Decoding the Mechanisms of Pigment Reduction and Skin Rejuvenation Induced by Picosecond Laser: Insights From a Porcine Model

OBJECTIVES: Picosecond lasers are extensively utilized to treat hyperpigmentation and signs of skin aging. However, the underlying biological mechanisms remain to be elucidated. This study was performed to explore the effects and mechanisms of picosecond laser therapy on melanin clearance and skin rejuvenation in a porcine model. METHODS: A dual-wavelength (532/1064 nm) picosecond laser was applied to both pigmented and non-pigmented skin of Bama miniature pigs. For pigmentation treatment, 532 nm irradiation was delivered with 3 and 4 mm spot sizes at fluences ranging from 0.4 to 1.0 J/cm², and 1064 nm irradiation was applied with 2, 3, and 4 mm spot sizes at fluences of 1.2 to 3.0 J/cm². For rejuvenation treatment, 1064 nm irradiation (8 mm spot size, 0.4 J/cm²) was employed. Skin samples were collected immediately after treatment and at 7 and 30 days post-irradiation. Hematoxylin and eosin staining and transmission electron microscopy were used to evaluate epidermal morphology and melanosome ultrastructure. Fontana-Masson staining was performed to assess melanin distribution and content, combined with CD163 immunofluorescence to visualize the colocalization of melanin and macrophages. Immunohistochemistry and Western blot analyses were conducted to determine tyrosinase expression levels. Multiplex immunohistochemistry was further applied to detect melanocytic markers (SOX10, MART-1), collagen I/III, growth factors (TGF-β1, GDF11), and barrier-related proteins (filaggrin, claudins). RESULTS: Picosecond laser treatment immediately induced epidermal vacuolization and melanosome disruption, followed by a progressive reduction in epidermal melanin over time, which was more prominent in the 532 nm treatment group compared with 1064 nm. Fontana-Masson and immunofluorescence staining revealed abundant CD163-positive macrophages colocalized with melanin in the dermis on Day 7, indicating active phagocytosis of pigment debris. SOX10- and MART-1-positive melanocytes were rare at Day 0 and absent thereafter in laser-treated skin. Western blot analysis showed that tyrosinase expression was significantly downregulated from Day 7 and remained suppressed through Day 30, suggesting inhibition of melanogenesis. Regarding rejuvenation, dermal collagen I and III slightly decreased at Day 7 but markedly increased by Day 30. TGF-β1 and GDF11 were upregulated following treatment. Filaggrin and claudins increased at Day 7 and declined at Day 30 but remained above baseline, indicating improved barrier integrity. CONCLUSIONS: Picosecond lasers effectively induced pigment reduction, immune-mediated clearance and melanogenesis inhibition, while concomitantly promoting dermal regeneration and epidermal barrier repair. These findings provide mechanistic insights into their therapeutic applications in pigmentation disorders and skin aging.