title: Species-specific modifications of mandible shape reveal independent mechanisms for growth and initiation of the coronoid.
abstract: BACKGROUND
  The variation in mandibular morphology of mammals reflects specialisations for different diets. Omnivorous and carnivorous mammals posses large mandibular coronoid processes, while herbivorous mammals have proportionally smaller or absent coronoids. This is correlated with the relative size of the temporalis muscle that forms an attachment to the coronoid process. The role of this muscle attachment in the development of the variation of the coronoid is unclear.
  
  
  RESULTS
  By comparative developmental biology and mouse knockout studies, we demonstrate here that the initiation and growth of the coronoid are two independent processes, with initiation being intrinsic to the ossifying bone and growth dependent upon the extrinsic effect of muscle attachment. A necessary component of the intrinsic patterning is identified as the paired domain transcription factor Pax9. We also demonstrate that Sox9 plays a role independent of chondrogenesis in the growth of the coronoid process in response to muscle interaction.
  
  
  CONCLUSIONS
  The mandibular coronoid process is initiated by intrinsic factors, but later growth is dependent on extrinsic signals from the muscle. These extrinsic influences are hypothesised to be the basis of the variation in coronoid length seen across the mammalian lineage.
authors.affiliation: Department of Craniofacial Development and Stem Cell Biology, Dental Institute, King's College London, London, SE1 9RT UK.
authors.forename: Neal
authors.initials: N
authors.lastname: Anthwal
authors.affiliation: Institute of Genetic Medicine, International Centre for Life, Newcastle University, Newcastle upon Tyne, NE1 3BZ UK.
authors.forename: Heiko
authors.initials: H
authors.lastname: Peters
authors.affiliation: Department of Craniofacial Development and Stem Cell Biology, Dental Institute, King's College London, London, SE1 9RT UK.
authors.forename: Abigail S
authors.initials: AS
authors.lastname: Tucker
keywords: Coronoid process; Dentary; Guinea pig; Mammal; Mammalian evolution; Mandible; Opossum mouse; Pax9; Sox9