Expression of β-actin is widely utilized as an internal control of mRNA expression in various cells. Here we show evidence that the expression level of β-actin mRNA in osteoclasts significantly differs in its intensity according to the position in bone tissues. By use of in situ hybridization, we obtained clear data showing that osteoclasts facing the distal part of a developing molar tooth germ expressed extremely high levels of β-actin mRNA in comparison with other osteoclasts observed in the mandibular bone surface. No signal was detected when β-nerve growth factor transcripts were observed. Electron microscopic analysis revealed that osteoclasts localized in these areas were functionally active, estimated from the expression of high levels of the osteoclast membrane antigen, Katl antigen, which associated with active osteoclasts. These data suggest that osteoclasts expressing extremely high levels of β-actin are highly related to active osteoclasts induced by the mechanical stress caused by physiological movement of molar tooth germ towards distal directions in the developing mandible.
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