Bone marrow aspiration is the procedure which involves taking the sample from soft tissue inside the bones. Bone marrow is a spongy tissue that is found inside the bones. It contains cells that produce white blood cells, red blood cells and platelets inside larger bones like spine, breastbone, ribs, hips and skull. If the complete blood count test shows that the number of red blood cells, white blood cells and platelets is abnormally high or low then your bone marrow needs to be examined to figure out the cause.