Prior to birth, hematopoiesis occurs in the yolk sac, then in the fetal liver, before taking place in the bone marrow and lymphatic system of normal adults. Hematopoiesis is the process of generating mature blood cells from pluripotent hematopoietic cells, otherwise known as "stem cells."
Hematopoietic activity starts with the initial formation of blood cells in the yolk sac, which is attached to the embryo. However, there is scientific evidence that the erythroblasts, or immature red blood cells, that emerge from the sac are primitive and not the product of definitive hematopoiesis, as claimed by a study published in the United States National Library of Medicine National Institute of Health by Dr. Kathleen E. McGrath and Dr. James Palis. McGrath and Palis believe that hematopoiesis actually begins in the aorta-gonad-mesonephros, or AGM, of the embryo, where the AGM pluripotent hematopoietic cells move on the fetal liver for further development to undergo differentiation in the lymphatic tissues and bone marrow.
Hematopoiesis is divided into two separate processes: erythropoiesis and granulocytopoiesis. Erythropoiesis involves the differentiation of red blood cells while granulocytopoiesis refers to the differentiation of white blood cells. The initial stages of red blood cells include proerythroblast, basophilic erythroblast, polychromatophilic erythroblast, orthochromatophilic erythroblast, reticulocyte and erythrocyte, which is the mature red blood cell. White blood cells develop from a promyelocyte, myelocyte, metamyelocyte, band neutrophil and granulocyte. A megakaryoblast develops into a megakaryocte before forming a blood platelet. A lymphoblast changes into a prolymphocyte prior to becoming a mature lymphocyte.