Bone marrow is the blood-forming tissue of the body. The hematopoietic organs of the human body include the bone marrow, thymus, lymph nodes, liver, and spleen. The hematopoietic organs of the human organism differ at different stages of life. During the fetal period, the hematopoietic tissues are, in chronological order, the yolk sac, the liver, the spleen, the lymph nodes, the bone marrow, and the thymus. Since the birth of infants, the main hematopoietic tissue is the bone marrow, while the spleen and lymphatic tissue also produce blood, but only a small number of monocytes and lymphocytes. In adults, the bone marrow is the main blood-producing organ. The human bone marrow consists of red marrow and yellow marrow. The red marrow contains hematopoietic cells that produce blood; the yellow marrow is not hematopoietic and is mainly composed of fat cells. Consult a hematologist for more information.