 Primitive hematopoiesis is first found in the blood islands (Pander's islands) of the yolk sac at E7.5 (embryonic day 7.5) in mice and 30dpc (30 days post-conception) in humans.As the embryo requires rapid oxygenation due to its high mitotic activity, these islands are the main source of red blood cell (RBC) production via fusing endothelial cells (ECs) with the developing embryonic circulation.The hemangioblast theory, which posits that the RBCs and ECs derive from a common progenitor cell, was developed as researchers observed that receptor knockout mice, such as Flk1-/-, exhibited defective RBC formation and vessel growth.