title: Incremental lines in mouse molar enamel.
abstract: OBJECTIVE
  The purpose of the present study was to investigate the occurrence and periodicity of enamel incremental lines in mouse molars in an attempt to draw attention to some key questions about the rhythm in the activity of the secreting ameloblasts during formation of mouse molar enamel.
  
  
  METHODS
  The mouse molars were ground, etched, and studied using scanning electron microscopy.
  
  
  RESULTS
  Lines interpreted as incremental lines generally appeared as grooves of variable distinctness, and were only observed cervically, in the region about 50-250μm from the enamel-cementum junction. The lines were most readily observable in the outer enamel and in the superficial prism-free layer, and were difficult to identify in the deeper parts of enamel, i.e. in the inner enamel with prism decussation. However, in areas where the enamel tended to be hypomineralized the incremental lines were observed as clearly continuous from outer into inner enamel. The incremental lines in mouse molar enamel exhibited an average periodicity of about 4μm, and the distance between the lines decreased towards the enamel surface.
  
  
  CONCLUSIONS
  We conclude that incremental lines are to some extent visible in mouse molar enamel. Together with data from the literature and theoretical considerations, we suggest that they probably represent a daily rhythm in enamel formation. This study witnesses the layered apposition of mouse molar enamel and supports the theory that circadian clock probably regulates enamel development.
authors.affiliation: Department of Oral Biology, Faculty of Dentistry, University of Oslo, P.O. Box 1052 Blindern, 0316 Oslo, Norway. amer.sehic@odont.uio.no
authors.forename: Amer
authors.initials: A
authors.lastname: Sehic
authors.forename: Minou
authors.initials: M
authors.lastname: Nirvani
authors.forename: Steinar
authors.initials: S
authors.lastname: Risnes
mesh_terms: D000565:Ameloblasts / Q000648:ultrastructure*; D000566:Amelogenesis / Q000502:physiology*; D000818:Animals; D003743:Dental Enamel / Q000648:ultrastructure*; D051379:Mice; D008855:Microscopy, Electron, Scanning; D008963:Molar / Q000648:ultrastructure*
keywords: Ameloblasts; Dental enamel; Molar; Mouse; Scanning electron microscopy