title: The enamel knot-like structure is eternally maintained in the apical bud of postnatal mouse incisors.
abstract: OBJECTIVE
  The boundary where inner and outer enamel epithelia meet is referred to as the cervical loop (CL) in molar tooth germs. In contrast, rodent incisors are continuously growing: the labial side of the teeth is covered with enamel (crown-analog), and the lingual side is covered with cementum (root-analog). These results in the appearance of CL in the frontal sections apart from the apical end. However, many researchers have used the term "labial CL" to indicate the apical end of incisors.
  
  
  METHODS
  This study investigated the gene expression patterns for the enamel knot signaling center in tooth morphogenesis to clarify the difference between "labial CL" and molar CL. We examined the three-dimensional expression patterns of markers for the enamel knot including fibroblast growth factor 4 (Fgf4), sonic hedgehog (Shh), Msx2, and P21 in frontal sections of murine mandibular incisors.
  
  
  RESULTS
  Serial frontal sections from the apical end of the postnatal incisor clearly demonstrated the existence of enamel knot-like structures composed of supra-inner enamel epithelium and stellate reticulum in the "labial CL". This structure includes the expression of all examined markers for enamel knots such as Fgf4, Shh, Msx2, and P21.
  
  
  CONCLUSIONS
  The molar tooth germ-like structure is maintained indefinitely in the "labial CL". Therefore, the "labial CL" is not equivalent to the molar CL. The existence of an EK-like structure in the apical end of incisors implies that the usage of "labial CL" may be inappropriate for indicating this region. The "apical bud" could be used as an alternative term.
authors.affiliation: General Dentistry and Clinical Education Unit, Niigata University Medical and Dental Hospital, Niigata, Japan; Division of Molecular Signaling and Biochemistry, Department of Health Promotion, Kyushu Dental University, Kitakyushu, Japan.
authors.forename: Chihiro
authors.initials: C
authors.lastname: Nakatomi
authors.affiliation: Division of Anatomy and Cell Biology of the Hard Tissue, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan; Division of Anatomy, Department of Health Promotion, Kyushu Dental University, Kitakyushu, Japan.
authors.forename: Mitsushiro
authors.initials: M
authors.lastname: Nakatomi
authors.affiliation: Division of Pediatric Dentistry, Department of Oral Health and Development Sciences, Tohoku University Graduate School of Dentistry, Sendai, Japan.
authors.forename: Kan
authors.initials: K
authors.lastname: Saito
authors.affiliation: Division of Developmental Biology and Regenerative Medicine, Department of Anatomy, Iwate Medical University, Yahaba, Japan.
authors.forename: Hidemitsu
authors.initials: H
authors.lastname: Harada
authors.affiliation: Division of Anatomy and Cell Biology of the Hard Tissue, Department of Tissue Regeneration and Reconstruction, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan. Electronic address: histoman@dent.niigata-u.ac.jp.
authors.forename: Hayato
authors.initials: H
authors.lastname: Ohshima
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics; D003743:Dental Enamel / Q000196:embryology / Q000378:metabolism*; D051519:Fibroblast Growth Factor 4 / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D053823:Hedgehog Proteins / Q000235:genetics; D018398:Homeodomain Proteins / Q000235:genetics; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D007180:Incisor / Q000166:cytology / Q000254:growth & development*; D019394:Ki-67 Antigen / Q000235:genetics; D051379:Mice; D009805:Odontogenesis / Q000235:genetics*; D014083:Tooth Germ / Q000166:cytology / Q000254:growth & development*
chemical_list: D050759:Cyclin-Dependent Kinase Inhibitor p21; C496446:Fgf4 protein, mouse; D051519:Fibroblast Growth Factor 4; D053823:Hedgehog Proteins; D018398:Homeodomain Proteins; D019394:Ki-67 Antigen; C084119:MSX2 protein; C465570:Shh protein, mouse
keywords: Mice (ICR); Regeneration; Rodentia; Stem cells; Tooth germ