Question

Deposition of osteoid by osteoblasts – the first step in establishment of the ECM of bone – requires a suitable substrate as well as presence of nearby blood vessels. These requirements suggest some features of the substrate and blood vessels are necessary to activate osteoblasts; although I do not know what these features are, presumably they include some sort of signal molecules necessary to activate osteoblasts.

Given the manner in which developing bone increases in size – particularly through appositional (from the outside) growth – what suggests that bone itself is a suitable substrate for deposition of osteoid? [Note: To answer the question you must also address presence of osteoblasts, since they are the source of osteoid.]

Intact cartilage is not a suitable substrate for deposition of osteoid. Since this is the case, how can we account for endochondral bone formation in the fetus and for cartilage replacement by bone in the epiphyseal plate as a bone is elongating?

What feature of a long bone (and remember a bone is an organ that includes multiple tissue types) seems to demonstrate that intact cartilage is not a suitable substrate for osteoid deposition, even if osteoblasts are in close proximity to intact cartilage?

Speculate…why does it seem to be a good idea that osteoblasts cannot be induced to deposit osteoid on intact cartilage?

Answer 1

In the process of bone formation, the ossification centers appear in fibrous connective tissue membrane.This happens when the selected centrally located mesenchymal cells cluster and differentiate into Osteoblasts, hence forming an ossification center.The bone matrix ( also known as the Osteoid ) is secreted within the fibrous membrane and calcifies in few days.The trapped Osteoblasts then become the Osteocytes.

Now, it actually seems a good idea that the Osteoblasts cannot be induced to deposit osteoid on intact cartilage since in the process of Endochondral Ossification cells in the perichondrium changes to osteoblasts(which then secretes the Osteoid) which is not possible with an Intact cartilage.Later on the blood vessels grow around the cartilage and the Osteoid is then mineralized and produces a layer of superficial bone around the shaft which continues to grow around the diaphysis and finally become a compact bone (appositional growth).