112
R. J.
GOSS
and bone are affected by the nearby injury. The cartilage matrix is
dissolved, releasing chondrocytes from their lacunae. Likewise, the
ossified matrix of bone is subjected to the erosive influences of osteoclasts, although this regressive process is considerably slower than the
changes already noted in softer tissues.
Two chief roles of the process of dedifferentiation suggest themselves.
The first appears to be a prelude to the distal migration of cells destined
to become part of the blastema. Thus, by the elimination of many of
their special characteristics, cells become better able to move toward
the site of amputation. The second is a possibility that the dedifferentiation of many kinds of cells in the stump facilitates the future
differentiation of cells in the blastema. If most of the dedifferentiated
cells in the stump ultimately become blastema cells, the blastema is a
very cosmopolitan structure. Its differentiation would be an easier
matter if the component cells were not restricted by their pasts.
Although it is as yet unproven, it may some day be conclusively
demonstrated that ex-fibroblasts may become chondrocytes, or even
that erstwhile chondrocytes may differentiate into muscle fibres, or
vice versa. The alternative method, which would be contrary to the bulk
of experimental evidence, would seem to be a strict correspondence
between stump and blastema, such that newly differentiated tissues
would be derived exclusively from their counterparts in the stump.
Were this the case, one might expect that dedifferentiation would be
superfluous.
C. Blastema Formation
While dedifferentiation is going on in the underlying mesodermal
tissues of the stump, the epidermal covering with which the wound was
healed increases considerably in thickness. Thus, many more layers of
epidermal cells usually occur over the stump than are normally present
elsewhere in the skin. The basal layer of epidermal cells is in intimate
contact with the immediately subjacent mesodermal tissues, for no
distinct basement membrane can be recognized beneath the apical cap
of epidermis overlying the stump. It is beneath this terminal epidermal
thickening that the blastema is formed; indeed, there is reason to
believe that the presence of the epidermal cap is responsible for determining the location of the blastema (Thornton, 1954, 1956, 1957).
The blastema is formed partly by the aggregation of cells and partly
by their subsequent proliferation. The cells which become incorporated
into the blastema are derived from the adjacent stump tissues. Because
of technical difficulties in tracing individual cells, it is currently impossible to determine the precise origins of the blastema cells. Nofcwith-
R. J.
GOSS
and bone are affected by the nearby injury. The cartilage matrix is
dissolved, releasing chondrocytes from their lacunae. Likewise, the
ossified matrix of bone is subjected to the erosive influences of osteoclasts, although this regressive process is considerably slower than the
changes already noted in softer tissues.
Two chief roles of the process of dedifferentiation suggest themselves.
The first appears to be a prelude to the distal migration of cells destined
to become part of the blastema. Thus, by the elimination of many of
their special characteristics, cells become better able to move toward
the site of amputation. The second is a possibility that the dedifferentiation of many kinds of cells in the stump facilitates the future
differentiation of cells in the blastema. If most of the dedifferentiated
cells in the stump ultimately become blastema cells, the blastema is a
very cosmopolitan structure. Its differentiation would be an easier
matter if the component cells were not restricted by their pasts.
Although it is as yet unproven, it may some day be conclusively
demonstrated that ex-fibroblasts may become chondrocytes, or even
that erstwhile chondrocytes may differentiate into muscle fibres, or
vice versa. The alternative method, which would be contrary to the bulk
of experimental evidence, would seem to be a strict correspondence
between stump and blastema, such that newly differentiated tissues
would be derived exclusively from their counterparts in the stump.
Were this the case, one might expect that dedifferentiation would be
superfluous.
C. Blastema Formation
While dedifferentiation is going on in the underlying mesodermal
tissues of the stump, the epidermal covering with which the wound was
healed increases considerably in thickness. Thus, many more layers of
epidermal cells usually occur over the stump than are normally present
elsewhere in the skin. The basal layer of epidermal cells is in intimate
contact with the immediately subjacent mesodermal tissues, for no
distinct basement membrane can be recognized beneath the apical cap
of epidermis overlying the stump. It is beneath this terminal epidermal
thickening that the blastema is formed; indeed, there is reason to
believe that the presence of the epidermal cap is responsible for determining the location of the blastema (Thornton, 1954, 1956, 1957).
The blastema is formed partly by the aggregation of cells and partly
by their subsequent proliferation. The cells which become incorporated
into the blastema are derived from the adjacent stump tissues. Because
of technical difficulties in tracing individual cells, it is currently impossible to determine the precise origins of the blastema cells. Nofcwith-