Scanning electron microscopy of final enamel formation in rat mandibular incisors following single injections of 1-hydroxyethylidene-1,1-bisphosphonate

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Abstract
Summary A single, high dose of 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) results in three different types of lesions along the enamel surface of the rat incisor, one of which is seen as a “bright band” crossing the final enamel surface in the scanning electron microscope (SEM). The present study presents the structural surface features of final enamel formation and its subsequent maturation in normal and HEBP-exposed rats. The position of the bright band is examined in relation to where the Tomes processes pits disappear (DTPP), where the boundary between “light” and “dark” enamel (LDB) as seen by SEM is located, and in particular, where the socalled opaque boundary (OB) is positioned. Groups of rats were given a subcutaneous dose of 0, 5, or 10 mg P/kg body wt of HEBP and killed at intervals of either 12 hours or 2 or 9 days. The mandibular incisors were processed for SEM after enzymatic digestion of enamel organ remains. Enamel surface nodules, 100–300 nm in diameter and composed of smaller units, were evident at the start of final enamel formation which was defined as the area from DTPP to LDB. With increasing maturation, the nodules merged to form a smooth surface. In HEBP-treated animals, growth and merging of these surface nodules became arrested at the time of injection resulting in an irreversible “porous” stage corresponding to that part of the surface enamel. This area—the bright band—developed corresponding to the start of the area of final enamel formation and was subsequently carried incisally during the eruption of the incisor. Adjacent surface areas appeared normal, with densely packed nodules in the maturation stage. The bright band did not coincide with either LDB or OB at the time of injection as these landmarks were located about 105 and 2000 μm incisally to DTTP, respectively. The width of the bright band was dose dependent (78 μm versus 105 μm at 5 and 10 mg P/kg body wt) which may indicate that HEBP carries on its effect in a few hours after administration. The findings are most likely a result of HEBP's physicochemical effect directly on crystal growth, although a cellular effect cannot be excluded.