The blood islands and vessels outside of the embryo is initially the sole source of blood cells and plasma, beginning 3 weeks after fertilization. Consequently, some blood islands end up inside the embryo proper, while others remain in the annexes. Over time, the network will continue to grow, and will eventually penetrate the embryo proper and fuse with the concurrently developing embryonic circulation. This forms a plexiform network of blood vessels. Such free blood cells continue to divide.īlood islands will sprout endothelial projections and these projections will fuse. Then the red blood cells break loose and are carried away in the plasma. Later the cells on the surface round-up, giving the mass a mulberry-like appearance. In either case the mass thus formed projects from and is attached to the wall of the vessel. They develop from undifferentiated hemangioblasts in blood vessels in the walls of the umbilical vesicle, allantois and chorion. Roughly 3 weeks after fertilization, red blood cells, still with a nucleus, and blood plasma develop outside the embryo. Mesenchymal cells exterior to this form the muscular and connective tissue components of blood vessels. The flattened cells at the periphery form the endothelium. Within the blood islands, lumens begin to appear by the growth of intercellular clefts. Clusters of angioblasts make up the blood islands. Vasculogenesis begins as mesodermal cells differentiate into hemangioblasts, which in turn differentiate into angioblasts. In humans, the formation of extraembryonic blood vessels starts at the beginning of the third week after fertilization. They are also known as Pander's islands or Wolff's islands, after Heinz Christian Pander or Caspar Friedrich Wolff. Blood islands arise external to the developing embryo on the umbilical vesicle, allantois, connecting stalk and chorion. Blood islands are structures around the developing embryo which lead to many different parts of the circulatory system.
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