Long-term histological comparison between near-infrared irradiated skin and scar tissues

BACKGROUND AND OBJECTIVE Our previous histological studies indicated that near-infrared (NIR) irradiation stimulates collagen proliferation in rat and human skin for 3 months. High collagen density in the dermis and smoothing of the epidermis were observed in irradiated rat skin, and appeared to last up to 6 months. Epidermal smoothness in irradiated rat skin seems to resemble scarring. Here, we performed a long-term histological comparison between NIR (1100 to 1800 nm) irradiated skin and scar tissues. MATERIALS AND METHODS Rat skin was irradiated using a NIR device. Scar tissues were harvested from wounded areas and were compared with irradiated skin. Histological changes up to 180 days post-treatment were evaluated with hematoxylin and eosin, Azan-Mallory staining, and collagen type I and III staining. RESULTS In nonirradiated control skin, the dermis showed a low density of type I and III collagen, the surface of the epidermis was rough, and no significant changes were observed over time. In irradiated skin, both type I and III collagen increased significantly, and persisted up to 180 days. The density of type I collagen was significantly higher than that of type III collagen, whereas type I and III collagen of the control group did not differ significantly. Epidermis was thickened for 30 days, and epidermal smoothness persisted up to 180 days. In scar tissues, the density of type III collagen was higher than that of type I collagen. The number of fibroblasts remained high and the glial fibrils were dense until 180 days after injury compared with irradiated skin. Significant increases in both type I and III collagen and epidermal flattering persisted until 180 days. CONCLUSIONS NIR irradiation induced high collagen density in the dermis, resulting in long-term epidermal smoothness without scar formation. Results indicated that NIR irradiation provides safe, consistent, and long-term effects of skin rejuvenation.