What is the main organ for blood production?

I believe everyone knows about leukemia, which is a very serious disease. People with leukemia have very poor hematopoietic ability. In addition to this disease, if a person's hematopoietic cells are damaged, it is easy to cause anemia and many other symptoms, which will also lead to a decline in hematopoietic function. Many people don’t know what the main organ for hematopoiesis is. Let’s learn about the professional knowledge in this area.

How does the human body produce blood?

The human body's hematopoietic organs include the bone marrow, thymus, lymph nodes, liver and spleen. The bone marrow, thymus, lymph nodes and spleen are also called lymphatic organs. The human body's hematopoietic organs are different at different stages. The hematopoietic cells of a fetus of 1 to 2 months old originate from the yolk sac, so the yolk sac is its hematopoietic organ. In fetuses aged 2 to 5 months, the liver, spleen, and lymph nodes begin to produce hematopoiesis, producing red blood cells, white blood cells, and platelets, replacing the hematopoietic function of the yolk sac. Bone marrow hematopoiesis begins in the fifth month of the fetus, and thymus hematopoiesis appears in the later stages of the fetus.

After a baby is born, the main hematopoietic cell is the bone marrow, which can produce various blood cells such as red blood cells, white blood cells, and platelets. The spleen, lymph nodes, and lymphoid tissue also produce hematopoiesis, but only a small amount of monocytes and lymphocytes.

The hematopoietic organ of adults is the bone marrow. Bone marrow is a spongy, gelatinous, fatty tissue enclosed in a hard marrow cavity. The bone marrow is divided into two parts: red marrow (hematopoietic cells) and yellow marrow (fat cells). Bone marrow hematopoiesis is initially distributed throughout the bones of the body, and then gradually becomes confined to the skull, ribs, sternum, spine, ilium, and part of the humerus and femur, with other parts gradually replaced by yellow marrow. The yellow marrow cannot produce blood.

Normal human blood cells are produced in the bone marrow and lymphatic tissue. Hematopoietic cells are generated in the embryonic mesoderm. As the embryo develops, the hematopoietic center shifts. Hematopoiesis before birth is divided into three stages:

① The yolk sac hematopoiesis period begins in the 3rd week of human embryo and ends in the 9th week. The blood islands on the yolk sac wall are the initial hematopoietic centers.

② Hepatic hematopoiesis begins in the 6th week of human embryonic development and reaches its peak in the 4th to 5th month. It is mainly composed of red blood cells and granulocytes, and no lymphocytes are produced. At this stage, the spleen, kidneys, thymus and lymph nodes are also involved in hematopoiesis. Lymphocytes begin to form in the spleen in the 5th month and become an organ of lymphocytes by birth. The human embryo at 6-7 weeks has a thymus and begins to form lymphocytes. The lymphoid stem cells in the thymus also come from the yolk sac and bone marrow.

③ The bone marrow hematopoietic period begins in the 4th month of the human embryo, and becomes the hematopoietic center after the 5th month. From then on, the hematopoiesis of the liver and spleen gradually decreases, and the hematopoietic function of the bone marrow increases rapidly, becoming the main organ for the production of red blood cells, granulocytes and megakaryocytes, while also producing lymphocytes and monocytes. Lymph nodes participate in red blood cell production for a very short time, and become organs that produce lymphocytes and plasma cells throughout life after the fourth month of human embryo. Their multipotent stem cells come from the embryonic liver and bone marrow, and lymphocyte stem cells also come from the thymus.