The epidermis: a sensory tissue.

The skin is an efficient barrier which protects our bodies from the external environment but it is also an important site for the perception of various stimuli. Sensory neurones of the peripheral nervous system send many primary afferent fibres to the skin. They pass through the dermis and penetrate the basement membrane to innervate epidermal cells or remain as free endings. Nerve fibres are clearly involved in somatosensation. However, they are not always so numerous, for example in distal parts of the limbs, and some kinds of sensors can be at a distance of hundreds of micrometers from each other. The skin can detect patterns at a very fine and smaller scale, which suggests that nerve terminals are helped by epidermal sensors. All epidermal cells (keratinocytes, melanocytes, Langerhans cells and Merkel cells) express sensor proteins and neuropeptides regulating the neuro-immuno-cutaneous system. Hence, they must play a part in the epidermal sensory system. This review will consider the epidermal components of this forefront sensory system and the stimulations they perceive. The epidermis can be considered a true sensory tissue where sensor proteins and neurone-like properties enable epidermal cells to participate in the skin surface perception through interactions with nerve fibres.