Tensile stress induces bone morphogenetic protein 4 in preosteoblastic andfibroblastic cells, which later differentiate into osteoblasts leading to osteogenesis in the mouse calvariae in organ culture

Mechanical stress is an important factor controlling bone remodeling, which maintains proper bone morphology and functions. However, the mechanism by which mechanical stress is transduced into biological stimuli remains uncle ar. Therefore, the purpose of this study is to examine how gene expression changes with osteoblast differentiation and which cells differentiate into osteoblasts, Tensile stress was applied to the cranial suture of neonatal m ouse calvaria In a culture by means of helical springs. The suture was exte nded gradually, displaying a marked increase in cell number including osteo blasts, A histochemical study showed that this osteoblast differentiation b egan in the neighborhood of the existing osteoblasts, which can be seen by 3 h, The site of osteoblast differentiation moved with time toward the cent er of the suture, which resulted in an extension of osteoid, Scattered area s of the extended osteoid were calcified by 48 h, Reverse-transcription pol ymerase chain reaction (RT-PCR) revealed that tensile stress increased bone morphogenetic protein 4 (BMP-4) gene expression by 6 h and it remained ele vated thereafter, This was caused by the induction of the gene in preosteob lastic cells in the neighborhood of osteoblasts and adjacent spindle-shaped fibroblastic cells. These changes were evident as early as 3 h and continu ed moving toward the center of the suture, The expression of Cbfa1/Osf-2, a n osteoblast-specific transcription factor, followed that of BMP-4 and thos e cells positive with these genes appeared to differentiate into osteoblast s, These results suggest that BMP-4 may play a pivotal role by acting as an autocrine and a paracrine factor for recruiting osteoblasts in tensile str ess-induced osteogenesis.