Neuro-immune-endocrine functions of the skin: an overview.

For many years, skin was just thought of as a barrier to protect against a variety of insults from the external environment. Our body’s largest organ is gradually revealing itself to be a complex organ involved in multiple neuro-immuno-endocrine functions [1,2]. Skin functionally consists of two compartments: the epidermis with keratinocytes, melanocytes and Langerhans cells and the dermis composed of fibroblasts/fibrocytes, nerve endings, vasculature and immune cells. It has been shown that the skin, with its various components, has the ability to communicate and regulate itself through the production of various cytokines, neurotransmitters, neuroendocrine hormones and their corresponding receptors. These neuro-immuno-endocrine functions are tightly networked to central regulatory systems [1]. Considering the fact that the skin is the front-line barrier of external stressors, such as solar radiation and bacteria, it seems logical that the skin has developed an effective sensory and signaling system to differentially react to changes in the external environment. These capabilities allow it to protect, restore and maintain the local and global homeostasis that is crucial for survival [2]. The skin complexity would be surprising if we did not remember that its embryologic ectodermal-derived sibling is the brain. We will briefly discus some of these axes here.