Ossification Osteoblast

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. 

Intramembranous ossification is responsible for most of the mineralization of the skull, including the maxilla and mandible. It begins with the differentiation and activation of osteoblasts from fibroblast-related precursors within a region of connective tissue that demarcates where the bone will develop. The osteoblasts secrete a nonmineralized protein-rich (osteoid) matrix and, as they move away, the matrix mineralizes (Fig. 9.3a). The periosteum remains uncalcified and contains latent and undifferentiated osteoblasts for bone remodeling. Odontoblasts (Ob) and cementoblasts secrete an osteoid-like matrix similar to that of intramembraneous ossification. 

Tortelli, F., Tasso, R., Loiacono, F., and Cancedda, R. 2010. The development of tissue-engineered bone of different origin through endochondral and intramembranous ossification foUowmg the Implantation of mesenchymal stem cells and osteoblasts in a murine model. Biomaterials 31 242-9. 

The formation and breakdown of bone are vital processes carried out by special cells. Mineralualion occurs in the osteoblasts, apparently involving alkaline phosphatase. The enzyme is especially abundant in areas of active ossification, while cartilage is free of it. The breakdown of bone in the osteoclasts serves to mobilize mineral material. The solubilization of apatite probably is a surface phenomenon, analogous to its formation. 

---