Per Request: How do bones regenerate?

I got a request to write a post on how bones regrow or grow for the first time, so here goes:

There are 3 different types of cells in bone.  Osteoblasts are responsible for making and putting down new bone, osteoclasts are responsible for taking up and getting rid of old bone, and osteocytes are osteoblasts that have been trapped inside the new bone and then work to maintain the bone after it has been constructed.  So, here’s what happens:

The osteoclasts drill through bone tissue to clean it out, and then osteoblasts follow behind depositing new bone tissue on the sides of the inside of the bone, creating this new tissue that surrounds either a blood vessel or a nerve.  When a bone is broken, the first thing osteoblasts lay down is what’s called “woven fiber” bone tissue; this tissue is much faster to create and lay down (providing quick structure) but is not as strong.  Later, because it takes more time, osteoblasts replace that woven fiber tissue with “lamellar” bone tissue, which is nicely organized in sheets that is very strong. 

So, a sensible follow up question would be: how does bone grow?  A bone begins as cartilage and then layers of bone begin to develop around it while cells destroy and take up the old cartilage.  Then, cells begin to move to the ends of the bone towards the “active growth centers,” where they stretch out the bones.  After the bone has finished growing, the cells mineralize the bone to make it tough and strong.  When a bone mineralizes, it ceases to grow forever.  So, when humans stop growing, that is because all of our bone has been mineralized.

Comment if there are any questions, but that explains the basics about how bone tissue can regenerate and how bones can be fixed.

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