91blastema cell recruitment during regeneration. In addition, endothelial cells in the
blastema and at non-regenerating amputation wounds are identifi ed as critical medi-
ators of BMP2 action by transducing the BMP2 signal into a functional cell recruit-
ment signal.
The third critical topic concerns one of the hallmarks of the regenerative pro-cess–the proliferative nature of the blastema. Endogenous blastema formation as
well as BMP-induced regeneration is associated with enhanced proliferation. Using
a transgenic BMP reporter mouse strain, Yu et al. [ 19 ] showed that BMP2 induced
P2 regeneration specifi cally enhanced proliferation of BMP responsive cells sug-
gesting that the mitogenic action of BMP2 was a rapid and direct effect on cells at
the amputation wound. In addition to BMP2, there is evidence that regenerative
proliferation is also controlled by WNT signaling that is linked to a neurotrophic
effect critical for the regeneration response [ 28 ].
The amputation level-specifi c nature of digit tip regeneration is associated withthe presence of the nail matrix in the stump. The nail organ is a continuously elon-
gating structure that consists of a nail plate that encases a proximal nail matrix of
proliferating nail stem cells , a distal nail matrix of transiently amplifying cells, and
the nail bed that extends to the distal digit tip [ 28 ]. Nail elongation is a process that
requires canonical WNT signaling , and nail dysmorphogenesis results when this
signaling pathway is disrupted [ 28 , 37 ]. Importantly, regenerative defects are also
observed when the canonical WNT signaling pathway is disrupted, thus providing
an explanation for the close link between regenerative capabilities and the nail in
both humans and mice [ 28 , 37 ]. In addition, studies in which the canonical WNT
pathway is constitutively activated in epidermal cells, including nail cells, show that
regeneration following amputation at a proximal P3 level can be induced [ 28 ], thus
canonical WNT signaling by epidermal cells is required for the endogenous regen-
eration response, and can induce regeneration from a normally non-regenerative
amputation injury. A secondary feature of canonical WNT signaling in epidermal
cells is evidence that inhibiting signaling causes a reduction in innervation associ-
ated with the regeneration response [ 28 ]. This raises the possibility that the nail
matrix effect on digit regeneration may be mediated via modifi cation of a neuro-
trophic effect on the regenerative response. The effect of denervation on cell prolif-
eration and blastema formation in regenerating salamander limbs is well documented,
resulting in the complete inhibition of regeneration [ 38 ]. Unlike the salamander
limb, however, denervation of the mouse digit tip does not completely inhibit the
regenerative response, but it does impair the normal formation of both nail and bone
of the regenerate [ 28 , 39 , 40 ]. Denervation of the digit tip inhibits mesenchymal
proliferation and the expression of Fgf2 which is mitogenic for blastema cells both
in vivo and in vitro [ 28 ], thus providing evidence that the FGF signaling pathway is
also an important trophic infl uence for mammalian regeneration. While there is
likely a laundry list of trophic infl uences important for the digit regenerative
response, the evidence to date point to two critical signaling pathways (BMP and
WNT) that are required for the endogenous regenerative response, and can also
induce regeneration at a non- regenerative amputation.
5 Digit Regeneration in Mammals