Correlation of ameloblastin with enamel mineral content

In enamel formation, the deposition of minerals as crystallites starts when the mineralization front first forms at the start of the secretory stage. During maturation, the enamel layer accumulates significant amounts of new mineral as the crystallites grow in volume. Inversely related to mineral gain is loss of protein and water from the forming enamel. Both ameloblastin (Ambn) and enamelin are essential components for formation of a functional enamel layer. The aim of this study was to quantify the proportion of mineral and non-mineral material present in developing enamel relative to Ambn concentration using Ambn mutant mice mated with others overexpressing full-length Ambn from the mouse amelogenin promoter at lower (+), similar (++) or higher (+++) concentration than normal. Mandibular incisors (age: 7 weeks, n = 8) were imaged by micro-computed tomography and the enamel was analyzed from the apical region to the incisal edge in sequential 1.0 mm volumes of interest. Mineral density was determined using a series of hydroxyapatite (HA) phantoms to calibrate enamel density measurements. At the site where the mandibular incisor emerged into the oral cavity, the enamel volume, mineral weight, and mineral density were reduced when Tg Ambn was expressed at lower or higher levels than normal. While in wild-type the % mineral was >95%, it was negligible in Ambn-/-, 22.3% in Ambn-/-, Tg(+), 75.4% in Ambn-/-, Tg(++), and 45.2% in Ambn-/-, Tg(+++). These results document that the deposition of mineral and removal of non-mineral components are both very sensitive to expressed Ambn concentrations.