OPN mRNA expression during tooth relocation as assessed by in situ hybridization with an OPN mRNA probe in sections of a mouse dentoalveolar complex. (a) A control, untreated site. The tooth was moved for 3 days by a controlled orthodontic force, resulting in bone formation in distal periodontal sites (b) and direct resorption in mesial sites (c) of the first molar. Following relocation, the OPN mRNA hybridization signal decreased in periodontal osteoblasts in bone formation sites (b, right side), and increased in cells (osteoblasts, osteoclasts, and their precursors) lining alveolar bone surface in resorption sites (c). The mechanically induced OPN hybridization signal is present in alveolar osteocytes within the bone adjacent to both resorption and formation sites (b, c). A control sense OPN probe did not yield any signal above background. F, direction of orthodontic force and tooth movement; TR, tooth root; PD, periodontium; AB, alveolar bone; OB, osteoblasts; OCY, osteocytes; CBL, cementoblasts; OCL and MN, osteoclasts and their mononuclear precursors. Note that the PD is enlarged in b, where the TR is being pulled away from the AB, but that it is reduced in c, where the PD is being compressed between the TR and the AB.