Collagen fibers in the dermis are closely related to skin aging because collagen is one of structural proteins and hence plays an important role to determine the morphology and mechanical properties of the skin. Although histological and/or biochemical studies have been used to reveal the age-related decrease and disorganization of dermal collagen fibers, such invasive methods cannot be used for simple assessment or serial monitoring of skin aging. Therefore, there is a considerable need for non-invasive assessment method of skin aging. Recently, second-harmonic-generation (SHG) microscopy has attracted attention for in situ visualization of human dermal collagen fibers. Due to its specific collagen selectivity without the need for staining, non-invasiveness, and high-resolution 3D imaging capability, collagen fiber structure can be visualized as high-contrast SHG image in situ. However, extraction of quantitative parameters reflecting the collagen fiber structure from SHG image presents difficulties. In this article, we applied two-dimensional auto-correlation analysis to SHG images of dermal collagen fibers in human cheek skin. The collagen fiber structure index obtained from the analysis decreased significantly with aging. Furthermore, we confirmed a moderate correlation between the collagen fiber structure index and skin elasticity measured by a Cutometer®.This correlation indicates that the dermal collagen fiber structure plays an important role to determine skin elasticity. In other words, the auto-correlation analysis of SHG image can be used for evaluation of skin elasticity from the viewpoint of dermal collagen. The proposed method will be a powerful tool in the fields of skin cosmetics and anti-aging dermatology.
