The skin is a multi-functional organ, the largest in the body, and its appearance generally reflects the health and efficacy of its underlying structures. The primary function of the skin is to act as a barrier against insults from the environment, and its unique structure reflects this. The skin is composed of two layers: the epidermal outer layer is highly cellular and provides the barrier function, and the inner dermal layer ensures strength and elasticity. It is relatively acellular and is primarily made up of complex extracellular matrix proteins, being particularly rich in collagen fibres, which make up to 75% of the dermis dry weight. The major cell type present in the dermis is fibroblasts, which are heavily involved in the synthesis of many of the extracellular matrix components. Blood vessels that supply nutrients to both skin layers are also present in the dermis.
Normal skin contains high concentrations of vitamin C, which supports important and well-known functions, stimulating collagen synthesis and assisting in antioxidant protection against UV-induced photodamage. Most of the vitamin C in the skin appears to be in intracellular compartments. It is transported into cells from the blood vessels present in the dermal layer. The high concentration of vitamin C in the skin indicates that it has a number of important biological functions that are relevant to skin health. Vitamin C also promotes collagen gene expression. In the skin, collagen formation is carried out mostly by the fibroblasts in the dermis.
Like the rest of the human body, the skin is subject to changes caused by the process of natural aging. All skin layers show age-related changes in structure and functional capacity. Aging of skin can be thought of as two distinct processes—natural or intrinsic aging and environmental aging. Lifestyle factors such as smoking and exposure to environmental pollutants increase the rate of environmental aging, and can have a marked impact on the function and appearance of skin. Exposure to chronic ultraviolet radiation from sunlight is also a major environmental factor that prematurely damages our skin. Acute exposure of skin to UV radiation can cause sunburn, resulting in a large inflammatory response causing characteristic redness, swelling and heat. By comparison, chronic long-term exposure to UV radiation causes premature aging of the skin. Termed photoaging, the most obvious features are wrinkles, hyperpigmentation and marked changes in skin elasticity that cause skin sagging, with the skin also becoming sallow and rougher with age. Topical application of vitamin C, in combination with vitamin E and other compounds, has also been shown to reduce injury due to UV irradiation.