Hematopoietic Organs

in animals and man, the organs in which the formed elements of the blood and lymph are produced.

In adult mammals and man, the principal hematopoietic organ is the bone marrow, where the red blood cells (erythrocytes), granular white blood cells (granular leukocytes), blood platelets (thrombocytes), and certain agranular white blood cells (lymphocytes) are formed. It is mostly lymphocytes that are formed in the other hematopoietic organs (that is, the lymph nodes, spleen, and thymus). Granular leukocytes and erythrocytes are formed otherwise in certain mammals only in the spleen. The yolk sac and liver also serve as hematopoietic organs in the mammalian embryo. The kidneys and liver perform the function in the lower vertebrates. In invertebrates, blood cells form directly in the perivisceral fluids and hemolymph.

Intensive proliferation and maturation of hematopoietic and lymphoid cells go on throughout the life of an organism. This compensates for the natural loss of blood cells and lymphocytes, which survive only a few days to a few months. Hematopoiesis is maintained by the stem cells, the element common to all hematopoietic tissues. Stem cells are found primarily in the bone marrow, but they can enter the other hematopoietic organs with the blood. The stem cells develop into erythrocytes, leukocytes, or thrombocytes, depending upon which of the hematopoietic organs they reach.

Hematopoietic organs are made up not only of hematopoietic cells but also of supporting tissue, or stroma. It is largely the interaction of the stroma with the stem cells that determines the type of blood cell production in the given organ. Immunologically active cells (lymphocytes and plasma cells) are also formed in the hematopoietic organs, which are the site of important stages in the influence on the cells of antigens.