Development of Bones
The formation of bone is called ossification. There are two types of ossification: intramembranous and endochondral (see Fig. 6.1) . Intramembranous ossification include the flat bones of the skull and some of the irregular bones, which are formed from connective tissue. Mesenchyme condensations differentiate into osteoblasts. Osteoblasts migrate to the membranes and deposit bony matrix around themselves without the requirement for a cartilage intermediate. When the osteoblasts are surrounded by matrix they are called osteocytes. Endochondral ossification occurs in long bones and most of the rest of the bones in the body, which involves the replacement of hyaline cartilage with bony tissue. Mesenchymal stem cells differentiate into chondrocytes that secrete cartilage matrix. Cartilage model is firstly
Fig. 6.1 Endochondral and intramembranous ossification. (a) Long bones form by endochondral ossification. (b) The skull vault forms by intramembranous ossification  (Copyright permission from Elsevier Science Ltd)
synthesized, the primary and secondary ossification center are then developed, articular cartilage and epiphyseal plate are finally formed . Progression of endochondral ossification are regulated by multiple systemic hormones and various cytokines and growth factors .