The chief function of osteoblasts is the formation of bone. Bone formation begins within cartilage and connective tissue membranes during the third prenatal month, ending sometime between late adolescence and early adulthood. Cartilage develops, becoming the long bones of the body, such as the femur and humerus. Flat, skull bones develop out of connective tissue membranes. Bone mineralization is another function of osteoblasts, which are found on the surface of bone.
Another function of osteoblasts is differentiating into osteocytes, which are mature bone cells that are no longer capable of cell division. Osteocytes become lodged within the lacunae of bone, playing a primary role in bone remodeling. Bone remodeling, which occurs throughout the entire span of life, is the process responsible for maintaining the balance of bone formation and resorption. Osteocytes are mechanosensors, capable of detecting mechanical stress, thereby initiating bone resorption. How osteocytes accomplish this was not fully understood as of 2011.
Osteoclasts are responsible for breaking down osteoid, the bone matrix. As osteocytes die, osteoclasts eat away dead bone cells. Minerals such as calcium move into the bloodstream for use by the body. As resorption occurs, new bone cells form, which is a function of osteoblasts.
Remodeling makes shaping and repairing of the skeleton possible. It also is a response to mechanical demands. Running, jumping, walking and standing are examples of mechanical demands commonly placed upon the body.
With age, remodeling becomes imbalanced. Bone resorption occurs more often than formation, causing osteoporosis, a condition of fragile, porous bone. This is the reason older people are at increased risk for falls and fractures.
Bone cells sometimes divide uncontrollably, becoming tumorous. These cells do not function normally and compete for space with healthy bone cells. Bone tumors might be benign, but others are malignant and cancerous.
Cancer that originates within bone is called primary bone cancer. Tumor growth caused by metastasis from another area of the body, such as the breast or lungs, is referred to as secondary bone cancer. People who have advanced cancer commonly develop secondary bone cancer because of the vascularity of the bone matrix.