Osseous, or bone, metastases are tumors caused by cancer cells that break off from the primary tumor and attach to the bones, the American Cancer Society explains. The bones they attach to depend on the type of cancer cell and where the cell began in the body.
In order for cancer cells to spread throughout other parts of the body, they must first bypass the body's immune system and attach to the wall of a new blood or lymph vessel, according to the American Cancer Society. The cancer cells often undergo changes after fighting through the immune system, and must be able to survive and grow in the new part of the body.
Depending on the type of cancer cell, some cells contain unique surfaces and release special chemicals that allow them to more readily attach to the target area or bone, causing the bone cells to release hormone-like factors that aid in the cancer cells' growth, notes the American Cancer Society. Once attached, the cancer cells work against the osteoclasts, one type of bone cell, to keep new bone from forming and break down the old bone, making it fragile. In some instances, the cancer cells work against the osteoblasts, cells that synthesize bone, keeping them from breaking down the old bone before laying down new bone, which causes a hardening of the bone or sclerosis.