Common molecular pathways in skeletal morphogenesis and repair.

The formation of bone is a continual process in vertebrate development, initiated during fetal development and persisting in adulthood in the form of remodeling and repair. The remarkable capacity of skeletal tissues to regenerate has led to the hypothesis that the molecular signaling pathways regulating skeletogenesis are shared during fetal development and adult wound healing. A number of key regulatory pathways that are required for endochondral ossification during fetal development are described, and their reintroduction in fracture repair demonstrated. Secreted proteins such as Sonic and Indian hedgehog exert their effect on pattern formation and chondrogenesis in the appendicular skeleton, partly through regulation of molecules such as bone morphogenic proteins (Bmps) and parathyroid hormone-related peptide (PTHrP). Once chondrocytes have matured and hypertrophied, they undergo apoptosis and are replaced by bone; the transcription factor Cbfal plays a critical role in this process of chondrocyte differentiation and ossification. Analyses of the expression patterns of these genes during fracture healing strongly suggest that they play equivalent roles in adult wound repair. Knowledge acquired through the study of fetal skeletogenesis will undoubtedly contribute to an understanding of fracture repair, and subsequently guide the development of biologically based therapeutic interventions.