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It is well known that epidermal keratinocytes form a water-impermeable barrier at the skin surface. However, recent studies suggest that keratinocytes also have a variety of sensory systems for a wide range of environmental factors, including humidity, temperature, mechanical stress, electric potential, visible light, and sound. Epidermal keratinocytes express a series of thermo-activated receptors, called the transient receptor potential (TRP) family. The TRP proteins in epidermal keratinocytes may serve as sensors not only of environmental temperature, but also of humidity, mechanical stress and chemical stimuli. Activations of some TRP influence the barrier recovery process. Voltage-gated calcium channel is expressed in the epidermal keratinocytes and external electrical potential and materials that produce an electrical double layer influence barrier homeostasis. Photo-receptor proteins (rhodopsin and opsin) and energy-transfer proteins (transducin and phosphodiesterase 6), which are expressed in retina, are also expressed in keratinocytes and appear to influence epidermal barrier homeostasis. Moreover, epidermal barrier homeostasis is influenced by sound in the 10 to 30 kilohertz range. Interaction between keratinocytes and nerve fibers has also been reported. Moreover, a series of neurotransmitter receptors, which play key roles in information processing in the central nervous system, are expressed in keratinocytes. Thus, epidermal keratinocytes might be have both sensory and information-processing functions at the surface of the body. In this paper, I deal with these new aspects of epidermal keratinocytes and also discuss the hypothesis that epidermal keratinocytes serve as a functional interface between the body and the environment.
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