SummaryA cell line with characteristics of the periodontal ligament fibroblasts is negatively regulated for mineralization and Runx2/Cbfa1/Osf2 activity, part of which can be overcome by bone morphogenetic protein-2

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 unclear. 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 mouse calvaria in a culture by means of helical springs. The suture was extended gradually, displaying a marked increase in cell number including osteoblasts. A histochemical study showed that this osteoblast differentiation began in the neighborhood of the existing osteoblasts, which can be seen by 3 h. The site of osteoblast differentiation moved with time toward the center of the suture, which resulted in an extension of osteoid. Scattered areas of the extended osteoid were calcified by 48 h. Reverse-transcription polymerase chain reaction (RT-PCR) revealed that tensile stress increased bone morphogenetic protein 4 (BMP-4) gene expression by 6 h and it remained elevated thereafter. This was caused by the induction of the gene in preosteoblastic cells in the neighborhood of osteoblasts and adjacent spindle-shaped fibroblastic cells. These changes were evident as early as 3 h and continued moving toward the center of the suture. The expression of Cbfa1/Osf-2, an osteoblast-specific transcription factor, followed that of BMP-4 and those cells positive with these genes appeared to differentiate into osteoblasts. 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 stressinduced osteogenesis. (J Bone Miner Res 2001;16:24 -32)

Astronauts experience osteoporosis‐like loss of bone mass because of microgravity conditions during space flight. To prevent bone loss, they need a riskless and antiresorptive drug. Melatonin is reported to suppress osteoclast function. However, no studies have examined the effects of melatonin on bone metabolism under microgravity conditions. We used goldfish scales as a bone model of coexisting osteoclasts and osteoblasts and demonstrated that mRNA expression level of acetylserotonin O‐methyltransferase, an enzyme essential for melatonin synthesis, decreased significantly under microgravity. During space flight, microgravity stimulated osteoclastic activity and significantly increased gene expression for osteoclast differentiation and activation. Melatonin treatment significantly stimulated Calcitonin (an osteoclast‐inhibiting hormone) mRNA expression and decreased the mRNA expression of receptor activator of nuclear factor κB ligand (a promoter of osteoclastogenesis), which coincided with suppressed gene expression levels for osteoclast functions. This is the first study to report the inhibitory effect of melatonin on osteoclastic activation by microgravity. We also observed a novel action pathway of melatonin on osteoclasts via an increase in CALCITONIN secretion. Melatonin could be the source of a potential novel drug to prevent bone loss during space flight.

Dental pulp is assumed to possess the capacity to elaborate both bone and dentin matrix under the pathological conditions following tooth injury. This study was undertaken to clarify the mechanism inducing bone formation in the dental pulp by investigating the pulpal healing process, after tooth replantation, by micro-computed tomography (mu-CT), immunocytochemistry for heat-shock protein (HSP)-25 and cathepsin K (CK), and histochemistry for both alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP). Under deep anesthesia, the upper right first molar of 4-week-old Wistar rats was extracted and immediately repositioned in the original socket. In control teeth at this age, the periphery of the coronal dental pulp showed intense ALP-positive and HSP-25-positive reactions, whereas there were no TRAP-positive or CK-positive cells. Tooth replantation weakened or terminated ALP-positive and HSP-25-positive reactions in the pulp tissue at the initial stages. At 3-7 days after operation, the ALP-positive region recovered from the root apex to the coronal pulp followed by HSP-25-positive reactions in successful cases showing tertiary dentin formation. In other cases, TRAP-positive and CK-positive cells appeared in the pulp tissue of the replanted tooth at postoperative days 5-10 and remained associated with the bone tissue after 12-60 days. Immunoelectron microscopy clearly demonstrated that CK-positive osteoclast-lineage cells made contact with mesenchymal cells with prominent nucleoli and well-developed cell organelles. These data suggest that the appearance of TRAP-positive and CK-positive cells is involved in the induction of bone tissue formation in dental pulp.

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