Scanning Electron Microscopy of Female Heterozygous Amelx Mice

Amelogenesis imperfecta (AI) is a group of inherited conditions featuring isolated enamel malformations. About 5-10% of AI cases show an X-linked pattern of inheritance and are caused by defects in AMELX, the amelogenin gene on the X-chromosome (Xp22.3). Enamel malformations are more severe in males (X*Y) than in heterozygous females (X*X). In some females, the enamel layer shows a characteristic pattern of vertical bands that are thought to be caused by Lyonization, that is, represent alternating bands of hypoplastic and normal enamel formed by cohorts of ameloblasts that have inactivated either the X or the X* chromosome respectively. Objective: To characterize the enamel phenotype in heterozygous female (Amelx^+/-) mice. Methods: The amelogenin null mice obtained from Jackson Laboratory were previously described by Gibson et al. (2001) J Biol Chem 276:31871-5. The Amelx null mouse was originally generated in 129SvJ mice and maintained in a C56Bl/6 X 129/Sv background. We continued to backcross the Jackson Lab mice with C56Bl/6, as their offspring lacking the Amelx null gene (wild-type) appeared to have defective enamel. After breeding with C56Bl/6 mice for three generations, mandibular incisors from perfused 9-week-old mice were carefully extracted and characterized by scanning electron microscopy (SEM). Age-matched mandibular incisors were fractured at the level of the alveolar crest, polished or not polished, coated and scanned. Results: The incisor enamel surface varied among individuals, but sometimes showed dramatic vertical bands of thicker enamel running parallel to the long axis of the incisor. Conclusion: As the decussating pattern of enamel rods clearly show that alternating rows of ameloblasts migrate in opposite directions, the linear vertical thickening of enamel in female heterozygous (X*X) mice cannot be explained by Lyonization. Histological investigations are underway to determine how the vertical bands form. This study was supported by NIDCR grant DE019775.