Objective: The aim of the present study was to analyze the effects of photobiomodulation using a 904-nm diode laser at two energy densities (6 J/cm2 and 50 mJ/cm2) on L929 fibroblast cells. Background: Low-power laser irradiation (LPLI) is a non-pharmacological resource that induces important in vitro photobiomodulation on cell cultures and tissues. Methods: Irradiation was performed for three days at 24-h intervals. After each interval, the cells were stained with MitoTracker Orange™ and DioC6 dyes to assess the photobiomodulatory effects of irradiation on mitochondrial activity and changes in the endoplasmic reticulum. The MTT assay [3-(4.5-dimethylthiazol-2-yl)-2.5 diphenyltetrazolium bromide] was used to evaluate cell proliferation. Results and Conclusions: The fluorescence microscopy assessment of mitochondria and endoplasmic reticulum in cells irradiated with 6 J/cm2 and 50 mJ/cm2 demonstrated intense mitochondrial activity, which was confirmed by DioC6 staining. Reticular activity was observed stemming from increased protein synthesis. Photobiomodulation with 50 mJ/cm2 was slightly higher than with 6 J/cm2, as demonstrated by fluorescence microscopy results. Photobiomodulation was also time-dependent, with better results 72-h after irradiation.
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