Low-energy irradiation stimulates formation of osteoclast-like cells via RANK expression in vitro.

Low-energy laser irradiation (Ga-Al-As semiconductor laser, output 50 mW) was applied to rat osteoclast precursor cells for 1, 3, 6, or 10 min at 24-h intervals during the culture period. The number of tartrate-resistant acid phosphatase positive multinucleate cells was increased by approximately 1.3-fold in the 3- and 6-min irradiation groups. Further, osteoclasts appeared on day 2 in the laser irradiation groups but not until day 3 in the control groups. In immunohistochemical staining for receptor activator of NF-kappaB (RANK), the laser irradiation groups showed significantly greater amounts of staining in comparison with the control group on days 2 and 3. Reverse transcription-polymerase chain reaction results also showed that the expressions of RANK were upregulated. In the pit formation assay, resorption pits were significantly more abundant in the laser irradiation groups than in the controls. The present results suggest that low-energy laser irradiation facilitates differentiation and activation of osteoclasts via RANK expression.