Hematopoiesis, also known as haematopoiesis, refers to the formation of blood cells from haematopoietic stem cells. This is necessary because the blood cells have a limited life span. Thus erythrocytes live for up to 120 days and thrombocytes for up to 10 days, after which renewal is necessary.
The 1st place of blood formation is in the yolk sac of the embryo. This is where the first erythrocytes (still with nucleus) are formed until the 3rd embryonic month, as well as megakaryocytes (precursors of thrombocytes), macrophages (scavenger cells) and haematopoietic stem cells (haematopoietic stem cells from which all blood cells are formed). From the 2nd embryonic month onwards, blood cells are also produced in the liver.
These are the first mature erythrocytes. The fetal liver is also responsible for the maturation and proliferation of stem cells, which later migrate to the bone marrow. Hematopoietic stem cells are produced in the embryo in the placenta, the AGM region (aorta, genitals, kidney region) and in the yolk sac.
From the 4th fetal month, blood formation takes place in the spleen and thymus and from the 6th fetal month in the spleen and bone marrow. After birth the so-called adult blood formation begins. This takes place mainly in the bone marrow.
There are different cell lines involved in blood formation. One is myelopoesis. From it the erythrocytes, thrombocytes, granulocytes and macrophages emerge.
The second cell line is lymphopoiesis. From it the different lymphocytes emerge.