RANK-L,binds RANK-L, and leads to inhibition of osteoclastic activity.
Those molecules create the link between osteoblasts and osteoclasts known
as theARF (activation-resorption-formation) cycle in which activation of
osteoclasts is inextricably linked to osteoblasts. This has been one of the
problems in treating osteoporosisin which osteoclastic activity dominates
osteoblastic activity. Growth factors such as transforming growth factor-
beta (TGF-β) and insulin-like growth factors also play a role in differentia-
tion of osteoblasts and osteoclasts. TGF-βis found in an inactive form in
the bone matrix and is activated by acid produced by osteoclasts. TGF-β
then inhibits osteoclast differentiation and stimulates osteoblastic activity.
High-Yield Facts 21Calcitoninopposes the actions of PTH, but plays a lesser role overall.
Bone is highly vascular and mineralized withhydroxyapatite. In con-
trast, the three types of cartilage are avascular and contain chondrocytes that
GF=growth factors, OPG =osteoprotegerin, PR =parathyroid hormone receptor,
PTH=parathyroid hormone, RANK =receptor for activation of nuclear factor
kappa B, RANK-L =ligand for RANK, M-CSF =macrophage colony-stimulating factor,
M-CSF-R=MCSF receptor.