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tip regeneration , treatment of amputated wildtype digits with Noggin, a BMP
signaling antagonist, was found to inhibit the regeneration response [ 11 ]. These
regeneration studies identifi ed both Msx1 function and BMP signaling as key
players in the control of mammalian digit regeneration. Just as important, analogous
studies focused on two other genes expressed in the fetal digit tip, Msx2 and Dlx5 ,
failed to yield a regeneration phenotype as single mutants or in combination indicat-
ing that regenerative failure is not simply a general phenotype of distally expressed
digit genes [ 11 , 17 ].
Shifting to neonatal digit studies, Msx1 and Bmp4 are also prominently expressedin the neonatal digit tip at the time of amputation , and re-expressed during digit tip
regeneration [ 8 , 18 ]. In this post-natal regeneration model, digit tip regeneration is
inhibited by targeting Noggin treatment to the amputation wound after epidermal
closure indicating that BMP signaling is essential for the regenerative response
[ 10 ]. This fi nding indicates that BMP signaling is critical for a successful regenera-
tive response. In addition to Bmp4 , the neonate digit regeneration blastema is asso-
ciated with prominent expression of Bmp2 and Bmp7 as well as known receptors for
BMP signaling ( Bmpr1a , Bmpr1b , and Bmpr2 ) so the specifi c BMP signal(s) impor-
tant for regeneration remains uncertain. To determine whether BMP signaling could
induce a regenerative response, studies were focused on amputations transecting the
proximal portion of the neonate P3 bone. Such amputations do not form a blastema ,
but undergo a wound healing response and resulted in truncated digits [ 10 ]. To test
for the induction of a regenerative response, purifi ed BMP2 , BMP4, or BMP7 were
absorbed onto agarose micro-carrier beads and implanted between the wound epi-
dermis and the digit stump after wound closure was complete. These tests showed
that BMP2 and BMP7, but not BMP4, were able to induce a regenerative response
that resulted in the restoration of the amputated digit tip. The BMP -induced regen-
eration response was associated with the accumulation of a blastema of proliferat-
ing undifferentiated cells that expressed two blastema-specifi c marker genes, Msx1
and Pedf (see below). Notably, whereas the endogenous regeneration of P3 forms a
blastema that re-differentiates bone by intramembranous ossifi cation [ 8 ], BMP-
induced regeneration created a blastema that formed an endochondral ossifi cation
center contiguous with the bone stump [ 10 ]. Moreover, the polarity of the induced
endochondral ossifi cation center was the same as the proximal P3 growth plate with
proliferating chondrocytes proximal to the distal hypertrophic chondrocytes
(Fig. 5.2a, b ). Taken together, proximal P3 amputation injuries respond to BMP
treatment via the formation of a transient blastema , comprised of proliferating cells
re-expressing relevant genes, reactivating a position-specifi c differentiation response
that results in the regeneration of a normally patterned terminal phalanx (Fig. 5.2e ).
A similar study was conducted to test the regeneration potential after amputationmidway through the neonate middle phalanx (P2). The P2 bone is similar to a typical
long bone with joint articulations both proximally (P1/P2 joint) and distally (P2/P3
joint), and a marrow region that extends the length of the bone. After amputation ,
wound closure occurs over the amputated bone and is consistently completed within
4–5 days [ 19 ]. BMP2 soaked micro-carrier beads were implanted between the wound
epidermis and the amputated bone stump after wound closure. The anatomical
L.A. Dawson et al.