Sclerostin is differently immunolocalized in metaphyseal trabecules and cortical bones of mouse tibiae

Sclerostin, an osteocyte-derived molecule, has been reported to serve as a negative regulator of osteoblastic activity as well as bone remodeling. However, there is no report that verified the regional difference for sclerostin synthesis, and in this study we have investigated immunolocalization of sclerostin by comparing dentin matrix protein (DMP) 1, an osteocyte-derived factor broadly expressed in tibial metaphyses and cortical bone. In metaphyseal primary trabecules, a site of bone modeling, strong DMP1-reactivity was observed in osteocytic lacunar-canalicular system (OLCS), while faint staining for sclerostin was visible only in a few osteocytes. In secondary trabecules, in which bone remodeling begins, some osteocytes showed intense sclerostin-immunopositivity, though there were many DMP1-positive osteocytes. In cortical bone, there were more osteocytes reactive for sclerostin, when compared with those in the secondary trabecules. Silver impregnation verified that immature, primary trabecules contained randomly-oriented OLCS, while mature, cortical bone showed geometrically well-arrangement of OLCS. Taken together, though DMP1 is broadly synthesized in bone, sclerostin appears to be abundantly synthesized in regular OLCS of cortical bone, but less produced in irregular OLCS as seen in primary trabecules, indicating the regional difference for sclerostin synthesis.