Root-analogue versus crown-analogue dentin: a radioautographic and ultrastructural investigation of the mouse incisor.

The present paper reports on differences between the root- and crown-analogue dentin portions of the continuously growing mouse incisor. Conventional light microscopy and radioautography were used to study dentin formation and the uptake of [3H]-proline and [3H]-serine. It was found that, although the dentin apposition rate along the crown-analogue part (covered by enamel) equalled or slightly exceeded that along the root-analogue part (covered by cementum), the processing of predentin into dentin was considerably faster in the root aspect. Comparison of the two dentin portions at the ultrastructural level revealed that differences occurred in the morphology of the secretory granules of the odontoblast layer. Two types of granules were observed: those that were and those that were not loaded with electron-dense particles of 30 nm diameter. While the former type was most frequent along the crown-analogue aspect of the incisor, the latter type was particularly found along its root-analogue aspect. This difference may reflect differences between the two dentin portions in the composition of the noncollagenous matrix.