Near-Infrared Irradiation Non-thermally Affects Subcutaneous Adipocytes and Bones

Objective: We previously reported that near-infrared irradiation simulating solar near-infrared with pre- and parallel-irradiational cooling can penetrate the skin and non-thermally affects dermis, superficial muscles, and so forth. To clarify the possible effect of NIR irradiation on other subcutaneous tissues, we evaluated how near-infrared non-thermally affects subcutaneous adipocytes and bones in rats. Methods: The central back tissues of rats were irradiated with a specialized near-infrared device that simulates solar radiation. The total energy emitted was equivalent to approximately 8.75 hours of sunbathing in North America. Histological evaluation was performed on subcutaneous adipocytes and the spinous process of the near-infrared-irradiated rat and compared with non-irradiated controls. Results: Subcutaneous and bone marrow adipocytes, CD34-positive hematopoietic stem cells in bone marrow, and the cortical bone mass were all significantly increased, whereas the bone marrow cell number was significantly decreased following near-infrared irradiation. Apoptotic cells were detected in the bone marrow at postirradiation days 7 and 30 but were not detected at day 60 or in the controls. Bone marrow cell numbers recovered gradually, whereas the increase in subcutaneous and bone marrow adipocytes, CD34-positive hematopoietic stem cells in bone marrow, and cortical bone mass remained elevated even at day 180. Conclusions: Near-infrared irradiation that simulated solar radiation non-thermally affected subcutaneous adipocytes and bones in rats. It induced putative, non-thermal damage of bone marrow, which was mediated by apoptosis. However, it increased subcutaneous and bone marrow adipocytes, CD34-positive hematopoietic stem cells in bone marrow, and cortical bone mass.