Bone formation (osteogenesis) begins during prenatal development and persists throughout adulthood. The bones of infants and children are softer than in adults because it has not yet been ossified (the process of synthesizing cartilage into bone).
There are two ways in which osteogenesis occurs: intramembranous ossification and endochondral ossification. Both types form by replacing existing cartilage however differ in the method they go about doing it. Two types of cells that are of great importance in the process are osteoblasts and osteoclasts. Osteoblasts, used mainly in intramembranous ossification, are the specialized cells in bone tissue that deposit calcium into the protein matrix of bone (collagen). Osteoclasts, used in endochondral ossification, dissolve calcium previously stored away in bone and carry it to tissues whenever needed.
One third of all of the bone's components is collagen; a flexible, gelatin-like matrix. Bones formed during intramembranous formation are called membranous bones, or occasionally dermal bone, and bones formed during endochondral formation are called cartilage bone.