Barrier functions of various kinds is the sublime function of the epidermis . The barriers of the skin are structurally located in the superficial epidermis. The interface between the superficial epidermis with stratum corneum and the profound epidermis with the stratum Malpighii is the important interface between ambient conditions and environment, including cosmetic product effects, and the internal milieu with many cellular and metabolic functions. The barriers in the superficial epidermis include a temperature moderator and barriers against evaporation of water, uptake of oxygen, expiration of carbon dioxide, penetration of chemicals from the environment (exposures related to occupation and lei-
sure), chemicals contained in products (cosmetics, cosmeceuticals, and drugs), and the penetration of ultraviolet light, which is reflected or scattered in the superficial epidermis. The superficial epidermis also protects against microbes such as bacteria and fungi.
These critically important interfaces in the epidermis are not a simple structure that can be visualized by histology, but by functions and gradients. The epidermis and the skin is also a neurosensory perceptive organ where negative sensations (pain, itching, stinging, burning, hot, cold) and positive sensations (touch, sexual stimulation) are elicited. Cosmetics, cosmeceuticals, and drugs are designed to interact with the different layers of the skin. Cosmetics primarily aim to influence the visible, superficial epidermis, whereas drugs typically aim to influence the inner layers of the skin and heal disease. Some drugs, namely the transdermals, permeate the skin and are absorbed into the blood stream to exert their action at a distant target organ.
It is not clear if cosmetic products and the chemicals ingredient they contain respect epidermal barriers and remain in the superficial epidermis or if they penetrate to deeper layers of the skin. For example, cosmetic products have to penetrate to the dermis in order to smoothen coarse wrinkles. Being present in the dermis, such ingredients or chemicals are expected to be systematically absorbed and reach the blood stream, maybe after metabolism in the skin to some unknow breakdown chemical with unknown action. However, for safety reasons, cosmetic products are normally claimed not to penetrate the dermis to any significant degree.
The interaction between epidermis and the cosmetic product with its various constituents is, as it may be understood, of crucial importance both for the claimed efficacy and the safety of product. Of course, ingredients are selected carefully, and limited to those expected to be harmless.
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