title: Evolutionary aspects of the development of teeth and baleen in the bowhead whale.
abstract: In utero, baleen whales initiate the development of several dozens of teeth in upper and lower jaws. These tooth germs reach the bell stage and are sometimes mineralized, but toward the end of prenatal life they are resorbed and no trace remains after birth. Around the time that the germs disappear, the keratinous baleen plates start to form in the upper jaw, and these form the food-collecting mechanism. Baleen whale ancestors had two generations of teeth and never developed baleen, and the prenatal teeth of modern fetuses are usually interpreted as an evolutionary leftover. We investigated the development of teeth and baleen in bowhead whale fetuses using histological and immunohistochemical evidence. We found that upper and lower dentition initially follow similar developmental pathways. As development proceeds, upper and lower tooth germs diverge developmentally. Lower tooth germs differ along the length of the jaw, reminiscent of a heterodont dentition of cetacean ancestors, and lingual processes of the dental lamina represent initiation of tooth bud formation of replacement teeth. Upper tooth germs remain homodont and there is no evidence of a secondary dentition. After these germs disappear, the oral epithelium thickens to form the baleen plates, and the protein FGF-4 displays a signaling pattern reminiscent of baleen plates. In laboratory mammals, FGF-4 is not involved in the formation of hair or palatal rugae, but it is involved in tooth development. This leads us to propose that the signaling cascade that forms teeth in most mammals has been exapted to be involved in baleen plate ontogeny in mysticetes.
authors.affiliation: Department of Anatomy and Neurobiology, Northeast Ohio Medical University, Rootstown, OH, USA.
authors.forename: J G M
authors.initials: JG
authors.lastname: Thewissen
authors.affiliation: Department of Anatomy and Neurobiology, Northeast Ohio Medical University, Rootstown, OH, USA.
authors.forename: Tobin L
authors.initials: TL
authors.lastname: Hieronymus
authors.affiliation: Department of Wildlife Management, North Slope Borough, Barrow, AK, USA.
authors.forename: John C
authors.initials: JC
authors.lastname: George
authors.affiliation: Department of Wildlife Management, North Slope Borough, Barrow, AK, USA.
authors.forename: Robert
authors.initials: R
authors.lastname: Suydam
authors.affiliation: Department of Wildlife Management, North Slope Borough, Barrow, AK, USA.|Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK, USA.
authors.forename: Raphaela
authors.initials: R
authors.lastname: Stimmelmayr
authors.affiliation: Department of Anatomy and Neurobiology, Northeast Ohio Medical University, Rootstown, OH, USA.
authors.forename: Denise
authors.initials: D
authors.lastname: McBurney
mesh_terms: D000818:Animals; D005075:Biological Evolution; D050642:Bowhead Whale / Q000033:anatomy & histology / Q000196:embryology*; D003818:Dentition, Mixed; D005260:Female; D007568:Jaw / Q000033:anatomy & histology / Q000196:embryology; D009055:Mouth / Q000033:anatomy & histology / Q000196:embryology*; D011247:Pregnancy; D014070:Tooth / Q000033:anatomy & histology / Q000196:embryology*
keywords: 
  FGF
  ; Cetacea; baleen; baleen whales; bowhead whale; embryology; keratin; mysticetes; ontogeny; tooth development