title: Human masseter muscle fiber type properties, skeletal malocclusions, and muscle growth factor expression.
abstract: OBJECTIVE
  We identified masseter muscle fiber type property differences in subjects with dentofacial deformities.
  
  
  METHODS
  Samples of masseter muscle were collected from 139 young adults during mandibular osteotomy procedures to assess mean fiber areas and percent tissue occupancies for the 4 fiber types that comprise the muscle. Subjects were classified into 1 of 6 malocclusion groups based on the presence of a skeletal Class II or III sagittal dimension malocclusion and either a skeletal open, deep, or normal bite vertical dimension malocclusion. In a subpopulation, relative quantities of the muscle growth factors IGF-I and GDF-8 gene expression were quantified by real-time polymerase chain reaction.
  
  
  RESULTS
  Fiber properties were not different in the sagittal malocclusion groups, but were very different in the vertical malocclusion groups (P ≤ .0004). There were significant mean fiber area differences for type II (P ≤ .0004) and type neonatal-atrial (P = .001) fiber types and for fiber percent occupancy differences for both type I-II hybrid fibers and type II fibers (P ≤ .0004). Growth factor expression differed by gender for IGF-I (P = .02) and GDF-8 (P < .01). The ratio of IGF-I:GDF-8 expression associates with type I and II mean fiber areas.
  
  
  CONCLUSIONS
  Fiber type properties are very closely associated with variations in vertical growth of the face, with statistical significance for overall comparisons at P ≤ .0004. An increase in masseter muscle type II fiber mean fiber areas and percent tissue occupancies is inversely related to increases in vertical facial dimension.
authors.affiliation: Department of Orthodontics, Temple University, Philadelphia, PA, USA. jjs6@dental.temple.edu
authors.forename: James Joseph
authors.initials: JJ
authors.lastname: Sciote
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Horton
authors.forename: Anthea M
authors.initials: AM
authors.lastname: Rowlerson
authors.forename: Joel
authors.initials: J
authors.lastname: Ferri
authors.forename: John M
authors.initials: JM
authors.lastname: Close
authors.forename: Gwenael
authors.initials: G
authors.lastname: Raoul
mesh_terms: D000293:Adolescent; D000328:Adult; D024722:Cardiac Myosins / Q000032:analysis; D005260:Female; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000032:analysis* / Q000235:genetics; D008297:Male; D008312:Malocclusion, Angle Class II / Q000473:pathology*; D008313:Malocclusion, Angle Class III / Q000473:pathology*; D008406:Masseter Muscle / Q000648:ultrastructure*; D008445:Maxillofacial Development / Q000502:physiology; D018656:Muscle Fibers, Fast-Twitch / Q000648:ultrastructure; D018485:Muscle Fibers, Skeletal / Q000648:ultrastructure*; D018657:Muscle Fibers, Slow-Twitch / Q000648:ultrastructure; D024461:Myosin Type I / Q000032:analysis; D024462:Myosin Type II / Q000032:analysis; D055435:Myostatin / Q000032:analysis* / Q000235:genetics; D024343:Open Bite / Q000473:pathology; D057887:Overbite / Q000473:pathology; D016133:Polymerase Chain Reaction; D012313:RNA / Q000032:analysis; D012737:Sex Factors; D014716:Vertical Dimension; D055815:Young Adult
chemical_list: D055435:Myostatin; D012313:RNA; D007334:Insulin-Like Growth Factor I; D024722:Cardiac Myosins; D024461:Myosin Type I; D024462:Myosin Type II