Ossification is the process in which cartilage is transformed into bone. Bone grows in three stages: first, tissue forms a mesh of collagen fibers, then the body creates a polysaccharide that acts like cement to hold the tissues together. Finally, calcium crystals salts are deposited to form bone.
The bone marrow contains osteoprogenitor cells that help in the formation and growth of new bone. Likewise, bone morphogenic proteins further stimulate new bone formation.
A particular form of ossification called heterotopic ossification involves abnormal bone growth in soft tissue that does not normally ossify which leads to pain and immobility at the site of ossification. The condition can arise from head injury, surgical trauma, and spinal cord injury.