What Is Chronic Leukemia? Chronic leukemia is a type of cancer that starts in blood-forming cells of the bone marrow, andthen can spread to other parts of the body including the lymph nodes, liver, spleen, skin, andbrain. Other types of cancer can start in organs such as the lungs, colon, or breast, and then spread to the bone marrow. These cancers that start elsewhere and then spread to the bonemarrow are not leukemia. Bone Marrow and Lymphoid Tissue Bone marrow is the soft inner part of some bones such as the skull, shoulder blade, ribs, pelvis,and backbones. The bone marrow is made up of hematopoietic (blood-forming) cells, fat cells,and supporting tissues that aid the growth of hematopoietic cells. It contains pluripotent stemcells that continually reproduce to form new cells. Some of these new cells remain as stem cellswhile others go through a series of changes. During this process, pluripotent stem cells stopreproducing and eventually become either lymphoid cells or hematopoietic stem cells. Thehematopoietic stem cells also can divide and form new hematopoietic stem cells or develop intoone of the three main types of blood cells - red blood cells, white blood cells, or platelets.