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During the end of the second week of gestation (12-14 dpc), the lower half of the primary yolk sac is pinched off to form the definitive yolk sac, while a second wave of hypoblast endoderm cells (yolk sac endoderm) form the inner lining of the definitive yolk sac.
Structure and Function:
The definitive yolk sac structure consists of a double-layered extraembryonic membrane with hypoblast-derived endoderm on the inside and mesoderm on the outside. The combination of endoderm and mesoderm layers is called splanchnopleure and appears in both the yolk sac and allantois.
The endodermal tissue supplies functioning epithelial cells, while the mesoderm generates the essential blood supply to and from this epithelium. Additionally, the outer mesodermal layer of the yolk sac constitutes an important site of hematopoiesis in the developing embryo. The yolk sac cavity is filled with fluid called the vitelline fluid. In humans, the yolk sac normally disappears before birth (its residues are rarely found postnatally, as an anomaly called Meckel's diverticulum).
Nutients are absorbed from the yolk sac through a complex capillary plexus and vitelline veins (the vitelline circulation) and conveyed to the embryo.
Several structures develop from the yolk sac. The allantois arises as a tubular diverticulum of the posterior part of the yolk sac, and a constriction in the yolk sac encloses a small part of yolk sac within the embryo and constitutes the primitive digestive tube. Communication with the embryonic digestive system is gradually reduced to a small tube called the vitelline duct (sometimes called the umbilical vesicle).