Rhinarium skin structure and epidermal innervation in selected mammals

The glabrous skin around the nostrils in mammals is called a rhinarium or planum nasale. Rhinarium skin has multiple epidermal domes that are generally assumed to form a tactile surface. The rhinarium is innervated by a branch of the trigeminal nerve which is associated with stimuli such as touch, chemical irritants and temperature. In this study, our aim was to correlate variation in rhinarium skin sensory innervation with different feeding behaviors while also covering a broad systematic spectrum. Using histological and immunohistological methods, we studied skin morphology, nerve fiber density and nerve fiber distribution in the rhinarium epidermal domes of four species: cow, ring-tailed lemur, brown bear, and dog, that all exhibit different feeding behaviors. All species share similar traits in rhinarium skin morphology, but glands were only found in cow rhinarium skin. The most substantial differences were observed in the innervation pattern. Mechanosensory skin organs were found only in the ring-tailed lemur. Dog epidermal domes possess a pronounced central dermal papilla containing a nerve bundle in its top, close to the skin surface. The abundance of free epidermal nerve fibers in epidermal domes of all species, suggest that the rhinarium skin is a sensory surface, that can be used to detect fine touch, chemical irritants or temperature. In the species where the whole epidermal dome was examined, the intraepidermal nerve fiber density is higher in the central part of the domes. The nerve distribution and the central positioning of a single gland duct in cow and the dermal papilla top organ in dog indicates that each epidermal dome can be considered a functional unit. The observed differences in innervation hint at different sensory functions of rhinaria in mammals that may be correlated to feeding behavior.