Fig. 4.

Hierarchical Level 6 (ordered and disordered repeats in the lamellar structure), and Level 7 (packing of co-aligned collagen fibrils into quasi-cylindrical bundles). These images were acquired using the FIB-SEM slice-and-view method on demineralized and stained lamellar bone. (A) Grey-scale 3D volume in which collagen fibrils appear dark grey, interfibrillar ground matter appears light grey, the lamina limitans lining canaliculi is white, and osteocyte processes are also white. (B) The same canaliculi as in A, surface-rendered and shaded to illustrate their meandering course across the ordered bundles, and their screw-shaped axial twist. (C) Segmentation of the ordered arrays of collagen, 3D rendered and superimposed on a semi-transparent grey-scale volume (same sample and same orientation as in A). Note the braided and gently twisted appearance of splitting and merging bundles of collagen fibrils. (D) Disordered collagen fibrils alternate with the ordered bundles and house the cellular processes. Here, the disordered phase has been segmented using deep learning-aided segmentation, and its content is about 30% with respect to the total volume of the extracellular matrix (original nonquantitative work by Reznikov et al. (Reznikov et al., 2013) underestimated the proportion of the disordered array by volume).