Osteoblasts bone-forming cells

Figure 7.2. Schematics of bone anatomy (a) the structure of a long bone demonstrating the distribution of the two different tissue structures, cortical and cancellous bone, and (b) the cells present in bone osteoblasts, bone-forming cells found on surfaces osteocytes, bone cells embedded in the mineralised matrix and osteoclasts, bone-removing cells.

The family of connective-tissue cells includes fibroblasts, chondrocytes (cartilage cells), and osteoblasts (bone-forming cells). They are specialized to secrete extracellular proteins, particularly collagens, and mineral substances, which they use to build up the extracellular matrix (see p. 346). By contrast, osteoclasts dissolve bone matter again by secreting and collagenases (see p. 342). 

Osteoblasts are the primary cells responsible for bone formation. They are derived from mesenchymal (stromal) cells that first differentiate into pre-osteoblasts and then into mature, bone matrix producing osteoblasts. Inactivated or resting osteoblasts become lining cells and thus a reservoir for bone forming cells to be activated at the next remodelling cycle. Osteoblasts trapped and embedded in the mineralised matrix are called osteocyts, and are important for many properties of living bone. 

In addition to mediating cell adhesion, it has been demonstrated that integrin expression by osteoblasts determines their phenotypic expression (see the section Osteoblasts The Bone-Forming Cells ). For example, addition of either soluble concentrations of RGD (Moursi et al, 1996) or antibodies of the integrin pair avp() (Moursi et al., 1997) to confluent osteoblast cultures, blocked initiation and formation of mineral nodules these results provided evidence that integrin-fibronectin interactions leading to extracellular matrix development play a crucial role in osteoblast function (Moursi etal., 1996,1997). 

Endochondral ossification is the gradual replacement of cartilage by bone during development. This process is responsible for formation of most of the skeleton of vertebrate animals. In this process, actively dividing bone-forming cells (osteoblasts) arise in regions of cartilage called ossification centers. The osteoblasts then develop into osteocytes, which are mature bone cells embedded in the calcified (hardened) part of the bone known as the matrix. 

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