In the previous post, I mentioned about bone remodeling. Now let’s go deeper into how bone tissue is resorbed and formed in this process.
How bone remodels?
Bone remodeling is a lifelong process to replace and reshape bone, that involves coupled activity of osteoclasts and osteoblasts to resorb and form bone at the simultaneous site. Basically a healthy bone remodeling consists of 5 phases: Activation, Resorption, Reversal, Formation and Quiescence.
For a clearer illustration, I’ve referred to some resources and drawn some figures to represent the process.
In Activation phase, mononuclear pre-osteoclasts are attached to the bone surface and fuse to form large (up to 100µm) and multinucleated (up to 50) osteoclasts. The membrane attached to the bone differentiates as ruffled membrane for further action.
During Resorption phase, the osteoclasts acidify the attached interface and slowly dissolve the bone matrix, result in deep cavities (cutting cones). The calcium is then released to blood for various body functions. Osteoclasts diminish when the resorption is complete.
During Reversal phase the resorbed surface is prepared for the subsequent Formation phase. A sugar-rich cement line is produced to help bonding old bone and new bone.
The Formation phase begins when the osteoprogenitor cells (precursors of osteoblasts) appear along the cavities. The cells undergo mitosis and proliferate (increase in large quantity). Eventually they differentiate into osteoblasts. Osteoblasts then deposit unmineralized bone matrix called osteoid at the cavities.
When osteoblasts are encapsulated by the matrix it synthetize, they become osteocytes and regulate the process of bone remodeling. Osteocytes are known as the most abundant cells found in the bone.
Calcification begins few days after osteoid is deposited: the process where osteoid becomes mineralized with calcium and phosphorus, thus forming the new bone.
The bone surface is restored and covered by the protective lining cells, and remains quiescent till the next cycle.
