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the bone stump, not distal to the amputation plane. The cartilaginous callus functions
as a template for the invading osteoblasts, which act to deposit woven bone in close
association with the periosteal surface, thus the bone repair response to P2 amputa-
tion is mediated via endochondral ossifi cation (Fig. 5.3f ). Sequential μCT 3-D
images illustrate periosteal bone deposition by 14 DPA, with continued bone depo-
sition corresponding to an increase in bone volume, yet new bone formation distal
to the amputation plane, i.e. regeneration, is not observed (Fig. 5.3j, k ). Endosteal-
derived osteoblasts function to cap the bone stump, separating the marrow space
from the wound site by 24 DPA (Fig. 5.3h , arrowhead). Remodeling of the bone
stump continues, complete with reconstitution of the marrow, evident at 24 and 45
DPA (Fig. 5.3h , i). In addition, there is clear evidence that soft tissues, such as the
ventral tendon which normally attaches to the P3 element, elongates across the
amputation wound and re-inserts into the dorsal aspect of the P2 bone stump
(Fig. 5.3i , arrowhead). The dynamic healing of P2 in response to amputation pro-
vides evidence that the injury response is not static but quite dynamic and sugges-
tive of an initiated but failed attempt at regeneration.
5.4 The Blastema
At the core of the problem of regenerative failure is the inability to form a blastema
rather than to proceed along a pathway of traditional wound repair. It is clear that
cells present at non-regenerating amputation wounds individually possess the
potential for involvement in a regeneration response, but what is missing is a mech-
anism whereby different cell types can interact to organize a coordinated multi-
tissue response. In regeneration-competent models the blastema functions as the
regenerative developmental site where morphogenesis and pattern formation occurs.
In addition, there is experimental evidence that patterning mechanisms guiding
development are similar to those guiding regeneration [ 25 , 26 ]. For this reason,
understanding how a regeneration blastema forms following amputation can pro-
vide critical clues for inducing or otherwise enhancing regenerative capabilities.
The fi rst question about blastema formation concerns the origin of cells. The blas-
tema is a heterogeneous population of undifferentiated cells that are proliferative
and have the potential to differentiate into the structures that make up the digit tip;
tissue types include bone, bone marrow, connective tissue, blood vessels, nerves and
epidermis. The question of cell origin can be addressed using cell type specifi c
markers coupled with a lineage marker. There are a number of important questions
that need to be addressed and some progress has been made in the past few years.
These questions include what tissue types contribute cells? Do cells arise from
stem/progenitors and/or from dedifferentiation? Are cells lineage restricted or can
they transdifferentiate during the re-differentiation process? Recent studies using
genetic approaches to label specifi c cell types and track them during digit regenera-
tion provide evidence for lineage-restriction in regeneration [ 18 , 27 , 28 ]. These
studies show that (1) epidermal cells gave rise to the nail organ and wound
L.A. Dawson et al.