The blood system consists of hematopoietic organs and hematopoietic cells. It is the entire system that makes blood in the body. It mainly includes the yolk sac, liver, spleen, kidney, thymus, lymph nodes and bone marrow. Hematopoietic cells all occur in the mesoderm of the embryo, and the hematopoietic center is transferred as the embryo develops.
Normal human blood cells are generated in bone marrow and lymphoid tissue. Hematopoietic cells all occur in the mesoderm of the embryo. As the embryo develops, hematopoietic centers are transferred. Hematopoietic cells before birth are divided into three stages:
① The yolk sac hematopoietic phase begins at the 3rd week of the human embryo and stops at the 9th week. The island of blood on the wall of the yolk sac was the original hematopoietic center.
② Liver hematopoietic phase begins at the 6th week of the human embryo and reaches its peak in the 4th-5th month. It is dominated by hematopoietic red and granulocytes and does not produce lymphocytes. At this stage, the spleen, kidney, thymus and lymph nodes are also involved in hematopoietic. Lymphocytes form in the spleen from the fifth month and become organs of lymphocytes at birth. At 6-7 weeks, the human embryo has a thymus and begins to form lymphocytes. Lymph stem cells in the thymus also originate from the yolk sac and bone marrow.
③ The bone marrow hematopoietic phase begins in the fourth month of human embryos, and becomes a hematopoietic center after the fifth month. From then on, hepatic and spleen hematopoiesis gradually decreases, and the bone marrow hematopoietic function increases rapidly, becoming the main organs that produce red blood cells, granulocytes and megakaryocytes. Lymphocytes and monocytes are also produced. Lymph nodes participate in erythropoiesis for a short time. From the fourth month of human embryos, they become organs of life-long lymphoblasts and plasma cells. Pluripotent stem cells come from the embryo's liver and bone marrow, and lymph stem cells also come from the thymus.
Hematopoietic System Development
In developing embryos, blood formation occurs in blood cell aggregates in the yolk sac, called blood islands. As development progresses, blood is formed in the spleen, liver, and lymph nodes. When the bone marrow develops, it eventually assumes the task of forming most blood cells for the entire organism. However, maturation, activation, and some proliferation of lymphoid cells occur in the spleen, thymus, and lymph nodes. In children, hematopoietic effects occur in the bone marrow of long bones such as the femur and tibia. In adults, it occurs mainly in the pelvis, skull, vertebra and sternum.
Hox genes and hematopoiesis
Homeodomain proteins figure prominently in embryonic development, particularly in pattern formation and segmental identity. The expression of various Hox genes in hematopoietic cell lines is consistent with the notion that genes of this class may play distinct roles in HSC development, progenitor commitment or proliferation, or lineage-specific differentiation. The selective expression of some members in particular lineages lends indirect support to this hypothesis; moreover, some Hox genes-such as HOXB3 and HOXBI are expressed at higher levels in populations containing immature hematopoietic cells. Despite these observations, mice with targeted mutations of various Hox genes have not been reported to exhibit striking clearcut hematological phenotypes