bone marrow

a soft tissue found in the spaces of BONES . In young animals, all bone marrow, the red marrow, produces blood cells. In older animals the marrow in long bones is replaced by yellow marrow, which contains a large amount of fat and does not produce blood cells. In mature animals, the red marrow occurs in the ribs, sternum, vertebrae and the ends of the long bones (e.g. the femur). The red marrow contains MYELOID tissue with ERYTHROBLASTS from which red blood cells develop. LEUCOCYTES also form the myeloid tissue and themselves give rise to other cells types.