AbstractsBiology & Animal Science

In this work, we employed a well-standardized mouse rib fracture model to assess the distribution of three important osteocyte-derived molecules-dentine matrix protein 1 (DMP1), sclerostin and fibroblast growth factor 23 (FGF 23). A transverse fracture was created on the right eighth rib of each mouse, and histochemical examinations were conducted at 2-, 4-, 7-days (early stage) and 2-, 3-and 4-weeks (late stage) after the fracture. Two days after the fracture, the periosteum thickened, and up to the seventh day post-fracture, the cortical surfaces were shown to form two tissue types depending on the distance from the fracture site: chondrogenesis was taking place near the fracture, whereas osteogenesis was seen distant from it. The cortical bones developing chondrogenesis featured several empty lacunae, while in the cortical bones underlying newly-formed woven bone, empty lacunae were hardly seen. Next, we examined immunolocalization of DMP1, sclerost in and FGF23 in the cortical bone close and distant from the fracture site. DMP1-immunopositivity was seen in most lacunae regardless the existence of osteocytes inside in both close and distant regions from the fracture. In the close region, some osteocytes seemed to be still alive though there were many empty lacunae. We examined the ratio of sclerostin/FGF23-immunopositive osteocytes among these living osteocytes; consequently, the region close to the fracture had only few sclerostin-, and FGF23-immunoreactive osteocytes, whereas the distant region revealed several osteocytes immunopositive for these markers. Statistical analyses showed the lower indices of sclerostin- and FGF23-immunopositive osteocytes in the close region compared to the distant region, implying that several osteocytes do not synthesize sclerostin and FGF23. Later stages of fracture healing at 2-, 3- and 4-weeks showed mature cortical bone encompassing the native cortical bone, and the distribution of DMP1, sclerostin and FGF23 appeared to have returned to normal. We concluded that the early stages of fracture healing seem to be important for triggering of chondrogenesis and osteogenesis, and osteocytic activities may take part in chondrogeensis/osteogenesis.