The human body contains over 200 bones of different shapes and sizes.
Bone is living tissue made up of calcium and various proteins that make it strong and rigid. It contains cells that continuously break down and remove old bone (osteoclasts), and other cells that build up new bone to maintain its strength (osteoblasts).
Each bone consists of a compact outer shell (the cortex) and a spongy inside. The inside contains the bone marrow, which produces blood cells (see diagram below).
The joints of the bones are covered in cartilage - a tough, flexible material like gristle. Cartilage is more stretchy than bone, and it allows the bones to move freely at the joints. It also cushions the bones at the joint to stop them rubbing against each other.
The bones have several important functions:
The skeleton gives the body rigid support.
The joints act as levers so the body can move.
The bones protect organs in the body; for example, the ribcage protects the heart and lungs, and the skull protects the brain.
The bones also store some of the body’s essential minerals, including calcium.
Although a secondary bone cancer can occur in any bone in the body, the most commonly affected bones are the spine, ribs, pelvis, skull, and the upper bones of the arms (humerus) and the legs (femur).