Spatial Distribution Density of MyoD− and MyoD+ Nuclei in Muscle Fibers of Regenerating Skeletal Muscle Tissue: The Effect of Photobiomodulation

BACKGROUND: Low-intensity laser exposure is used as a universal method for stimulating cellular activity, with photobiomodulatory effects directly dependent on laser wavelength and the absorption of radiation by specific tissue chromophores. It is known that infrared laser irradiation can stimulate cell proliferation and differentiation. The effects of green laser irradiation on tissue are poorly studied, and no studies have been found on the influence of low-intensity green photobiomodulation on cambial reserve cells of skeletal muscle. Meanwhile, the search for effective methods to restore skeletal muscle tissue after injury remains highly relevant. AIM: To analyze the effects of infrared and green laser irradiation on the total number of nuclei in injured skeletal muscle fibers, as well as on the counts of MyoD+ (satellite cell nuclei) and MyoD− nuclei. METHODS: The study was conducted on male Wistar rats divided into four experimental groups: Group 0, intact control (n = 8); Group I, rats with an incised skeletal muscle wound (n = 40); Group II, animals with an incised muscle wound treated with infrared laser irradiation (980 nm wavelength; n = 40); Group III, rats with an incised wound treated with green laser irradiation (520 nm wavelength; n = 40). Laser exposure was applied once in continuous mode for 180 seconds. The number of nuclei in the intact and injury zones of the skeletal muscle was assessed at 1, 3, 7, 14, and 30 days post-injury. Histological sections were stained with hematoxylin and eosin, as well as using immunohistochemistry with anti-MyoD antibodies. RESULTS: The use of infrared and green photobiomodulation contributed to an increase in the spatial density of nuclei within skeletal muscle fibers in the injury zone on days 1 and 7 post-injury compared to animals in experimental Group I. Immunohistochemical analysis using anti-MyoD antibodies showed that the spatial density of MyoD+ and MyoD− nuclei increased in the injury zone on day 3 of the experiment compared to animals in Group I. Laser exposure resulted in an increased proportion of MyoD+ nuclei among the total nuclear population compared to animals in control Group I. CONCLUSION: The application of infrared and green photobiomodulation promotes an early increase (on days 1 and 7) in the total number of nuclei in skeletal muscle fibers, with the effect of the 980 nm long-wavelength laser being more pronounced. In addition, the spatial density of MyoD+ satellite cells increased, with a more pronounced effect observed following green laser exposure. Activation of satellite cells by green laser light is early and short-term, whereas infrared irradiation induces a delayed but longer-lasting response. The overall findings indicate a stimulatory effect of laser exposure on regenerative processes in injured skeletal muscle.