Hematopoiesis

• trigger for heart and vascular system development → too large embryo size
• formation of circulatory and vascular systems starts IN → the mesodermal layers of the yolk sac and chorion
• circulatory or vascular system development starts with blood island formation
• Function of yolk sac hematopoiesis → serves as a temporary adaptation to accommodate the immediate needs of the embryo
• The formation of blood islands begins with the differentiation of cells of the extraembryonic splanchnic mesoderm into hemangioblasts
• Hemangioblasts subsequently form hemangioblastic aggregates
• hemangioblastic aggregates give rise to two cell lineages:

  • primitive hematopoietic stem cells (HPCs)
  • endothelial precursor cells (EPCs)
• The hemangioblasts in the center of the blood islands will form HPCs
• HPCs will form primitive blood cells through hematopoiesis
• The hemangioblasts on the outside of the blood islands differentiate from EPCs into endothelial cells
• hematopoiesis will shift to the liver, where the original extraembryonic HPCs join with a second source of HPCs.
• Definitive HPCs, programmed within the liver, will eventually colonize the bone marrow and other lymphatic organs between the tenth and eleventh week of development.