Provisional mineralization layer in the predentin of murine teeth

Background: Mineralization of circumpulpal dentin has been shown to progress at the mineralization front, where predentin matrix is abruptly converted to almost fully mineralized dentin. Goldberg et al. (1996) claimed existence of lightly mineralized layer in predentin immediately adjacent to the mineralization front, and suggested that dentin mineralization is a rather transient process. These authors described the lightly mineralized layer as “metadentin” and that metadentin is recognizable in anhydrously processed specimens. Objectives: The purpose of the study is to clarify the rather illusive, transient predentin layer along the dentin mineralization front by light and electron microscopy, in an aim to gain more insights in the mechanism of dentin mineralization in rodent teeth, based on fine structural analyses.

Methods: Young and adult Wistar rats were fixed by conventional vascular perfusion with 4% paraformaldehyde (PFA) or a mixture of 2% glutaraldehyde and 3% PFA in 0.1M cacodylate buffer (pH 7.4). Lower incisors were dissected, immersed in the same fixative overnight and embedded in EPON 812 with or without OsO4 post-fixation. Semi-thin sections were stained with toluidine blue and some were processed for von Kossa staining to test mineral deposition in predentin layers. Ultrathin sections were cut with ethylene-glycol in trough to prevent mineral loss from the specimen.

Results: Light microscopy confirmed lightly metachromatic and weakly von Kossa-positive layer in the predentin of crown-analogue portion of incisors, and its absence in the root-analogue. Thickness of the transient layer varied from none to 3 micrometer in the crown-analogue of the individual teeth. Electronmicroscopy confirmed mineral deposition in von Kossa positive portions. Specialized predentin layer was not confirmed in the rat molars.

Conclusions: These results indicate region-specific as well as tooth-type-specific diversity in the process of appositional mineralization of circumpulpal dentin in rat teeth.