Name: Ni Ketut Dewi Aprilianti
Nim: 2470121167
Bone Remodeling
Bone remodeling is a process that is repeated and occurs continuously. This process is a
replacement of old bone tissue into new bone to maintain bone strength and structure. Old
tissue will be destroyed by osteoclasts and rebuilt by osteoblasts. Bone remodeling can help
repair microscopic damage to bones, adjust the shape and structure of bones, and maintain
calcium and phosphate levels in the blood through bone resorption and mineralization.
The bone remodeling process requires the main cells that play a role. Cells that play a
role in bone remodeling are osteocytes, osteoblasts, osteoclasts, and macrophages. Osteocytes
are bone cells that are in the matrix and function as mechanical sensors and the main
regulators of bone remodeling. Osteoclasts are multinucleate cells that function in bone
resorption. Macrophages are cells that will clean up bone debris after being destroyed by
osteoclasts. Then there are osteoblasts that will later form new bones.
There are several phases in bone remodeling. This phase is divided into five and will
occur sequentially. The first phase is activation, in this phase osteocytes will send signals in
the form of molecules such as RANKL and reduce OPG levels. This will recruit osteoclast
progenitors from the hematopoietic line so that it will trigger the change of progenitors into
mature osteoclasts. When osteoclasts are mature, they will begin to enter the resorption
phase. In this phase osteoclasts will attach to the bone surface and will form a ruffled border
then release proteolytic enzymes and acids to dissolve minerals and organic matrix in the
bone. Bone debris will be cleaned by macrophages, this phase is called reversal. In this phase
there will also be the release of growth cells such as TGF-β and IGF-1 which will recruit
osteoblasts. Osteoblasts are cells derived from mesenchymal progenitor cells that will
become osteoblast cells and begin to produce osteoid (organic matrix) which is composed of
type-1 collagen and other bone proteins. Osteoblasts after the formation phase can be trapped
in the matrix and become osteocytes, settle on the bone surface and become bone lining cells,
and apoptosis. After this phase is complete, the mineralization phase will occur. The osteoid
that has been formed will experience gradual crystal deposition to strengthen the new bone
structure. After completion, it will enter the quiescence phase or resting phase. Bone
remodeling will begin if the osteocytes emit signals again.
Bone remodeling is an important physiological process in the body to maintain bones.
Each phase of bone remodeling has its own role to ensure that old bones are replaced by
healthy new bones. In this case there is also the influence of hormonal regulation such as
parathyroid hormone which can increase resorption, vitamin D which can stimulate
osteoclasts, and other hormones. In bone remodeling there are also clinical aspects such as
Paget's disease which occurs due to irregular and excessive remodeling and osteoporosis
which occurs due to an imbalance in formation and resorption So bone remodeling success of
bone remodeling depends on the balance between destruction and formation of the bone, as
well as hormone regulation and other body factors.
Reference:
Chen, Y., et al. 2021. SIRT1, a Promising Regulator of Bone Homeostasis. Life sciences, 269.
p. 119041
Guyton, A.C. & Hall. 2021. Guyton and Hall Textbook of Medical Physiology. 14th ed.
Philadelphia: Elseiver
Weivoda, M. M., & Bradley, E. W. (2023). Macrophages and bone remodeling. Journal of
Bone and Mineral Research, 38(3), 359-369.
