Adaptations in the structure and innervation of follicle-sinus complexes to an aquatic environment as seen in the Florida manatee (Trichechus manatus latirostris)

Florida manatees are large‐bodied aquatic herbivores that use large tactile vibrissae for several purposes. Facial vibrissae are used to forage in a turbid water environment, and the largest perioral vibrissae can also grasp and manipulate objects. Other vibrissae distributed over the entire postfacial body appear to function as a lateral line system. All manatee vibrissae emanate from densely innervated follicle‐sinus complexes (FSCs) like those in other mammals, although proportionately larger commensurate with the caliber of the vibrissae. As revealed by immunofluorescence, all manatee FSCs have many types of C, Aδ and Aβ innervation including Merkel, club, and longitudinal lanceolate endings at the level of the ring sinus, but they lack other types such as reticular and spiny endings at the level of the cavernous sinus. As in non‐whisking terrestrial species, the inner conical bodies of facial FSCs are well innervated but lack Aβ‐fiber terminals. Importantly, manatee FSCs have two unique types of Aβ‐fiber endings. First, all of the FSCs have exceptionally large‐caliber axons that branch to terminate as novel, gigantic spindle‐like endings located at the upper ring sinus. Second, facial FSCs have smaller caliber Aβ fibers that terminate in the trabeculae of the cavernous sinus as an ending that resembles a Golgi tendon organ. In addition, the largest perioral vibrissae, which are used for grasping, have exceptionally well‐developed medullary cores that have a structure and dense small‐fiber innervation resembling that of tooth pulp. Other features of the epidermis and upper dermis structure and innervation differ from that seen in terrestrial mammals. J. Comp. Neurol. 504:217–237, 2007. © 2007 Wiley‐Liss, Inc.