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The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc.This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though yolk sac is far more widely used.
Blood is conveyed to the wall of the sac by the vitelline arteries (a branch of the dorsal aorta), and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo. This constitutes the vitelline circulation, and by means of it nutritive material is absorbed from the yolk-sac ...
The vitelline veins drain the yolk sac during early embryonic development. [ 1 ] [ 5 ] They also drain the gut tube in embryos once this has formed from the yolk sac. [ 2 ] [ 6 ]
Structures called blood islands form in the mesoderm layer of the yolk sac by cellular differentiation of hemangioblasts into endothelial and red blood cells. [7] Next, the capillary plexus forms as endothelial cells migrate outward from blood islands and form a random network of continuous strands. [ 7 ]
The development of blood formation takes place in clusters of blood cells, known as blood islands, in the yolk sac. Blood islands develop outside the embryo, on the umbilical vesicle, allantois, connecting stalk, and chorion, from mesodermal hemangioblasts.
The definitive yolk sac contributes greatly to the embryo during the fourth week of development, and executes critical functions for the embryo. One of which being the formation of blood, or hematopoiesis. Also, primordial germ cells are first found in the wall of the yolk sac before primordial germ cell migration. After the fourth week of ...
Like the veins, they play an important role in the vitelline circulation of blood to and from the yolk sac of a fetus. They are a branch of the dorsal aorta. They give rise to the celiac artery, superior mesenteric artery, and inferior mesenteric artery. [1]
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 ...