title: Characterization of the temporomandibular joint of the harbour porpoise (Phocoena phocoena) and Risso's dolphin (Grampus griseus).
abstract: OBJECTIVE
  The temporomandibular joint (TMJ) in cetaceans is largely uncharacterized. This study aims to describe the macroscopic, microscopic, biochemical and biomechanical features of the TMJ of two species of the suborder Odontoceti: the harbour porpoise (Phocoena phocoena) and Risso's dolphin (Grampus griseus). Furthermore, we aim to elucidate the structure-function relationship of their TMJs and their possible role in echolocation.
  
  
  METHODS
  The TMJs from fresh cadaver heads of harbour porpoise (n=4) and Risso's dolphin (n=2) acquired from stranding were examined. Following macroscopical evaluation, the TMJs were investigated for their histological, mechanical and biochemical properties.
  
  
  RESULTS
  The TMJs of the studied odontocetes were found to be fundamentally different from other mammals. Macroscopically, the TMJ lacks the typical joint cavity found in most mammals and is essentially a syndesmosis. Histological and microstructural analysis revealed that the TMJ discs were composed of haphazardly intersecting fibrous-connective tissue bundles separated by adipose tissue globules and various calibre blood vessels and nerve fibres. The collagen fibre composition was primarily collagen type I with lesser amounts of collagen type II. Sulphated glycosaminoglycan (sGAG) content was lower compared to other studied mammals. Finally, mechanical testing demonstrated the disc was stronger and stiffer in the dorsoventral direction than in the mediolateral direction.
  
  
  CONCLUSIONS
  The spatial position of the TMJ, the absence of an articulating synovial joint, and the properties of the TMJ discs all reflect the unique suction-feeding mechanism adopted by the harbour porpoise and Risso's dolphin for underwater foraging. In addition, the presence of unique adipose globules, blood vessels and nerves throughout the discs may indicate a functional need beyond food apprehension. Instead, the disc may play a role in neurological sensory functions such as echolocation.
authors.affiliation: Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
authors.forename: M
authors.initials: M
authors.lastname: McDonald
authors.affiliation: Department of Biomedical Engineering, College of Engineering, University of California, Davis, CA 95616, USA.
authors.forename: N
authors.initials: N
authors.lastname: Vapniarsky-Arzi
authors.affiliation: Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
authors.forename: F J M
authors.initials: FJ
authors.lastname: Verstraete
authors.affiliation: Institute of Veterinary-Anatomy, -Histology and -Embryology, Faculty of Veterinary Medicine, Justus-Liebig-University Giessen, Gießen, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Staszyk
authors.affiliation: Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Leale
authors.affiliation: Department of Pathology, Microbiology, and Immunology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
authors.forename: K D
authors.initials: KD
authors.lastname: Woolard
authors.affiliation: Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA. Electronic address: barzi@ucdavis.edu.
authors.forename: B
authors.initials: B
authors.lastname: Arzi
mesh_terms: D000818:Animals; D001669:Biochemical Phenomena; D002102:Cadaver; D004289:Dolphins / Q000033:anatomy & histology* / Q000502:physiology*; D050741:Phocoena / Q000033:anatomy & histology*; D013704:Temporomandibular Joint / Q000033:anatomy & histology* / Q000502:physiology*
keywords: Cetacea; Harbour porpoise; Odontocete; Risso's dolphin; Temporomandibular disc; Temporomandibular joint