Orthogonal design and dual-mode construction of oriented bundles of fluorapatite from organic fibrils in shark enameloid

-axis-oriented fluorapatite (FA) fibers were studied to clarify the hierarchical architecture in the tooth enameloid of sharks in the Lamnidae family. Here, we focused on the orthogonal configuration of radial bundles (RBs) and parallel bundles (PBs) in the outer layer. From multidirectional characterization of mature and immature teeth, RBs and PBs were found to be constructed through different modes from collagen and non-collagenous organic fibrils, respectively. In the previous stage of calcification (stage 0), collagen bundles are produced as platy bands bridging from the dentin to the tooth surface, and then the non-collagenous fibrils are arranged perpendicularly to the bundles. In stage 1, the FA fibers of PBs are formed with the gradual transformation of non-collagenous fibrils with the infiltration of calcium and phosphate ions (Mode NC). In stage 2, RBs calcify through the crystal growth of FA fibers along with collagen fibrils that disappear gradually (Mode C). In stage 3, the orthogonal architecture matures through the widening of the FA fibers of RBs and decreasing organic matter. The FA bundles in other parts of the enameloid were deduced to be constructed from non-collagenous fibrils through Mode NC. Dual-mode construction is essential for the structural design of the sophisticated architecture consisting of crystal bundles with controlled directions.