Molecular aspects of healing in stabilized and non‐stabilized fractures

Abstract

Bone formation is a continuous process that is initiated during fetal development and persists in adults in the form of bone regeneration and remodeling. These latter two aspects of bone formation are clearly influenced by the mechanical environment. In this study we tested the hypothesis that alterations in the mechanical environment regulate the program of mesenchymal cell differentiation, and thus the formation of a cartilage or bony callus, at the site of injury. As a first step in testing this hypothesis we produced stabilized and non-stabilized tibial fractures in a mouse model, then used molecular and cellular methods to examine the stage of healing. Using the "molecular map" of the fracture callus, we divided our analyzes into three phases of fracture healing: the inflammatory or initial phase of healing, the soft callus or intermediate stage, and the hard callus stage. Our results show that indiun hrdgehog(ihh), which regulates aspects of chondrocyte maturation during fetal and early postnatal skeletogenesis, was expressed earlier in an non-stabilized fracture callus as compared to a stabilized callus. ihh persisted in the non-stabilized fracture whereas its expression was down-regulated in the stabilized bone. IHH exerts its effects on chondrocyte maturation through a feedback loop that may involve bone morphogenetic protein 6 [hmpb; (S. Pathi, J.B. Rutenberg, R.L. Johnson, A. Vortkamp, Developmental Biology 209 (1999) 239-253)] and the transcription factor gli3. bmp6 and gli3 were re-induced in domain adjacent to the ihh-positive cells during the soft and hard callus stages of healing. Thus, stabilizing the fracture, which circumvents or decreases the cartilaginous phase of bone repair, correlates with a decrease in ihh signaling in the fracture callus. Collectively, our results illustrate that the ihlz signaling pathway participates in fracture repair, and that the mechanical environment affects the temporal induction of ihh, bmph and gli3. These data support the hypothesis that mechanical influences affect mesenchymal cell differentiation to bone.

Relationships between tissue dilatation and differentiation in distraction osteogenesis

“…4, these findings suggest that very brief periods of pressure and compaction may be sufficient to shunt mesenchymal tissue down a chondrogenic pathway. While it has been shown previously that commitment of mesenchymal stem cells to a certain cell fate can occur within several days (Le et al, 2001), no experimental studies to our knowledge have investigated the effects of periods of mechanical stimulation that last only a fraction of an hour. Consequently, the impact of such short periods of stimulation on bone and cartilage formation is not known.…”

PlGF: A Multitasking Cytokine with Disease-Restricted Activity

“…Angiogenesis and inflammation are involved in bone fracture healing (Le et al 2001;Carano and Filvaroff 2003). In a mouse model of semistabilized bone fracture healing, PlGF deficiency impairs healing, cartilage accumulation, angiogenesis, and inflammatory cell recruitment (Maes et al 2006).…”

Mepe is expressed during skeletal development and regeneration

“…By 3 days post-fracture ( Fig. 2A-C), which corresponds to the inflammatory phase of fracture healing, mesenchymal cells at the site of injury began to differentiate into either chondrocytes or osteoblasts (Le et al, 2001). This time point precedes proteoglycan staining.…”