Bone marrow is a specific tissue found inside bones. Its main purpose is to produce stem cells, which give rise to the various blood cell types, especially the white blood cells, the red blood cells and the platelets. Each stem cell can further form millions of stem cells. Most stem cells are found inside bones in the bone marrow of the sternum, hips, skull, upper arms and legs. If the stem cells develop into fully functional cells, they are released into the blood. A smaller number of undifferentiated stem cells are also found in the bloodstream.