Ossification is the process of building up the bone particles from the bone cells of osteoblasts. Generally, bone grows from two ways: intramembranous ossification and endochondral ossification.
Intramembranous bone growth is the formation of flat bones. Bones are formed inside the membrane where it is attached in the connective tissues. The connective tissue first form sheets where the bone will be deposited eventually. These membranes are made up of blood vessels so they can assist in the formation of the flat bones. When the sheet is established, the osteoblasts begin adding up matter in order to form matrix which eventually become the spongy bone structure. Examples of bones formed through intramembranous are the skull, clavicle, maxilla and the mandible.
Endochondral bone growth is essential in formation of the long bones and the bone healing. Unlike the former type, Endochondral bone growth form cartilage from the products of chondrocyte cell division. Periosteum will start to form around the cartilaginous material which contains stem or undifferentiated cells. Lengthening of the bone happens as the bone collar is produced until it becomes the compact bone. Most of the bones in the body form via endochrondal bone growth.