BMP2 induces segment-specific skeletal regeneration from digit and limb amputations by establishing a new endochondral ossification center.

Bone morphogenetic proteins (BMPs) are required for bone development, the repair of damage skeletal tissue, and the regeneration of the mouse digit tip. Previously we showed that BMP treatment can induce a regeneration response in mouse digits amputated at a proximal level of the terminal phalangeal element (P3) (Yu et al., 2010). In this study, we show that the regeneration-inductive ability of BMP2 extends to amputations at the level of the second phalangeal element (P2) of neonatal digits, and the hindlimb of adult limbs. In these models the induced regenerative response is restricted in a segment-specific manner, thus amputated skeletal elements regenerate distally patterned skeletal structures but does not form joints or more distal skeletal elements. Studies on P2 amputations indicate that BMP2-induced regeneration is associated with a localized proliferative response and the transient expression of established digit blastema marker genes. This is followed by the formation of a new endochondral ossification center at the distal end of the bone stump. The endochondral ossification center contains proliferating chondrocytes that establish a distal proliferative zone and differentiate proximally into hypertrophic chondrocytes. Skeletal regeneration occurs from proximal to distal with the appearance of osteoblasts that differentiate in continuity with the amputated stump. Using the polarity of the endochondral ossification centers induced by BMP2 at two different amputation levels, we show that BMP2 activates a level-dependent regenerative response indicative of a positional information network. In summary, our studies provide evidence that BMP2 induces the regeneration of mammalian limb structures by stimulating a new endochondral ossification center that utilizes an existing network of positional information to regulate patterning during skeletal regeneration.