Functional morphology of mammalian rhinariumn skin

The naked skin surrounding the nostrils in most mammals is called a rhinarium. Rhinarium skin exhibits several unique characteristics, including an ultrastructure of pits or furrows on surface corneocytes, formed in a unique epidermal differentiation process (Paper I). Rhinarium skin is often assigned a mechanosensory function because the rhynoglyphic pattern of epidermal domes, ridges or polygons resemble the digital skin in higher primates. This is corroborated by the presence of mechanosensory Eimer’s organs in the rhinarium skin of various insectivores, indicating that it is indeed a tactile surface in these species. Interestingly, the rhinarium skin of a distantly related prosimian primate, the ring-tailed lemur also contains Eimer’s-like mechanosensory organs (Paper II). Although rhinarium skin structure in all studied mammals exhibits comparable features, differences in innervation pattern suggest that its sensory function varies amongst species (Paper III). The rhinarium function becomes even more curious if we consider its peculiar temperature dynamics in carnivorous mammals. In alert dogs, the rhinarium skin surface is normally kept a few degrees below ambient temperature. The rich vascularization and the arrangement of blood vessels in the canine rhinarium (Paper IV) suggests that it may be cooled actively and that the low tissue temperature is of functional importance. Due to the risk of tissue damage, this relative reduction in skin temperature cannot be sustained in freezing climates. Nevertheless, the rhinarium surface temperature in cold-acclimatized dogs can decrease below the cold pain threshold previously measured in mammals without eliciting any behavioural signs of distress (Paper V). In addition, the cold-transducing channels TRPM8 and TRPA1 in dog behave similarly to their human orthologs, suggesting that the cold tolerance observed in naked rhinarium skin does not rely on the innate characteristics of these channels (Paper V). Considered together, the findings presented in this thesis lay the groundwork for future studies into this enigmatic structure.