The human skeleton is made up of more than 200 bones of different shapes and sizes.

Bones are made of collagen (a type of protein) and minerals such as calcium and phosphate. The combination of collagen and minerals makes the bones strong.

Bones do lots of important things:

• They support our bodies. The joints at the end of bones act as levers so we can move around.
• They protect parts of our bodies from injury. For example, the ribs protect the heart and lungs.
• They store important minerals that our bodies use, such as calcium.
• Some bones are filled with a soft, spongy material called bone marrow, which makes blood cells.

Our bones are constantly breaking down and being replaced with new bone. This helps maintain their strength and shape. There are two main types of cell that break down and replace bone:

• Osteoclasts break down and remove old bone.
• Osteoblasts make new bone.

A secondary bone cancer can occur in any bone in the body. But the most commonly affected bones are the:

• spine
• ribs
• pelvis
• upper bones of the arms (humerus) and legs (femur)
• skull.