Human amelogenesis I: High resolution electron microscopy study of ribbon-like crystals

Abstract

Ribbon-like crystals, from developing enamel of human fetuses, were studied by high resolution electron microscopy. These crystals were classically described as the first organized mineral formed during amelogenesis. They were characterized by a mean width-to-thickness ratio (W.T-1) of 9.5, and 40% were bent. On lattice images we noted the presence of the central dark line (CDL) associated with white spots. Both structures were found in crystals with a minimum thickness of 8-10 nm. CDLs were localized in the center of the crystals and seemed to be linked to the initial growth process, but their exact structure and function were not fully determined. We were able to study the structure of the ribbon-like crystals with a Scherzer resolution close to 0.2 nm. The good correspondence between experimental and computed images showed that their structure was related to hydroxyapatite (HA). In addition, the presence of ionic substitutions and deficiencies were also compatible with HA. In this study, about 50% of the crystals showed structural defects. Screw dislocations were the most often noted defects and were observed within crystals aligned along five different zone axes. Low- and high-angle boundaries were also detected. Low-angle boundaries, found in the center of the crystals, could thus be related to CDLs and be implicated in the nucleation step of crystal formation, whereas high-angle boundaries could result from the fusion of ribbon-like crystals. Such mechanisms could induce an acceleration of the growth in thickness of the crystal observed during the maturation stage of amelogenesis.

First Experimental Evidence for Human Dentine Crystal Formation Involving Conversion of Octacalcium Phosphate to Hydroxyapatite

“…This mechanism suggests that the ®rst step is the formation of a two-dimensional nucleus of OCP, one unit-cell thick, which subsequently grows to HA (Brown, 1965;Nelson & McLean, 1984). The thin plate-like morphology of OCP crystals is consistent with the ribbon-like structure of the initial enamel crystal Cuisinier, Steuer et al, 1992;Weiss et al, 1981) and with the plate-like structures of bone (Cuisinier et al, 1987) and dentine crystals (Schroeder & Frank, 1985). The fact that OCP hydrolysis forms carbonated HA strengthens this hypothesis.…”

“…Nanometer-sized particles were observed in human enamel Cuisinier, Steuer et al, 1992;Cuisinier et al, 1993), dentine (Houlle  et al, 1997) and chicken bone (Cuisinier et al, 1995). A four-step mechanism was proposed to describe the conversion of the nanometer-sized HA crystals to ®nal adult HA crystals (Cuisinier et al, 1995).…”

Enamelysin (Matrix Metalloproteinase 20)-deficient Mice Display an Amelogenesis Imperfecta Phenotype

“…Enamel development (amelogenesis) consists of several stages that include the secretory, transition, and maturation stages. During the secretory stage, enamel crystallites elongate into long thin ribbons that are only a few apatitic unit cells in thickness (about 10 nm) with a width of ϳ30 nm (4,5). The ribbons are evenly spaced, are oriented parallel to each other, and grow in length but very little in width and thickness.…”

Effect of Experimental Fluorosis on the Surface Topography of Developing Enamel Crystals

“…The decrease in surface roughness with increasing developmental age seen in control crystals suggests that incremental crystal growth during normal enamel development is accompanied by a decrease in surface features such as steps and grooves. These features are in general less than 5 nm in height, which represents only few unit cells for hydroxyapatite [Kay et al, 1964] and may correspond to the lattice defects detected using high resolution electron microscopy (HR-TEM) [Cuisinier et al, 1992;Shibahara et al, 1994] and high resolution secondary electron I scanning microscopy (HR-SEM) [Apkarian et al, 1990]. It is possible that such irregularities reflect discontinuities and areas of strain in the crystal which may represent growth sites.…”