Cells that have the capacity to develop into red blood cells, white blood cells, and platelets and have a key role in hematopoiesis (producing blood cells). Hematopoietic progenitor cells are located principally in the red bone marrow, which in adults is restricted to the pelvis, ribs, spine, skull, sternum (breastbone), humerus (upper arm bone), and femur (thigh bone).