| Summary: | The skin’s sensory afferents have a myriad of varieties, tactile functions and specialized end‐organ receptors enabling species to exploit ecological niches. In typical vertebrate skin, from fish to humans, axons innervate relatively small discreet receptive fields, providing fine‐scale discrimination of texture or location of a pin prick. The cetacean suborders of Mysticeti (baleen) and Odontoceti (toothed) whales are fully‐aquatic glabrous‐skinned mammals, descendants of terrestrial hairy‐skinned arteriodactyls. Shaped by dynamic forces of the marine environment, nerves in the skin of cetaceans (whales, dolphins and porpoises) have adapted to assess critical aquatic information including current flow, swim turbulence, fish movement and boundary layers, as well as tactile touch, temperature and nociceptive stimuli. The anatomy of this highly‐derived sensory system was investigated in the flank skin of humpback whales ( Megaptera novaeangliae , NMFS permits 14809, 15240) using antibodies against highly‐conserved molecules of physiologically‐distinct afferents identified in other mammalian species. Humpback whales inherited a genomically‐diverse suite of cutaneous axons, including low‐threshold mechanoreceptors that sense innocuous environmental signals, as well as high‐threshold thermoreceptors, chemoreceptors and nociceptors. However, during their transition from terrestrial artiodactyls into obligate marine mammals, axon morphology and organization diverged. Unlike other vertebrates, heterogenous axons in whale skin assemble into thread‐like bundles. These ensembles divide into smaller bundles, follow circuitous trajectories that form unpredictable shapes, and lack structural endings. Developmental research has shown that axonal growth in vertebrates is guided by the skin’s signaling molecules and external physical stimuli. As environmental forces are transmitted through the epidermis they convey stretch at the dermal‐epidermal junction, a mechanically unstable boundary where mechanoreceptors typically reside. ...
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