title: The effects of increasing occlusal vertical dimension on the deep masseter of rat at different ages.
abstract: OBJECTIVE
  To investigate the influence of increasing the occlusal vertical dimension (iOVD) on the fibre-type distribution and ultrastructure of deep masseter of rat at different ages.
  
  
  METHODS
  A total of forty-eight male Wistar rats were divided into two groups according to age: 'teenage' group (n=24, 1.5 months) and 'young adult' group (n=24, 8 months). Both the teenage and the young adult rats were then randomly divided into the control group (n=12) and the experimental group (n=12). The occlusal vertical dimensions of the rats in the experimental groups were increased by placing composite resin on all maxillary molars. The fibre-type distribution and ultrastructure of the deep masseter were subsequently observed on day 7 and day 14 after iOVD.
  
  
  RESULTS
  In the teenage experimental group, the proportion of type IIa fibres increased, while the proportion of type IIb and type IIx fibres decreased by day 7 after iOVD (P<0.05). However, no significant fibre phenotype transformation was observed in the young adult experimental group until day 14 after iOVD. In addition, the proportion of type IIa in the teenage experimental group was higher than that of the young adult experimental group on day 7 and 14 (P<0.05). Under the transmission electron microscope, muscle fibre reconstruction and the compensatory increase in the number and volume of mitochondria appeared earlier in the teenage experimental group. The cellular traumatic reaction was less than that in the young adult experimental group.
  
  
  CONCLUSIONS
  The teenage rat alters masseter muscle structure to a slower phenotype earlier and to a greater degree than that of the young adult rat when increasing the occlusal vertical dimension.
authors.affiliation: Department of Orthodontics, College of Stomatology, Hebei Medical University, Shijiazhuang 050017, PR China. Electronic address: 82115@163.com.
authors.forename: Wen
authors.initials: W
authors.lastname: Wang
authors.affiliation: Department of Oral Pathology, College of Stomatology, Hebei Medical University, Shijiazhuang 050017, PR China. Electronic address: wangjiephd@163.com.
authors.forename: Jie
authors.initials: J
authors.lastname: Wang
authors.affiliation: Department of Orthodontics, College of Stomatology, Hebei Medical University, Shijiazhuang 050017, PR China. Electronic address: luhaiyan67@163.com.
authors.forename: Hai-Yan
authors.initials: HY
authors.lastname: Lu
authors.affiliation: Department of Orthodontics, College of Stomatology, Hebei Medical University, Shijiazhuang 050017, PR China. Electronic address: wsma02@sina.com.
authors.forename: Wen-Sheng
authors.initials: WS
authors.lastname: Ma
authors.affiliation: Department of Oral & Maxillofacial Surgery, College of Stomatology, Hebei Medical University, Shijiazhuang 050017, PR China. Electronic address: dongfusheng@hotmail.com.
authors.forename: Fu-Sheng
authors.initials: FS
authors.lastname: Dong
authors.affiliation: Department of Orthodontics, College of Stomatology, Hebei Medical University, Shijiazhuang 050017, PR China. Electronic address: zhz20047576@sina.com.
authors.forename: Xiao-Ying
authors.initials: XY
authors.lastname: Hu
authors.affiliation: Department of Orthodontics, College of Stomatology, Hebei Medical University, Shijiazhuang 050017, PR China. Electronic address: liuchunyan0313@163.com.
authors.forename: Chun-Yan
authors.initials: CY
authors.lastname: Liu
mesh_terms: D000222:Adaptation, Physiological; D000367:Age Factors; D000818:Animals; D003188:Composite Resins; D007568:Jaw / Q000000981:diagnostic imaging / Q000502:physiology; D008297:Male; D008406:Masseter Muscle / Q000000981:diagnostic imaging / Q000201:enzymology / Q000502:physiology* / Q000648:ultrastructure*; D008437:Maxilla; D046529:Microscopy, Electron, Transmission; D008928:Mitochondria; D023421:Models, Animal; D008963:Molar; D018485:Muscle Fibers, Skeletal / Q000145:classification / Q000201:enzymology / Q000502:physiology / Q000648:ultrastructure; D051381:Rats; D017208:Rats, Wistar; D014716:Vertical Dimension
chemical_list: D003188:Composite Resins
keywords: Enzyme histochemistry; Increasing the occlusal vertical dimension; Masseter; Ultrastructure