Bone marrow contains hematopoietic stem cells and stromal cells (mostly adipocytes). Marrow cellularity is the volume ratio of hematopoiesis and fat. Cellularity is age dependent — in newborns, all marrow is hematopoietic (shows 100% cellularity), with age hematopoiesis diminishes, and the amount of fat increases. In adults hematopoiesis takes place in axial skeleton only. Long bones contain only white (fatty) marrow with no hematopoiesis taking place (except proximal parts of the humerus and femur).
Normal cellularity of an adult hematopoietic bone marrow ranges between 30 – 70% and changes under pathological conditions — we talk of hypercellular (over 70%), normocellular (30 – 70%) or hypocellular (under 30%) bone marrow.
Under pathological conditions hematopoiesis may also expand into long bones of the limbs and even out of the marrow (into the liver, spleen and other organs).