Bone tissue structure
Bone is hierarchically organized. Zooming in the bone structure (Fig. 4a), we find the osteons or Haversian systems, which are the basic unit of structure of compact bone. Osteons consist of concentrically arranged layers, or lamellae, of compact bone tissue, that surround a central canal, the Haversian canal, where bone’s blood supplies and nerves can be found (Fig. 4b). Sequential concentric lamellae have alternate fiber orientations with each other, spiralling around the central canal [12]. These fundamental units are aligned on the direction in which force is applied to bone, and the concentric layers give bone tissue an extra resistance to breaking, as a crack cannot be easily spread from lamellae to lamellae.
Figure 4: a) Hierarchical structure of bone ranging from the macroscale skeleton to nanoscale collage and HA. Nair, A.K., et al., Molecular mechanics of mineralized collagen fibrils in bone. Nat Commun, 2013. 4: p. 1724. b) Structure of
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It is physiologically essential, as it maintains normal skeleton mass and repairs possible microdamages to the skeleton. The whole procedure occurs in discrete foci, called basic multicellular units (BMUs) (see Fig.6). It starts with a resorption phase which can last up to 4 weeks, where osteoclast (OC) precursors are recruited to the active site from precursors in bone marrow, and excavate a resorption cavity, limited by a border called cement line. Following this excavation, OCs die by apoptosis. After a brief reversal phase, osteoblast (OB) precursors come to the active site and fill the resorption cavity. They slowly start secreting osteoid which, after a long period, will end up mineralizing and forming new bone. After 4-6 months, the formation phase is completed and a brand new osteon is formed