Bending Fibers of Hydroxyapatite for Ordered Parallel Architecture in Bovine Tooth Enamel

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