title: Why does enamel in Klk4-null mice break above the dentino-enamel junction?
abstract: BACKGROUND
  The enamel layerof kallikrein 4 (Klk4)-null mice has a normal thickness and a decussating pattern of enamel rods, but it contains residual enamel proteins, is less highly mineralized, and fractures in its deepest part just above the dentino-enamel junction (DEJ). The plane of fracture is puzzling because the deepest enamel is deposited earliest and, through the action of the secretory stage enamel protease (Mmp20), is the most mature part of the enamel layer at the time of the onset of Klk4 expression.
  
  
  OBJECTIVE
  To characterize the planes of fracture in Mmp20- and Klk4-null mice and to localize Klk4 expression in developing teeth.
  
  
  METHODS
  Klk4- and Mmp20-null mice were sacrificed at 7 weeks and their mandibular incisors were characterized by scanning electron microscopy. Klk4(+/)(lac)(Z) mice were mated with Klk4(+/)(lac)(Z) mice. Offspring were genotyped by polymerase chain reaction. Klk4(+/)(+), Klk4(+/)(lac)(Z), and Klk4(lac)(Z/)(lac)(Z) (null) littermates on postnatal days 5, 8, 11, and 14 were processed for β-galactosidase histochemistry.
  
  
  RESULTS
  The enamel layer fractures at the DEJ in Mmp20-null mice, and fractures occur in enamel above the DEJ in Klk4-null mice. Klk4 is not expressed by secretory-stage ameloblasts, murine odontoblasts beneath the secretory stage, or maturation-stage ameloblasts. Klk4 is specifically expressed by transition and maturation-stage ameloblasts.
  
  
  CONCLUSIONS
  The breakage of enamel near the DEJ in Klk4-null mice is not due to a failure of odontoblasts to express Klk4, but it relates to a progressive hypomineralization of enamel with depth.
authors.affiliation: Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Mich., USA.
authors.forename: James P
authors.initials: JP
authors.lastname: Simmer
authors.forename: Yuanyuan
authors.initials: Y
authors.lastname: Hu
authors.forename: Amelia S
authors.initials: AS
authors.lastname: Richardson
authors.forename: John D
authors.initials: JD
authors.lastname: Bartlett
authors.forename: Jan C-C
authors.initials: JC
authors.lastname: Hu
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism; D003743:Dental Enamel / Q000378:metabolism / Q000473:pathology* / Q000648:ultrastructure; D003804:Dentin / Q000378:metabolism*; D007150:Immunohistochemistry; D007180:Incisor / Q000378:metabolism / Q000473:pathology / Q000648:ultrastructure; D007610:Kallikreins / Q000172:deficiency* / Q000378:metabolism; D008334:Mandible / Q000378:metabolism / Q000473:pathology / Q000648:ultrastructure; D051379:Mice; D018345:Mice, Knockout; D001616:beta-Galactosidase / Q000378:metabolism
chemical_list: D001616:beta-Galactosidase; D007610:Kallikreins; C120801:kallikrein 4