Fracture healing and bone repair are postnatal

Fracture healing and bone repair are postnatal processes that mirror many of the ontological events that take place during embryonic development of the skeleton and have been extensively reviewed. The recapitulation of these ontological processes is believed to make fracture healing one of the few postnatal processes that is truly regenerative, restoring the damaged skeletal organ to its pre-injury cellular composition, structure and bio- mechanical function. Interestingly, a comparison of the transcriptome of mouse callus tissues across a 21-day period of fracture healing showed that about one-third of the mouse homologues of the genes expressed by human embryonic stem cells are preferentially induced. Many of the homeotic genes that control appendicular limb development also show increased expression during fracture healing.Finally, all the primary morphogenetic pathways that are active during embryonic skeletal development are expressed in fracture calluses,and have therefore become the focal point of efforts to develop new therapies. In this Review, we place these biological processes in the context of how trauma and the immune system, as a component of the injury response, are related to the developmental aspects of fracture healing. We then review the relationships between ontogeny and the recovery of skeletal function. Finally, we focus on specific biophysical, local and systemic therapies that have been used to promote fracture healing and on the various biological processes they promote.

Phases of fracture healing

Fracture healing and skeletal tissue repair involve an initial anabolic phase characterized by an increase in tissue volume related to the de novo recruitment and differentiation of stem cells that form skeletal and vascular tissues. Immediately adjacent to the fracture line, a cartilaginous callus will form. Peripheral to this central region, at the edges of the new cartilage tissues, the periosteum swells and primary bone formation is initiated. Concurrent with cartilage tissue development, cells that will form the nascent blood vessels that supply the new bone are recruited and differentiate in the surrounding muscle sheath. The increases in the vascular bed that surrounds and then grows into the callus are further reflected by the increased blood flow into the area of tissue repair. As chondrocyte differentiation progresses, the cartilage extracellular matrix undergoes mineralization and the anabolic phase of fracture repair terminates with chondrocyte apoptosis. The histological and cellular progression of these events .The anabolic phase is followed by a prolonged phase in which catabolic activities predominate, and is characterized by a reduction in the volume of the callus tissues. During this phase of predominately catabolic activity, such as cartilage resorption, specific anabolic processes continue to take place; secondary bone formation is initiated as the cartilage is resorbed and primary angiogenesis continues as the nascent bone tissues replace the cartilage. Subsequently, when bone remodelling begins, the first mineralized matrix produced during primary bone formation is resorbed by osteoclasts, and then the secondary bone laid down during the period of cartilage resorption is also resorbed. As the bony callus tissue continues to be resorbed, this prolonged period is characterized by coupled cycles of osteoblast and osteoclast activity in which the callus tissues are remodelled to the bone's original cortical structure (termed `coupled remodelling' here). During this period, the marrow space is re-established and the original marrow structure of haematopoietic tissue and bone is regenerated. In the final period of the catabolic phase, extensive vascular remodelling takes place in which the increased vascular bed regresses and the high vascular flow rate returns to its pre-injury level. Although these processes take place consecutively, they overlap substantially and are a continuum of changing cell populations and signalling processes within the regenerating tissue.

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