The hard connective tissue that, together with cartilage, forms the skeleton of humans and other vertebrates. It is made of calcium phosphate crystals arranged on a protein scaffold. Bone performs a variety of functions: it has a structural and mechanical role; it protects vital organs; it provides a site for the production of blood cells; it serves as a reserve of calcium. Bone tissue is constantly renewing itself and has great capacity to respond to altered stresses and loads and to repair fractures. The formation and remodeling of bone is accomplished by the action of bone cells of the osteoblast and osteoclast lineages. The activities of these cells are directed by hormones and other molecules that circulate in the blood as well as regulators that are produced locally in the bone. There are a number of bone diseases which are the subject of vigorous research.