bone marrow bone marrow bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. in birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). it is composed of hematopoietic cells, marrow adipose tissue, and supportive stromal cells. in adult humans, bone marrow is primarily located in the ribs, vertebrae, sternum, and bones of the pelvis. bone marrow comprises approximately 5% of total body mass in healthy adult humans, such that a man weighing 73 kg (161 lbs) will have around 3.7 kg (8 lbs) of bone marrow. bone marrowa section of bone marrow tissue (prussian blue-stained)detailssystemhematopoietic system immune system lymphatic systemidentifierslatinmedulla ossiummeshd001853ta98a13.1.01.001ta2388fma9608anatomical terminology human marrow produces approximately 500 billion blood cells per day, which join the systemic circulation via permeable vasculature sinusoids within the medullary cavity. all types of hematopoietic cells, including both myeloid and lymphoid lineages, are created in bone marrow; however, lymphoid cells must migrate to other lymphoid organs (e.g. thymus) in order to complete maturation. bone marrow transplants can be conducted to treat severe diseases of the bone marrow, including certain forms of cancer such as leukemia. several types of stem cells are related to bone marrow. hematopoietic stem cells in the bone marrow can give rise to hematopoietic lineage cells, and mesenchymal stem cells, which can be isolated from the primary culture of bone marrow stroma, can give rise to bone, adipose, and cartilage tissue. structure the composition of marrow is dynamic, as the mixture of cellular and non-cellular components (connective tissue) shifts with age and in response to systemic factors. in humans, marrow is colloquially characterized as "red" or "yellow" marrow (latin: medulla ossium rubra, latin: medulla ossium flava, respectively) depending on the prevalence of hematopoietic cells vs fat cells. while the precise mechanisms underlying marrow regulation are not understood, compositional changes occur according to stereotypical patterns. for example, a newborn baby's bones exclusively contain hematopoietically active "red" marrow, and there is a progressive conversion towards "yellow" marrow with age. in adults, red marrow is found mainly in the central skeleton, such as the pelvis, sternum, cranium, ribs, vertebrae and scapulae, and variably found in the proximal epiphyseal ends of long bones such as the femur and humerus. in circumstances of chronic hypoxia, the body can convert yellow marrow back to red marrow to increase blood cell production. hematopoietic components