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U N I T 1 2
Musculoskeletal Function
Bone remodeling constitutes a process of skeletal maintenance once skeletal growth
is complete. It takes place in the (1) osteons of mature bone and consists of a cycle
of (2) bone resorption by osteoclasts, (3) followed by bone formation by osteoblasts.
Bone remodeling is (4) controlled by cytokines and growth factors that interact with
a paracrine system consisting of the RANK ligand (RANKL), the RANK receptor, and
osteoprotegerin.
U N D E R S T A N D I N G
Bone Remodeling
Bone Remodeling Cycle.
Mature bone is made up of osteons
or units of concentric lamellae (bone
layers) and the haversian canal they
surround. Bone remodeling consists
of a sequence of bone resorption
within an osteon by osteoclasts, fol-
lowed by new bone formation by
osteoblasts. In the adult, the length
of one sequence (i.e., bone resorption
and formation) is approximately 4
months. Ideally, the replaced bone
should equal the resorbed bone. If it
does not, there is net loss of bone.
In the elderly, for example, bone
resorption and formation no lon-
ger are perfectly coupled, and bone
mass is lost.
1
Concentric
lamella
Osteon
Osteocytes
Vessel of
haversian
canal
Bone Resorption.
The osteo-
clasts, which are bone-resorbing cells
derived from monocyte/macrophage
precursors, are the cells involved in
the initiation of bone remodeling.
The sequence of bone resorption
and bone formation is activated by
many stimuli, including the action
of parathyroid hormone and cal-
citonin. It begins with osteoclastic
resorption of existing bone, during
which the organic (protein matrix)
and inorganic (mineral) components
are removed, creating a tunnel-like
space in the osteon. Soluble factors
released during resorption aid in
the recruitment of osteoblasts to the
site, thereby linking bone resorption
to bone formation.
2
Cutting
cone
Osteoclasts
