blastemas made up of former perichondrial cells. From these cited
examples, it is evident that there is no single source of blastema cells in
regenerating vertebrate appendages. Indeed, in some kinds of appendages, such as the amphibian limb, if one kind of tissue which normally
provides some cells for blastema formation is absent or deficient, other
tissues can apparently make up the difference. Thus, if some of the
skeletal elements or part of the musculature of a limb is removed, a
perfectly normal regenerate can still be produced from the residual
component tissues of the stump. In contrast, however, removal of the
cartilaginous rod from a taste barbel is sufficient to inhibit regeneration,
for in this instance the principal source of blastema cells (perichondrium)
is no longer present. Nevertheless, in most vertebrate appendages there
are several different sources of blastema cells, each of which is capable
of compensating for the loss of others. In the process of blastema
formation, there is usually a rather wide margin of safety.