Abstract
The nanometer-scale bending structure in the enamel–dentin interfacial region of bovine teeth was clarified using detailed electron micrographs. We found that enamel prisms of width ∼5–10 μm in the enamel are regarded as a bundle of nanometer-scale hydroxyapatite (HA) fibers of width ∼20–30 nm that are connected to the enamel–dentin interface. The radial arrangements of HA fibers elongated in the c-axis were formed on the dentin surface at the boundary. The orientation adjustment of the HA fibers was observed through stepwise bending with a gradual change of the c direction in the ordered parallel array of the enamel prisms. Our observation suggests that the crystallographic ordering in the enamel layer originates from the growth-induced orientation of HA crystals in the interfacial region.
Highlights
Mammalian teeth are covered with hard layers called enamel that are mainly composed of hydroxyapatite (HA, Ca10(PO4)6(OH)2).[1]
The enamel is basically comprised of three layers: an outer enamel surface (OES), Hunter−Schreger bands (HSBs), and the enamel−dentin junction (EDJ) (Figure S1 in the Supporting Information).[11−14] Whereas HA crystals are randomly arranged in the dentin,[11] HA fibers are highly oriented in the OES and HSBs of enamel
The bilayer prisms were observed near the enamel−dentin boundary of a bovine incisor, detailed crystallographic structures have not been characterized in a previous work.[24]
Summary
Mammalian teeth are covered with hard layers called enamel that are mainly composed of hydroxyapatite (HA, Ca10(PO4)6(OH)2).[1] Parallel arrangement of enamel prisms of width ∼3−5 μm is observed in the enamel layers.[2] The prisms elongated in the c direction are regarded as a bundle of nanometer-scale HA fibers or rods of width ∼30−40 nm.[2−4] The specific mechanical properties of the enamel are ascribed to their hierarchical architecture.[5,6] Dentin as a base of the enamel layer is composed of 70 wt % HA, 20 wt % collagen, and 10 wt % water.[7] Its elastic nature supports the outermost hard layer of the teeth.[8−10]. Ordering mechanism of parallel arrangements of nanometerscale fibers observed in biological mineralization systems
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