Involvement of α-Smooth Muscle Actin-Positive Cells in Bone Regeneration Induced by Rib Fractures

α-Smooth muscle actin (SMA) has been found in the pericytes of blood vessels. In addition, it has recently been reported that α-SMA-positive cells also appear during tissue repair, suggesting that this protein is a marker for undifferentiated cells. In this study, to analyze the involvement of α-SMA-positive cells in bone regeneration, the localization of α-SMA and osteoblast differentiation markers, including runt-related transcription factor 2 (Runx2) and osterix (Osx), was examined using an experimental animal model of rib bone fracture. Twenty-five Wister rats were used. The ribs were cut vertically and were collected on day 0 (Day 0) (before fracture) and days 4 (Day 4), 8 (Day 8), 12 (Day 12), and 28 (Day 28) (after fracture); α-SMA-positive reactions were observed around the fractured bone, except for the perivascular area at Day 4. At Day 8, α-SMA-positive cells were localized around the cartilage tissue at the fracture site. This localization pattern was similar to those of Runx2 and Osx. At Day 12, α-SMA-, Runx2-, and Osx-positive cells were localized on the surface of the new bone. At Day 28, few α-SMA-positive cells were found in the bone marrow or on the cortical bone surface. These findings suggest that α-SMA is related to the early stages of osteoblast differentiation and could be involved in new bone formation in endochondral ossification.