Hematopoiesis is the term for the formation of blood cells out of pluripotent stem cells that originate in the aorto-gonado-mesonephric region. From these arise on the one hand the lymphoid stem cells that differentiate during the fetal period in the primary lymphatic organs - thymus and bone marrow – into immunocompetent B and T cells. On the other hand, myeloid stem cells, out of which all the other blood cells arise, also differentiate.
Initially, erythropoiesis is mainly in the foreground because O2 diffusion is soon insufficient to nourish the embryo. An initial extraembryonic phase of erythropoiesis occurs in the umbilical vesicle. These erythrocytes contain nuclei.
Intraembryonically erythropoiesis continues in the hepato-lienal phase, whereby these erythrocytes no longer have nuclei. In the second half of the pregnancy erythropoiesis occurs almost only in the bone marrow (myeloid phase). The composition of the embryo-fetal hemoglobin of the erythrocytes is optimally fitted to the intrauterine requirements. All other blood cells of these myeloid stem cell series also differentiate, though somewhat later, in the liver and in the bone marrow, respectively.
The lymphatic system is responsible for the defense of the body against antigens. The «microenvironment» of the thymus for the T cells (cell mediated immunity) and of the bone marrow for the B cells (humoral immunity), respectively, are crucial for achieving immunocompetence. The mature lymphatic cells emigrate afterwards into the secondary lymphatic organs, among which the lymph follicles of the mucous membranes, the lymph nodes and the white splenic pulp are included.