Electron microscopy of matrix formation and calcification in rat enamel

Abstract

Enamel from incisors and molars of 4-day, 10-day and 1-year old rats has been cut with a diamond knife, as non-decalcified ultra-thin sections, and examined by electron microscopy and diffraction, with the following findings: 1. (1) Enamel matrix elaboration starts in the ameloblastic cytoplasm, next to the amelo-dentinal junction membrane. Fibrogenesis occurs in two steps: first by an elaboration of diffusely scattered fibrillar protein bundles, embedded in ground substance; secondly, by the appearance of elongated ovoid or frequently cord-like structures, which by coalescence form the enamel matrix. When completed, the fibrillar matrix assumes a three-dimensional reticular appearance. 2. (2) The apatite crystal growth starts selectively within and on the longitudinal fibrils of the reticulum, in the form of small crystallization centres, which later fuse together, giving rise to strips of calcifying fibrils. Grouping of adjacent calcifying fibrils is followed by partial and then complete fusion, ultimately forming adult apatite crystals, which contain in their core calcified protein fibrils. These crystals take the shape of elongated hexagonal prisms. 3. (3) In the rat incisor, matrix differentiation and calcification of enamel proceeds progressively from the enamel-dentine junction toward the enamel surface. Three distinct layers can be recognized: inner, middle and outer enamel. The narrow inner and outer enamel contain aggregates of parallel oriented apatite crystals, without any distinct rod structure, whereas the wider middle enamel exhibits a complicated rod interrelationship characterized by alternating layers of rods running in different directions. 4. (4) In adult rat teeth, a difference in apatite crystallization has been noted both in incisor and molar enamel which is characterized by zones of well-formed apatite crystals intermixed with zones of unfused calcified fibrils.