Hematopoiesis is the formation and development of blood cells. Today one assumes that all the cells of the hematopoietic system stem from a stem cell line. The first of such stem cell lines forms in the mesoblast (extraembryonic mesoderm) of the umbilical vesicle (stage 7, ca. 19 days). Investigations have shown that definitive stem cell lines for hematopoiesis probably come from a center within the embryo. This is located in the splanchnopleura around the dorsal aorta at the level of the navel.
Blood and vessel formation therefore first occurs extraembryonally on the umbilical vesicle and in the chorionic villi (extraembryonic phase of blood and vessel formation).
Subsequently follows the intraembryonic phase, which is subdivided into hepatolienal and myeloid phases. First vessels and pluripotent stem cells arise within the embryo. These migrate from the dorsal aorta near the navel and mainly colonize the liver (hepatic phase). A small portion - primarily for erythropoiesis - appears after the 12th week in the spleen (lienal phase).
In the myeloid phase daughter cells of these stem cells emigrate via the vessel system into the bone marrow and increasingly take over the formation of the blood.
These stem cells are lifelong able to renew and differentiate themselves; they remain in the bone marrow and, according to need, can differentiate into every type of lymphoid and also myeloid cells.
|Overview of the various phases of hematopoiesis|