Enamel maturation: a brief background with implications for some enamel dysplasias

Abstract

The maturation stage of enamel development begins once the final tissue thickness has been laid down. Maturation includes an initial transitional pre-stage during which morphology and function of the enamel organ cells change. When this is complete, maturation proper begins. Fully functional maturation stage cells are concerned with final proteolytic degradation and removal of secretory matrix components which are replaced by tissue fluid. Crystals, initiated during the secretory stage, then grow replacing the tissue fluid. Crystals grow in both width and thickness until crystals abut each other occupying most of the tissue volume i.e. full maturation. If this is not complete at eruption, a further post eruptive maturation can occur via mineral ions from the saliva. During maturation calcium and phosphate enter the tissue to facilitate crystal growth. Whether transport is entirely active or not is unclear. Ion transport is also not unidirectional and phosphate, for example, can diffuse out again especially during transition and early maturation. Fluoride and magnesium, selectively taken up at this stage can also diffuse both in an out of the tissue. Crystal growth can be compromised by excessive fluoride and by ingress of other exogenous molecules such as albumin and tetracycline. This may be exacerbated by the relatively long duration of this stage, 10 days or so in a rat incisor and up to several years in human teeth rendering this stage particularly vulnerable to ingress of foreign materials, incompletely mature enamel being the result.

Keywords: Porosity; enamel dysplasias; enamel maturation; matrix protein loss; uptake of foreign materials.

Figure 1

(A) Diagram of enamel organ at each stage of development. OEE; Outer Enamel Epithelium, SI; Stratum Intermedium, SR; Stellate Reticululm, Am; ameloblasts. T; Transition Stage, RA; Ruffle Ended ameloblasts, SA; Smooth Ended ameloblasts, P; papillary layer. With permission, modified from: Josephsen et al. (2010). Am. J. Physiol. Cell Physiol. 1299–1307. (B) Images of developing incisors from rat, human and cow, showing appearance (after drying) and position of secretory and maturation sages.

Figure 2

(A) Mineral content at each stage of development in developing teeth of sheep, cow, coypu, rat, human and pig. Steep increase in mineral content can be seen at the beginning of the maturation stage in each species. (B) Mineral content of developing teeth from pig, cow and rat in relation to duration of each stage. Maximum mineral content is achieved in about 2 weeks in the rat, 10 weeks in the cow and a minimum of about 13 weeks in the pig.

Figure 3

(A) Rat incisor showing secretory and white opaque maturation stages. In rat, enamel. (B) Electron micrographs of apatite crystals at secretory and maturation stages in the rat incisor at the same magnification (size bars = 30 nm). Much larger, growing crystals (~30 nm in width) can be seen in the maturation stage tissue. (C) Protein expressed as total amino acid content at each developmental stage of the rat incisor. Dramatic fall in protein content can be seen in the maturation stage. (D) Fluoride content at each stage of development of rat incisor enamel. Peak concentrations can be seen across the boundary between transition and maturation stages. The peak indicates some loss of free fluoride during the maturation stage.