Quantitative analysis of collagen and capillaries of 3.8-μm laser-induced cutaneous thermal injury and wound healing

The biological effects of cutaneous thermal injury and wound healing after 3.8-μm laser radiation were investigated by observing the effects of different radiation doses on in vivo cutaneous tissue. A 3.8-μm laser with a radiation dose that changes from small (5.07) to large (15.74 J/cm²) was used to irradiate mouse skin with the 2 × 4 grid array method. The healing progress of laser-injured spots, pathological morphology (H&E staining), and collagen structure changes (Sirius Red staining) were dynamically observed from one hour to 21 days after laser radiation, and the capillary count and collagen content were quantitatively and comparatively analyzed. When the radiation doses were 5.07, 6.77, 8.21, and 9.42 J/cm², a white coagulation spot predominantly occurred, and when the radiation doses were 11.09 12.23, 14.13, 15.74 J/cm², a small injured spot predominantly occurred. One hour after radiation, the collagen structure was obviously damaged. Three to fourteen days after radiation, the hyperplasia and morphology of the collagen in the 5.07 J/cm² group were significantly better than those in the other dose groups. The number of capillaries in the 5.07 J/cm² and 6.77 J/cm² groups was significantly higher than that in the normal group (P < 0.01 or P < 0.05). Twenty-one days after radiation, only the collagen in the 5.07 J/cm² group was densely arranged, and it was basically close to the normal group level. The collagen content in the 5.07 J/cm² group was approximately 10.7%, but it was still lower than that in the normal group (with a collagen content of approximately 14.1%). The collagen in the other dose groups was diminished and had not returned to the normal group level. As the dose of the 3.8-μm laser increased, skin thermal injury gradually increased, the full-thickness skin increased, and the collagen content decreased, showing better dose-dependent and time-dependent effect relationships. The increase in capillaries in the early stage of laser radiation and the significant increase in collagen content in the middle and late stages of laser radiation were two important factors that promoted wound healing.