We have outlined that human skin is an emitter of olfactory compounds that have the capability to direct our behaviour, consciously or unconsciously. In addition, physicians formerly used olfactory information for diagnosis. Unfortunately, in today′s high-tech medicine, this knowledge seems to have vanished. Among physicians of different medical specialties, the dermatologist is the one who approaches the patients closest. In this context, the nose can still provide valuable information in respect to disease, emotion, age and diet. Furthermore, there is accumulating evidence that odours from skin glands exert physiological and behavioural effects. The original definition of a pheromone implies a stereotypical stimulus-response reaction, as known from many insects. In contrast, the effects in humans seem to be more complex. In the majority of cases, the reaction towards an odour is dependent on individual history and actual motivation and, therefore, hardly ever stereotypical. Furthermore, it is an open question what social and biological impact the increasing personal hygiene measures typical for modern societies will have on human life. Could it be that olfaction in humans is just an aesthetic sense? In most mammals, odours are perceived by two distinct organs within the nasal cavity, namely the main olfactory epithelium (MOE), primary responsible for the perception of odours, and the vomeronasal organ (VNO), mainly specialized on pheromone-driven effects . The VNO binds volatile and non-volatile chemicals and transmits their signal to areas of the limbic system . Interestingly, recent findings describe exceptions from the functional separation between VNO and MOE by detecting pheromone receptors also in the MOE and vice versa vomeronasal responses to non-pheromonal stimuli [90, 91]. In this light, the probable absence of a functional VNO in adult humans  does not necessarily exclude communication via pheromones. However, it seems that olfactory communication in humans plays not that essential role as in most other mammals (see Fig. 1). Of note, recent studies add a new aspect on olfaction by showing the extra-nasal expression of olfactory receptors. In particular, functional olfactory receptors are expressed in human gastrointestinal cells , sperm  and prostate cells . These amazing findings amend the concept of olfaction. In this context, it would be intriguing to investigate whether those receptors are also functionally expressed in human skin.
Figure 1. Evolution of odour and pheromone receptors in mammals. Mouse, dogs and new-world monkeys have the highest repertoire of functional genes for odorant receptors. Interestingly, in dogs, intact pheromone receptor genes are decimated. The red asterisk marks the development of trichromatic colour vision typical for hominoidea, a superfamily including humans and old-world monkeys such as chimpanzees, gorillas, orangutans and gibbons. It seems that this evolutionary step is correlated with pseudogenization of many odorant and pheromone receptor genes. Moreover, the loss of a functional vomeronasal organ in this superfamily seems likely. Modified from Rouquier and Giorgi . n.a., not available.
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