Identification of important interactions between subchondral bone ...
Identification of important interactions between subchondral bone ...
Identification of important interactions between subchondral bone ...
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CHAPTER 2: Introduction<br />
follows normal development or healing. Lamellar <strong>bone</strong> is characterized by organized fibres,<br />
which are forming arches for optimal <strong>bone</strong> strength. The lamellae can be parallel to each other if<br />
deposited along a flat surface (trabecular <strong>bone</strong>) or concentric if deposited on a surface<br />
surrounding a channel centred on a blood vessel 17,22,23 .<br />
2.2.3 The cells <strong>of</strong> <strong>bone</strong> and their communications<br />
19<br />
Fig. 4. Lamellar and woven <strong>bone</strong>.<br />
Histological cut from fibula <strong>of</strong> rabbit<br />
showing details <strong>of</strong> lamellar <strong>bone</strong><br />
concentrically organized and woven<br />
<strong>bone</strong> mixed with cartilage and calcified<br />
cartilage tissues 24 .<br />
Bone contains several types <strong>of</strong> cells, which are <strong>important</strong> during development, repair, and<br />
maintenance. During <strong>bone</strong> development, the woven <strong>bone</strong> is modelled by resorption <strong>of</strong> calcified<br />
cartilage by osteoclasts and formation <strong>of</strong> <strong>bone</strong> by osteoblasts, independently <strong>of</strong> each other (not<br />
directly coupled) 16,25,26 . This is different in adult <strong>bone</strong>, where the tissue is constantly being<br />
remodelled by a communication/coupling <strong>between</strong> osteoclasts and osteoblasts, which enable<br />
<strong>bone</strong> to regenerate old or damaged tissue itself, adapt to changing stress environments, and<br />
control calcium homeostasis.<br />
Activation <strong>of</strong> the <strong>bone</strong> remodelling cycle is believed to be initiated by the<br />
osteocytes 15,21,27,28 , which comprises the most abundant cell type in <strong>bone</strong>. Osteocytes are cells<br />
trapped in the <strong>bone</strong> matrix. They rest in fluid-filled lacunae and communicate with surrounding<br />
cells via extensions called canaliculi 23,29 . When <strong>bone</strong> ages, the tissue eventually suffers<br />
microdamage and the surrounding osteocytes undergo apoptosis, which leads to signalling to the<br />
nearest <strong>bone</strong> lining cells (fig. 5A). This results in increased expression <strong>of</strong> receptor activator <strong>of</strong><br />
nuclear factor kappa-B ligand (RANKL), which attracts osteoclast precursors to the damaged<br />
area and initiates osteoclastogenesis. Osteoclasts derived from hematopoietic stem cells that are<br />
differentiated into monocytes and subsequently pre-osteoclasts, which then fuse into large<br />
multinucleated osteoclasts. The two cytokines, macrophage colony stimulating factor (M-CSF)<br />
and RANKL, are produced by the cells <strong>of</strong> the osteoblast lineage, and are very <strong>important</strong> for the<br />
differentiation <strong>of</strong> the osteoclasts (fig. 5A).