In biology and medicine, and the specialty of hematology, bone marrow literally is the the soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Red marrow is the most significant type, because it is the site of hematoepoesis: generation of erythrocytes, leukocytes and platelets from hematopoetic stem cells.
At a basic level, marrow disorders principally involve inappropriate blood cell production. In aplastic anemia, for example, the marrow stops producing erythrocytes. In myeloproliferative disorders, such as polycythemia vera, the marrow makes excessive numbers of often abnormal blood cells.
Morpholologically, bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. The second type is yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells.