85response to BMP2 was the elongation of the P2 skeletal element to almost 90 % of
stage-matched unamputated control length, however there is no evidence for the
regeneration of the P2/P3 joint or any part of the P3 element. Thus, the induced
response was specifi c to the amputated skeletal segment. The BMP2 -induced regen-
eration response included the re-expression of blastema marker genes ( Msx1 a n d
Pedf ), an enhanced proliferation response, and the accumulation of mesenchymal
cells associated with the bead and distal to the amputation plane. This mesenchymal
cell aggregate initially expressed Col2a1 indicative of chondrocyte differentiation,
and then transitioned to Col10a1 expression indicative of differentiation into hyper-
trophic chondrocytes. The organization of this induced endochondral ossifi cation
center was consistent with an elongating skeletal element: proliferating chondrocytes
distal to the hypertrophic chondrocytes associated with the stump bone, and resulted
in the deposition of new bone onto the amputated stump (Fig. 5.2c, d ). The spatial
Fig. 5.2 ( a ) In-situ hybridization probing for the cartilage proliferation marker Col2a1 in proxi-
mally amputated P3 digits show transcripts within the regenerating tissue, yet not in close associa-
tion ( arrowheads ) with the BMP2 soaked bead ( asterisk ). ( b ) In situ hybridization probing for the
hypertrophic cartilage marker Col10a1 in proximally amputated P3 digits show transcripts in
direct association ( arrow ) with the BMP2 soaked bead ( asterisk ). ( c ) In situ hybridization for
Col2a1 in P2 amputated digits treated with BMP2 shows robust transcript localization directly
adjacent ( arrow ) to the bead ( asterisk ). ( d ) Col10a1 in situ hybridization detected transcripts in the
regenerating P2 stump, yet not in close association ( arrowheads ) with the BMP2 soaked bead
( asterisk ). ( e ) Schematic diagram illustrating the in-situ results following BMP2-induced regen-
eration of proximal P3 amputation and P2 amputation. Amputation level shown as arrow. The
BMP2-induced P3 regeneration response is characterized by blastema formation and the subse-
quent differentiation into chondrocytes to regenerate the digit in a distal to proximal fashion, thus
following P3 developmental mechanisms. The BMP2 -induced P2 regeneration response is charac-
terized by blastema formation and the regeneration of a growth plate, resulting in proximal to distal
bone regeneration, therefore following the mechanism of P2 development. Distal is to the top.
Reprinted from Yu et al. [ 19 ]
5 Digit Regeneration in Mammals