What is the role of osteoclasts in the pathogenesis of osteoporosis?

Updated: Jan 20, 2021
  • Author: Rachel Elizabeth Whitaker Elam, MD, MSc; Chief Editor: Herbert S Diamond, MD  more...
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Answer

Osteoclasts, derived from hematopoietic precursors, are responsible for bone resorption, whereas osteoblasts, from mesenchymal cells, are responsible for bone formation (see the images below). The 2 types of cells are dependent on each other for production and linked in the process of bone remodeling.

Osteoblasts not only secrete and mineralize osteoid but also appear to control the bone resorption carried out by osteoclasts. Osteocytes, which are terminally differentiated osteoblasts embedded in mineralized bone, direct the timing and location of bone remodeling. In osteoporosis, the coupling mechanism between osteoclasts and osteoblasts is thought to be unable to keep up with the constant microtrauma to trabecular bone. Osteoclasts require weeks to resorb bone, whereas osteoblasts need months to produce new bone and on average bone formation takes 4 to 6 months to be completed. Therefore, any process that increases the rate of bone remodeling results in net bone loss over time. [13]

Bone remodeling has 4 sequential phases: activation precedes resorption, which precedes reversal, which precedes the formation of a new osteon and the process is the same in both cortical spongy trabecular or cancellous bone.  [19]  'Activation' phase defined as the conversion of bone surface from quiescence to active, which is followed by the differentiation of osteoclast precursors into mature osteoclasts in the ‘resorption’ phase. In the ‘reversal’ phase, osteoclasts complete the resorption process and produce signals that directly or indirectly initiate bone formation, and in the final ‘formation’ phase, mesenchymal cells differentiate into functional osteoblasts to make the bone matrix.

This image depicts bone remodeling with osteoclasts resorbing one side of a bony trabecula and osteoblasts depositing new bone on the other side.

This image depicts bone remodeling with osteoclast This image depicts bone remodeling with osteoclasts resorbing one side of a bony trabecula and osteoblasts depositing new bone on the other side.

Osteoclast, with bone below it. This image shows typical distinguishing characteristics of an osteoclast: a large cell with multiple nuclei and a "foamy" cytosol.

Osteoclast, with bone below it. This image shows t Osteoclast, with bone below it. This image shows typical distinguishing characteristics of an osteoclast: a large cell with multiple nuclei and a "foamy" cytosol.

In this image, several osteoblasts display a prominent Golgi apparatus and are actively synthesizing osteoid. Two osteocytes can also be seen.

In this image, several osteoblasts display a promi In this image, several osteoblasts display a prominent Golgi apparatus and are actively synthesizing osteoid. Two osteocytes can also be seen.

 


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