Electron Microscopy Analysis of the Central Dark Line Defect of the Human Tooth Enamel
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Electron Microscopy Analysis of the Central Dark Line Defect of the Human Tooth Enamel A. G. Rodríguez-Hernández1, M.E. Fernández2, G. Carbajal-De-La-Torre3, R. García-García3 and J. Reyes-Gasga3 1
Facultad de Odontología, UNAM. Circuito de la Investigación s/n, Ciudad Universitaria, Coyoacán 04510, México D.F., MEXICO. 2 Instituto Nacional de Investigaciones Nucleares, ININ. Km. 36.5 Carretera México-Toluca Ocoyoacac Edo. de México 52045. MÉXICO. 3 Instituto de Física, UNAM. Apartado Postal 20-364, 01000 México, D.F., MÉXICO. E-mail: jreyes @fisica.unam.mx
ABSTRACT After some experimental results that indicated that HA is able to growth in an epitaxial way on the surface of OCP, it has been suggested that the central dark line (CDL) observed in the nanometric-sized grains of human tooth enamel corresponds to a one-unit-cell-thick layer of octacalcium phosphate (OCP). Based on this consideration, in this work we propose a model for CDL and we carried out the chemistry and structural analysis of the CDL with high resolution microscopy techniques such as Electron Energy Loss Spectroscopy (EELS) and Z-contrast (HAADF) with the aim of find the agreements and/or differences between the human tooth enamel HA and its CDL.
INTRODUCTION Human tooth enamel structure is formed by prisms in the size range of tenths of microns, which run from the enamel-dentin junction to the enamel surface. These prisms are formed by many elongated-plate-like crystallites which have a diameter from 50 to 100 nm in transversal section, and from 300 to 500 nm in longitudinal section, approximately. When the enamel crystallites are observed with the transmission electron microscope (TEM), they exhibit a line of 1 to 1.5 nm wide along their centers [1-4]. This line has been named “dark line”, although certainly its contrast is focusing dependent: it appears dark in underfocus, disappears when the image goes through focus, and is white in overfocus [1]. The dependence of its contrast with the focus value suggests that the term “dark” is not correct: the out-of-focus contrast resembles more the Fresnel fringes behavior in a phase-contrast image [5]. Evermore, the observance of this “line” both in plan view (transversal) and cross-sectioned (longitudinal) enamel samples indicates that it corresponds to a plane and not to a line. However, “central dark line” (CDL) is the name historically used to identify it. The analysis of the occurrence of this “line” in the human tooth enamel crystallites is of particular interest because it represents a zone that undergo into preferential dissolution during early stages of the carious process [6,7]. Therefore the knowledge of its structure, properties and
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chemical composition is very important to understand the role that it plays in the enamel structure and, maybe, to combat and prevent the carious process. Based on some experimental results on the epitaxial growth of HA on OCP surfaces [8] and the reported suggestions that CDL corresponds to a HA-OCP interface, in this work we comment a pos
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