Bone marrow is a semi-solid tissue found in bones of spongy or spongy parts.  In birds and mammals, the bone marrow is the main site of new blood cell production or hematopoiesis. It is composed of hematopoietic cells, spinal adipose tissue and supporting stromal cells. In adult men, the bone marrow is mainly located in the ribs, vertebrae, sternum and pelvic bones.  Bone marrow accounts for approximately 5% of total body mass in healthy adult men, so a man weighing 73 kg (161 lb) will have approximately 3.65 kg (8 lb) of bone marrow.

The human marrow produces around 500 billion blood cells per day, which join the systemic circulation through permeable vascular sinusoids.  All types of hematopoietic cells, including myeloid and lymphoid lines, are created in the bone marrow; However, lymphoid cells must migrate versus other lymphoid organs (for example, the thymus) to complete maturation. Bone marrow transplants can be treated with bone marrow, including certain forms of cancer such as leukemia. In addition, bone marrow stem cells have been transformed into functional neural cells  and can also be used to treat inflammatory bowel disease.