- Yolk sac
- Not all yolk has to do with birds eggs. Human embryos have a yolk sac, a membrane outside the embryo but connected by a tube (the yolk stalk) though the umbilical opening to the embryos midgut. The yolk sac serves as an early site for the formation of blood and in time is incorporated into the primitive gut of the embryo.
* * *yolk sac n a membranous sac of most vertebrates that is attached to an embryo and encloses the yolk, that is continuous in most forms including humans through the omphalomesenteric duct with the intestinal cavity of the embryo, that is abundantly supplied with blood vessels which transport nutritive yolk products to the developing embryo, and that in placental mammals is nearly vestigial and functions chiefly prior to the formation of the placenta
* * *the membranous sac, composed of mesoderm lined with endoderm, that lies ventral to the embryo. It is one of the extraembryonic membranes. Its initially wide communication with the future gut is later reduced to a narrow duct passing through the umbilicus. It probably assists in transporting nutrients to the early embryo and is one of the first sites where blood cells are formed.
* * *the extraembryonic membrane that connects with the midgut; formed during the development of higher vertebrates. In human embryos, the yolk sac is formed in the second week of development from the exocoelomic membrane and exocoelomic cavity as the primary yolk sac. As the extraembryonic coelom forms, extraembryonic endodermal cells migrate from the hypoblast inside the primary yolk sac and form a smaller secondary yolk sac, pinching off a large portion of the primary yolk sac, which decreases in size and eventually disappears. In human embryos, the yolk sac contains no yolk and does not serve a primary nutritive function; it is often called the umbilical vesicle. However, it does serve several functions; it is the first hematopoietic organ of the embryo and its vitelline circulation plays an important role in the early embryonic circulation. It is also the site of origin of the primordial germ cells. During organogenesis, the yolk sac provides endodermal cells that form the primordial midgut; lateral folding of the embryo reduces the connection between the midgut and yolk sac to a yolk stalk. The yolk sac shrinks and by 20 weeks is usually undetectable. In marsupial and placental mammals, the yolk sac produces a complete vitelline circulation in the early embryo and then undergoes regression; in oviparous vertebrates, it encloses the yolk mass, breaks down yolk, and makes it available to the developing organism.
Yolk sac of an embryo at Carnegie stage 14, about 32 days.
Medical dictionary. 2011.