Functional characteristics of the stratum corneum in photoaged skin in comparison with those found in intrinsic aging

Among various functions of the skin, the most vital one is carried out by the stratum corneum (SC), because the SC effectively protects our body from desiccation even in a dry environment as well as from external invasion of injurious agents. Despite the general decline of various bodily functions in advanced age, the barrier function of the SC does not deteriorate but rather improves with aging, reflecting the reduced epidermal proliferation associated with slower desquamation of the SC. Although the intercellular lipid production that is crucial for the SC barrier is reduced in aged epidermis, it is compensated by the thicker SC, consisting of larger corneocytes covering the aged skin surface due to the retardation of the desquamating process. However, such SC is deficient in water-binding capacity, another important function of the SC that keeps the skin surface soft and smooth, due to decreased amounts of water-binding substances in the SC. Thus, large portions of the covered skin begin to develop xerotic changes in a dry environment of winter, being frequently accompanied by pruritus. In contrast, most elderly individuals display the unique features of photoaging on their exposed skin such as the face and hands due to chronic exposure to the ultraviolet light (UV) of sunlight. However, functional derangements of the SC are rather mild in the photoaged skin. Our functional analyses of the SC of the chronically sun-exposed skin found in the symmetrical located areas, i.e., the dorsa of the hands in middle-aged Japanese golf players who always wore a glove only on the one of the hands demonstrated significant impairment in SC water-binding capacity in the sun-exposed side, while its barrier function was well retained. Despite the decreased water content of the SC, elderly people can live without any inconvenience even when they expose the facial skin to the dry environment of winter, because there take place sebum excretion and non-apparent sweating by comparison with the skin of the sun-protected areas such as the trunk and limbs that easily develop xerotic changes in cold seasons.