Identification of important interactions between subchondral bone ...

CHAPTER 2: Introduction
20
Fig. 5. The remodelling cycle on trabecular
bone. A) A microcrack or old bone causes
osteocytic apoptosis, which induces RANKL
expression by bone lining cells. This attracts
monocytes to the damaged bone area, and
osteoclastogenesis starts. The osteoclasts will
start to resorb bone, which is followed by
formation of new bone matrix by the osteoblasts.
Osteoblasts that are trapped in the remodelled
matrix become osteocytes, whereas the rest
either die or become flattened osteoblast lining
cells. B) The resorbing osteoclast seals tightly to
the bone surface to create a closed local
environment known as the resorption lacuna.
This is where the bone resorption takes place, by
lowering the pH (secretion of H + ) and by
releasing proteases by exocytosis. The figures
were produced by Madsen, S.H.
Active osteoclasts can be characterized by their ruffled cell borders, which seal and degrade the
underlying bone in the resorption pit, through the release of hydrogen ions (lowering of pH) and
proteases (such as cathepsin K and matrix metalloproteases). The resorbed bone tissue is
transported across the osteoclast in small vesicles and is released into the extracellular fluid (fig.
5B.) 21 .