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Wrinkles, dryness, and a translucent and fragile appearance are hallmarks of old skin, caused by the natural aging of skin cells. But while most of us can recognize the signs of lost youth when we peer into the mirror each morning, scientists do not have a standardized way to measure the extent of age damage in skin. Now a group of Taiwanese researchers has used a specialized microscope to peer harmlessly beneath the skin surface to measure natural age-related changes in the sizes of skin cells.
The results, which are published in the Optical Society's (OSA) open-access journal Biomedical Optics Express, can be used to study the general phenomenon of skin aging and may help provide an index for measuring the effectiveness of anti-aging skin products.
In the study, Chi-Kuang Sun, a distinguished professor at National Taiwan University and chief director of the university's Molecular Imaging Center, along with medical researcher and dermatologist Yi-Hua Liao and colleagues, evaluated 52 subjects ranging in age from 19- to 79- years-old. The researchers focused a brief burst of infrared laser light into the skin of the subjects' inner forearms, an area that is generally protected from sun damage, which accelerates natural aging. The beam penetrated to a depth of about 300 millionths of a meter, or approximately where the epidermis (the upper layer of skin) and the dermis (the lower layer) meet.
The researchers used a technique known as harmonic generation microscopy (HGM), which has previously been used to study developing embryos. In the procedure, a concentrated beam of photons is sent into a material. The photons naturally oscillate at a particular frequency, and as they interact with the material, they generate "harmonics"— vibrations that are multiples of the original frequency, which are characteristic of the material structure and properties. For example, the second harmonic is twice the original frequency and the third harmonic is three times the original frequency. In an imaging system, harmonics can reveal different structures at very high resolution. In their study, the team scanned for reflected second and third harmonic photons, and from those measurements, produced a high-resolution 3-D map of the tissue that revealed structures within the skin cells.
Natural aging, the scanning showed, caused a significant increase in the overall size of cells known as basal keratinocytes—the most common cells in the outermost layer of skin—as well as in the sizes of their nuclei. However, other types of skin cells, known as granular cells, did not show a similar pattern. Thus, says Sun, the relative changes in the two types of cells can serve as an index for scoring natural or "intrinsic" skin aging -the aging of skin caused by programmed developmental or genetic factors.