EXTRAMEDULLARY BLOOD PRODUCTION

Abstract

In the case of mammals throughout post-natal life blood cells are produced under normal conditions exclusively in the bone marrow. This statement ap-plies only to two of the three predominating and essential hemocytes, the eryth-rocytes and t.he granulocytes. The third type of cell which occurs in great numbers in the circulating blood, namely, the lymphocyte, is produced chiefly in the lymph nodes and the spleen, to a relatively small extent in the bone mar-row. It is essentially extramedullary in origin, The lymphocytes formed in lymph nodes enter the blood stream via the lymphatic radicles of the thoracic duct and the right lymphatic duct; those produced in the spleen and the bone marrow pass directly into sinusoidal venous channels. Minor contributions of lymphocytes are made also from the thymus and lymphoid areas in the wall of the gastro-intestinal canal. This mixture of lymphocytes with erythrocytes and granulocytes in the blood, and especially the presence of lymphocytes in the bone marrow, at once raises the questions of the function of lymphocytes and of a possible genetic relationship of lymphocytes to erythrocytes and granulocytes.