All blood cells arise from a type of cell called the hematopoietic stem cell (HSC). Stem cells are cells that can differentiate into other cell types; they are self-renewing—they maintain their population level by cell division. In humans, hematopoiesis, the formation and development of red and white blood cells, begins in the embryonic yolk sac during the first weeks of development. Here, yolk-sac stem cells differentiate into primitive erythroid cells that contain embryonic hemoglobin. In the third month of gestation, hematopoietic stem cells migrate from the yolk sac to the fetal liver and then to the spleen; these two organs have major roles in hematopoiesis from the third to the seventh months of gestation. After that, the differentiation of HSCs in the bone marrow becomes the major factor in hematopoiesis, and by birth there is little or no hematopoiesis in the liver and spleen.
It is remarkable that every functionally specialized, mature blood cell is derived from the same type of stem cell. In apter 2
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