A joint team of researchers from the Biological Science Research Laboratory and the Skin Care Products Research Laboratory of Kao Corporation (President: Yoshihiro Hasebe) and Computational Optics Group, University of Tsukuba, led by Prof. Yoshiaki Yasuno, has succeeded in elucidating how the structural changes that occur at various depths in the skin affect wrinkle formation, using JM-OCT*1 that can obtain cross-sectional images of the skin non-invasively.
Further improvement of this method will be helpful for understanding the relationship between wrinkles and the depth of the associated skin structural changes in individuals, and the method could be used for providing an appropriate skin care approach for each person.
The findings from this research project have been published in Skin Research and Technology.*2
The skin has a multi-layered structure, consisting of the stratum corneum, epidermal cell layer, dermis (papillary and reticular layers), and subcutaneous tissue, from the outer surface inward (Figure 1).
Previous researches have indicated that the process of wrinkle formation is influenced by structural changes in the tissue density and collagen fibers of each of the skin layers due to various deteriorating factors such as aging and chronic exposure to ultraviolet light. However, the extent to which the structural changes in each tissue layer affect the wrinkle formation had not been determined prior to this research project.
OCT technology is a method of observing the inside of the skin without damaging it, by irradiating light onto the skin and analyzing the returning light (backscattering light), thereby obtaining cross-sectional images of the skin, from the stratum corneum to the subcutaneous tissue. However, with the conventional OCT technology, it was difficult to quantitatively evaluate the optical properties of the specific depth part of the skin with sufficient accuracy, due to the influence of increased light absorption and scattering with deeper penetration of the light through the skin.
JM-OCT was developed to overcome this shortcoming. JM-OCT uses algorithms for acquiring local data in the skin by dividing up the skin into small sections by depth. The method allows researchers to obtain quantitative local data on the optical properties of the skin at various depths in a single scan sequence, regardless of the light penetration distance (from the surface of the skin inward) (Figure 2). The joint research team used this JM-OCT method to obtain local attenuation coefficient,*3 local birefringence,*4 and local degree of polarization uniformity,*5 and investigate the tissue density and collagen fiber structure at each skin depth, in an attempt to identify local optical properties associated with wrinkle formation.
This study, involving a group of 21 female volunteers in their 70s, measured and calculated the average depth of the wrinkles detected within a 10-mm square of the eye corner area of their eyes, and investigated how the wrinkle data was related to the optical properties of their skin obtained using JM-OCT.
(1) Relationship between local attenuation coefficient (tissue density) and mean wrinkle depth
The study found a correlation between the local attenuation coefficients at the depth of 13–19 μm (i.e., stratum corneum and upper epidermis) and the depth of 189–460 μm (i.e., reticular dermis) from the skin surface and the mean wrinkle depth (Figure 3). It suggests that the deeper the wrinkles, the lower the tissue density of those areas.
(2) Relationship between local birefringence (condition of collagen fiber structure) and mean wrinkle depth
The study observed a correlation between the local birefringence at the depth of 88–139 μm from the skin surface (i.e., papillary dermis) and the mean wrinkle depth (Figure 4). It suggests that the deeper the wrinkles, the greater the degeneration of the collagen structure of those areas.
The process of wrinkle formation is affected by multiple factors such as the density of different tissues and collagen fiber structure. To estimate the degree to which each local optical property affects wrinkle formation, the research team performed a multiple linear regression analysis (stepwise method) using the local attenuation coefficient, local birefringence, and local degree of polarization uniformity measured in each skin depth range as independent variables, and the mean wrinkle depth as an explanatory variable.
The analysis showed that mean wrinkle depth could be explained roughly 65% (r = 0.805, R2 = 0.649, p < 0.001) by (1) the local attenuation coefficient obtained at 252 μm depth (reticular dermis), (2) the local birefringence at 107.1 μm depth (papillary dermis), and (3) the local degree of polarization uniformity at 170.1 μm depth (papillary dermis).
It is thought that the mean wrinkle depth can be predicted from these local optical properties.
In addition, (1) and (2) are also regions showing a correlation between the optical properties and the mean wrinkle depth as described above. It was suggested that alteration of tissue density of the dermal reticular layer (corresponding to (1)) and degeneration of collagen fiber structure of the dermal papillary layer (corresponding to (2)) are strongly related to wrinkle formation.
Measurement of wrinkles of people of various ages using JM-OCT will clarify how the structure of each skin layer changes for wrinkle formation with aging. Further research will be helpful for identifying the internal structural change contributing to individual wrinkles and for providing an appropriate skin care approach for each person.
This research project was partially funded by the subsidy granted by the Japan Science and Technology Agency's (JST) Future Society Creation Program (Accelerated Exploration Type) in the Common-Platform Area (on the key publicly-solicited research topic of "Invention of Common-Platform Systems and Devices Conducive to the Creation of Innovative Knowledge and Products," operation supervised by Nobuyuki Osakabe) on the R&D topic "Virtual Aperture Microscope: High Depth-of-Field Tomography Enabled by Computational Optics" (R&D Project Leader: Yoshiaki Yasuno) and also the research grant awarded by the Japan Society for the Promotion of Science on the project titled "Development of a Light Coherence Microscope Capable of Simultaneous Measurement of Mechanical, Light-Polarization, and Blood-Flow Characteristics of Organisms" (R&D Project Leader: Yoshiaki Yasuno).
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