Fig. 8.

Contemporary understanding of the hierarchical structure of bone. According to the recent inventory, there are approximately twelve levels: 1) skeletons, made of 2) bones, made of 3) cortical and trabecular tissue, made of 4) cortical osteons, fibrolamellar bone packets, and trabecular lamellar packets, all of which contain 5) lamellar bone, made of 6) lamellae, made of 7) ordered collagen motifs that form 8) bundles, surrounded by the disordered collagen motif. The bundles are made of 9) collagen fibrils, made of 10) triple helices, made of 11) alpha-helices, made of 12) amino acids. The mineral organization in 3D shows its own hierarchical organization starting at level 8) of mineralized collagen bundles, that contain 9) tessellated prolate ellipsoids of mineral, made of 10) mineral platelets, made of 11) laterally merging acicular crystals, made of 12) unit cells. Because the cascade of hierarchical levels splits at the micrometer level for organic and inorganic matter, and because same-level mineral and collagen units have different shapes and even scale (for example, the tesselles, and the collagen fibrils, which are both level 9), and also for visual flow and continuity between levels, we intentionally did not number the levels in the figure.