The process of development of bones is called osteogenesis or ossification. Though all bones develop from mesenchyme, osteogenesis is traditionally described as occurring by two methods: (1) intramembranous method, ie, bone development in membrane and (2) intracartilaginous or endochondral method, ie, bone development in cartilage. However, it is important to know that in each case the fundamental process of bone formation is the same. This process begins in the second month of intrauterine life and consists of the following steps: (1) the mesenchymal cells differentiate into bone-forming cells called osteoblasts, (2) the osteoblasts secrete a special type of intercellular substance called osteoid, which consists of collagenous fibers embedded in an amorphous ground substance, (3) under the influence of the enzyme alkaline phos-phgitase (which is also secreted by the osteoblasts) mineral salts (mainly calcium phosphate) are deposited in the osteoid. Deposition of mineral salts converts the osteoid into bone matrix.
In this process some osteoblasts become surrounded by the bone matrix; these cells lose the capability of bone formation and are now designated as osteocytes. The osteocytes are lodged in small spaces called lacunae.
Having understood the basic process of bone tissue formation, the abovementioned two methods of bone formation will now be described. At this stage it is also important to note that some bones of the body develop by a combination of the intramembranous and intracartilaginous methods of ossi- fication