Metastatic bone cancer. Light micrograph of a large multi-nucleated osteoclast (red) resorbing bone matrix (orange) adjacent to cancer cells (blue). When cancer cells metastasize (spread) to the bone from the primary cancer site, they secrete factors that stimulate osteoclasts (cells that break down bone tissue) to resorb mineralized bone matrix and to release stored growth factors. The growth factors stimulate growth of cancer cells.