title: Initiation of primary myogenesis in amniote limb muscles.
abstract: BACKGROUND
  Vertebrate muscles are defined and patterned at the stage of primary myotube formation, but there is no clear description of how these cells form in vivo. Of particular interest is whether primary myotubes are "seeded" by a unique myoblast population that differentiates as mononucleated myocytes, similar to the founder myoblasts of insects.
  
  
  RESULTS
  We analyzed the cell populations and processes leading to initiation of primary myogenesis in limb buds of rats and mice. Pax3(+ve) myogenic precursors migrate into the limb bud and initially consolidate into dorsal and ventral muscle masses in the absence of Pax7 expression. Approximately a day later, Pax7(+ve) cells appear in the central aspect of the limb base and subsequently throughout the limb muscle masses. Primary myogenesis is initiated within each muscle mass at a time when only Pax3, and not Pax7, protein can be detected. Primary myotubes form initially as elongate mononucleated myocytes, well before cleavage of the muscle masses has occurred. Multinucleate myotubes appear approximately a day later. A similar process is seen during initiation of chick limb primary myogenesis.
  
  
  CONCLUSIONS
  Primary myotubes of vertebrate limb muscles are initiated by mononucleated myocytes, that appear structurally analogous to the founder myoblasts of insects.
authors.affiliation: Department of Anatomy, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand.
authors.forename: Antonio S J
authors.initials: AS
authors.lastname: Lee
authors.forename: John
authors.initials: J
authors.lastname: Harris
authors.forename: Michael
authors.initials: M
authors.lastname: Bate
authors.forename: Krishnaswamy
authors.initials: K
authors.lastname: Vijayraghavan
authors.forename: Lorryn
authors.initials: L
authors.lastname: Fisher
authors.forename: Shahragim
authors.initials: S
authors.lastname: Tajbakhsh
authors.forename: Marilyn
authors.initials: M
authors.lastname: Duxson
mesh_terms: D000818:Animals; D002642:Chick Embryo; D018507:Gene Expression Regulation, Developmental / Q000502:physiology; D006614:Hindlimb / Q000196:embryology*; D051379:Mice; D024510:Muscle Development / Q000502:physiology*; D018482:Muscle, Skeletal / Q000196:embryology*; D000072041:PAX3 Transcription Factor; D051763:PAX7 Transcription Factor / Q000096:biosynthesis; D051761:Paired Box Transcription Factors / Q000096:biosynthesis; D051381:Rats
chemical_list: D000072041:PAX3 Transcription Factor; C530820:PAX3 protein, rat; D051763:PAX7 Transcription Factor; C530821:PAX7 protein, rat; D051761:Paired Box Transcription Factors; C497322:Pax7 protein, mouse; C068686:Pax3 protein, mouse
keywords: Pax3/7; amniote; limb muscles; muscle precursors; primary myogenesis