Cytotoxicity tests on FPCLs were carried out by using a WST-1 commercially available cell proliferation reagent. The assay is based on cleavage of the tetrazolium salt WST-1 by active mitochondria to produce a soluble colored formazan salt. As the conversion is performed only by viable cells, it directly correlates with the cell number. At 2 days after RF treatment, the growth medium on the contracted FPCLs was removed and replaced with 0.3 ml test solution (DMEM). Then, 30 μl of the cell proliferation reagent WST-1 was added to each well. The FPCLs were incubated for 2 hours at 37°C in a humidified atmosphere with 5% CO2, then the microplate was thoroughly shaken for 1 minutes and the absorbance was measured at 450 nm by using a microtiter reader (model 550; Bio-Rad Laboratories, Hercules, CA, USA). The background absorbance was measured on wells containing only the dye solution. The results were expressed as percent optical density of RF treated vs. control untreated, serum-containing cultures.