Describe the process of bone remodeling. Why does this process exist?
Bone remodeling, in brief, is the process by which osteoclasts eat old bone and stimulate osteoblasts to make new bone.
The activity of osteoblasts is easy to comprehend: make bone where needed. Osteoclasts are bit trickier: why resorb bone?
The process of resorption exists for two reasons:
- first, to liberate calcium and other ions; and
- second, to clear out worn out pieces of the skeleton and promote the deposition of newer, better material.
Osteoclastic resorption occurs by secretion of acid and proteolytic enzymes which digest the bone matrix; Ca2+ and PO43- are then taken up by the osteoclasts and released into the circulation.
Bone formation occurs by osteoblasts secreting an organic matrix (osteoid) and then mineralizing the matrix.
- When the remodeling process is skewed such that, over time, there is more eating than replenishing, you get osteoporosis.
- When the remodeling process is aborted, say in avascular necrosis,
bad bone accumulates. (Seen here as density). This can lead to collapse and failure of the subchondral ("under the cartilage") bone thus can lead to arthritis.
- When the remodeling process just can't keep up with (new) mechanical demands, like over-exercising, you get a stress fracture
as shown on the bone scan to the right (xrays do not detect this)
- When you get a long bone fracture, bone remodeling kicks in to literally remodel the callus and lay down new bone (not scar). This is the final step of the fracture healing cascade, shown to the lower left
- And to be sure, when the bone needs to liberate calcium and other ions, it employs osteoclasts and invokes the process of bone remodeling; as such the invocation of the bone remodeling program is a key feature of metabolic bone disease (such as hyperparathyroidism).
Bone resorption occurs from osteoclastic breakdown of trabecular bone via the secretion of hydrolytic enzymes. This process occurs throughout life and is tightly regulated by several factors: serum vitamin D, serum calcium, growth hormone, PTH (increase resorption), and calcitonin (increase bone formation) levels, to name a few.
Two things to recall:
- you cannot "de-mineralize" the bone as you would in this party trick. . You have to "de-bone" the bone, as Dr Fred Kaplan termed it: you cannot simply dissolve out some ions when you need them and replenish the bone later; you must break down the matrix to get the mineral out. Thus, even if the body needs calcium 'only for a minute', it takes a while get the skeleton restored. Think of it as having to get a home equity load if you wanted to borrow even a small amount; it's a much bigger hassle than a credit card overdraft! Implication: if you have a state of high mineral flux, there will be lots of immature bone, as every instance of mineral withdrawal necessitated some 'de-boning'.
- Metabolic needs trump skeletal needs. This makes sense: calcium is needed for cardiac contractility and nerve transmission. Thus, in metabolic diseases or states of nutritional deficiency, the skeletal system can be harmed.