## 4 Discussion  
In this comparative in vitro analysis, we provide compelling experimental evidence showing the ability of fibrobl cells to respond differently, morphologically and functionally, to three PBM treatments performed by red, NIR, and violet-blue light centered at 635 nm, 808 nm, and 405 nm, respectively, delivered at a constant energy density of 0.4 J/cm\({}^{2}\). In particular, the findings of the present study highlight that red PMB treatment mainly promotes fibroblast proliferation and the cellular capability to degrade/remodel ECM components such as collagen, whereas violet-blue and even more NIR PBM stimulate the synthetic and secretive activities of the cells, in terms of collagen expression and deposition and likely the cell differentiation towards proto- or myofiblabats. Myofibroblasts possess the immunophenotypical and ultrastructural features of either collagen-synthetically active fibroblasts with a prominent rough endoplasmic reticulum or of contractile cells exhibiting de novo expression of \(\alpha\)-sma within well assembled actin-myosin bundles namely stress fibers and peculiar electrophysiological properties typical of smooth muscle cells. However, with respect to fibroblasts, differentiated myofiblabats are much larger with a more polygonal shape and secrete higher quantity of proteoglycans, type-1 and type-3 collagen and other components of the ECM [37; 38; 39; 40; 41]. In particular, our above-reported conclusions are supported by the following observations. We have demonstrated that the percentage of fibroblasts displaying the nuclear expression of Ki67 marker--which is expressed by the cells undergoing G1, S, G2, and mitosis cycle phases and absent in quiescent cells during the resting G0 phase--increased after red PBM exposure. These results agree with our previous observations in fibroblasts and other mesenchymal cells [4; 34] and are in line with studies from other groups performed on normal or wounded fibroblasts, aimed to evaluate the proliferative cell response to red light [42; 43; 44; 45; 46; 47; 48; 49]. By contrast NIR light induced a decrease of fibroblast proliferation, whereas violet-blue light did not modify substantially the cell proliferation rate as compared to untreated cells, according to previous reports [47; 50; 51; 52; 53; 54; 55]. Red PBM treated cells showed a reduced assembly of F-actin, whereas violet-blue and even more so NIR PBM treated ones displayed well assembled and defined F-actin filaments, likely stress fibers, together with a more polygonal shape and an increase in cell surface area, suggestive of a proto- or myofiblabatic phenotype. These morphological features may be coherent with the proliferation data, in line with theory stating that an increase in proliferation results in a reduction of cell size and conversely, a decrease in proliferation produces larger cells [56]. Moreover the reduction of stress fiber-like structures observed in red PBM treated cells may be indicative of motile migrating cells, based on findings showing that cells with highly motility typically display few, thin and more dynamic stress fibers and that stress fiber are not essential for cell migration [57; 58]. Our results concerning the impact of red PBM on cytoskeleton assembly properties of NIH/3T3 fibroblasts are in accordance with a very recent paper by [PERSON] and co-workers [59]. However, differently from our findings, these authors did not find any difference between the cell response, in terms of actin filament content, to red and NIR light. This may be explained, in addition to the different PBM parameters applied, by the difference in cell growth substrate rigidity, i.e., gel in the work by [PERSON] et al. versus plastic or glass in our work [59; 60; 61].