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

Abstract

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, Adelta and Abeta 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 Abeta-fiber terminals. Importantly, manatee FSCs have two unique types of Abeta-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 Abeta 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.

The Morphology of the Rat Vibrissal Follicle-Sinus Complex Revealed by Three-Dimensional Computer-Aided Reconstruction

“…This basic innervation pattern of the deep vibrissal nerve is similar to that described in previous studies [Waite and Li, 1993;Rice et al, 1997;Ebara et al, 2002]. A recent study has demonstrated the dual innervation of the deep vibrissal nerve in the Florida manatee [Sarko et al, 2007]; however, the deep vibrissal nerve innervation pattern was singular in the present study, not dual ( fig. 5 ).…”

“…Many studies have investigated the vibrissal FSC, and the results have been presented schematically [Fundin et al, 1999;Ebara et al, 2002], on electron micrographs [Sakita et al, 1994;Sarko et al, 2007], and using immunofluorescence analysis . Thus, the basic structure, innervation pattern, and distribution of blood vessels within the vibrissal FSC are known.…”

Aquatic Environment and Differentiation of Vibrissae: Comparison of Sinus Hair Systems of Ringed Seal, Otter and Pole Cat

“…In another semi-aquatic mammal, the Australian water rat (Hydromys chrysogaster) , the number of myelinated nerve fibers in DVN is about the same as in otters . In the pinnipeds and sirenians that have been examined so far, the vibrissa system and its innervation is even more developed than in the studied semi-aquatic mammals [Yablokov and Klevezal, 1964;Ling, 1968Ling, , 1977Stephens et al, 1973;Hyvärinen and Katajisto, 1984;Hyvärinen, 1989Hyvärinen, , 1995Reep et al, 1998Reep et al, , 2001Reep et al, , 2002Marshall et al, 2006;Sarko et al, 2007]. In the bearded seal the number of mystacial vibrissae is greatest (about 244 mystacial vibrissae per seal) and the innervation of one F-SC is also slightly greater or about the same as in the ringed seal [Hyvärin- en and Katajisto, 1984;Marshall et al, 2006].…”

“…In pinnipeds and sirenians the progressive development of the vibrissal system was possibly the most important adaptation of the sensory system to the aquatic environment [Stephens et al, 1973;Hyvärinen and Katajisto, 1984;Sokolov, 1986;Hyvärinen, 1989Hyvärinen, , 1995Dehnhardt and Kaminski, 1995;Dehnhardt et al, 1998Dehnhardt et al, , 2001Reep et al, 2001Reep et al, , 2002Marshall et al, 2006;Sarko et al, 2007]. Furthermore, in pinnipeds the vibrissae are used not only in tactile exploration but also in many social displays [Miller, 1975;Kastelein and van Gaalen, 1988;Dehnhardt and Kaminski, 1995].…”

“…The vibrissae can also be used as a speedometer during dives [Hyvärinen, 1995]. In sirenians vibrissae are distributed over the entire body of the animals [Bryden et al, 1978;Sokolov, 1986;Reep et al, 2002] and it has been suggested that the vibrissal systems of both sirenians and pinnipeds have functions analogous to the lateral line in fishes [Hyvärinen, 1995;Reep et al, 2002, Sarko et al, 2007.…”