Stratum corneum (SC) is located at the outermost of the skin and plays pivotal roles in barrier function and moisturizing function of the skin. SC is composed of piled-up dead corneocytes with 10–20 layers and their intercellular spaces are filled with lipids. Major components of corneocytes are keratin fibers providing mechanical stiffness of the SC. Natural moisturizing factors (NMF) are a group of water-soluble small molecules, including amino acids, minerals, lactate and so on, which can retain water molecules to keratin fibers. Cornified envelope is a membrane-like insoluble structure surrounding corneocytes and acts as a scaffold for organization of intercellular lipids. Intercellular lipids are consisted of ceramides, free fatty acids, and cholesterol, which form packing crystal and lamellar structure. Any of these composite components is essential for healthy SC, and their defects may result in deterioration of SC function and dry skin. Mechanisms of moisturizers are classified into two groups. Emollients, such as oil, cover the skin surface without penetration into the skin, and moisturize the SC by their occlusive properties. Humectants, such as polyols and amino acids derivatives, can penetrate into the SC, and moisturize the SC like NMF. Thus, moisturizing effect of skincare products is well-recognized. However, hyper-moisturized SC may lead to deteriorated barrier function with pros and cons. Diaper dermatitis is a typical trouble with hyper-moisturized SC. On the other hand, a defect in barrier function is helpful for penetration of certain drugs into the skin known as occlusive dressing technique. Recent study shows that slow evaporation of water from SC make SC structure tightly packed with optical transparency and barrier function. In addition, this water evaporation-induced rearrangement process of SC can reduce formation of expression-induced residual wrinkle. These new concepts provide valuable information to understand real mechanisms of moisturization and suggest future innovation of moisturizers.

Many cosmetics, not only skin care products but also make-up products, are employed together with pharmaceutical products in dermatology. We dermatologists recommend the use of moisturizer for preventing the development of eczematous lesions from xerotic skin such as atopic xerosis and senile xerosis. We also utilize sunscreen products together with physical protection such as hats and long sleeved-shirts for the prevention of photoaging such as solar lentigines and wrinkles as well as various skin tumors that are caused by a history of prolonged sun exposure. Moreover, strict photo-protection with sunscreen products is required in patients with photosensitive dermatoses and with diseases whose skin symptoms are easily exacerbated by sunlight exposure. The former includes xeroderma pigmentosum, chronic actinic dermatitis and polymorphous light eruption, the latter includes systemic lupus erythematosus, dermatomyositis and rosacea. In aesthetic dermatology, we recommend to the use of retinol or retinoic acid products for facial wrinkles due to photoaging. Whitening agents such as hydroquinone are utilized for treating melasma. Make-up products are utilized to camouflage mottled pigmentation and telangiectasia caused by photoaging and skin disorders whose symptoms are mainly color changes such as birthmarks, vitiligo and scars. Camouflaging lesional skin is helpful for improving the quality of life of patients.

Many studies of risk factors for atopic dermatitis (AD) onset have reported about skin barrier dysfunction such as filaggrin gene mutation and rise in transepidermal water loss in addition to family history and genetic background. There are also reports of the influence of risk factors given by westernized living environment such as change of intestinal microbiome, reduction of sunlight exposure, change of diet as immunological background, and it is thought that each factor interacts complicatedly. Various studies have been conducted to try to prevent the onset of AD, such as eliminating specific foods in mothers during pregnancy or lactation, or in infants undergoing weaning, and avoiding inhaled antigens such as house dust mites or pets in the environment. However, it has not been possible to demonstrate efficacy in many prevention studies using these allergen removal measures. Currently, moisturizing skin care from early infancy is expected to be most effective as a prophylactic method for AD. Furthermore, as a compromised skin barrier and the presence of dermatitis constitute percutaneous sensitization risk to food antigens, the possibility that skin cares aiming at active eczema control against atopic dermatitis decrease allergen sensitization and subsequent allergy march has been considered, however, at present, supporting evidence is scarce.