yolk sac

(noun)

A membranous sac attached to an embryo that provides early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and mammals. It functions as the developmental circulatory system of the human embryo before internal circulation begins.

Related Terms

Examples of yolk sac in the following topics:

  • Yolk Sac Development

    • The yolk sac is vascularized and contributes nutrients to the embryo.
    • The yolk sac is the first element seen in the gestational sac during pregnancy.
    • This vitelline circulation absorbs nutritive material from the yolk sac that is conveyed to the embryo.
    • The yolk sac starts forming during the second week of embryonic development, at the same time of the shaping of the amniotic sac.
    • The yolk sac is a membranous sac attached to the embryo that provides nourishment in the form of yolk.

  • Development of Metabolism

    • As a result, a piece of the yolk sac (the endoderm-lined structure in contact with the ventral aspect of the embryo) is then "pinched off" to become the primitive gut.
    • The yolk sac remains connected to the gut tube via the vitelline duct.

  • Development of the Extraembryonic Coelom

    • During the formation of the primitive yolk sac, some of the migrating hypoblast cells transdifferentiate into mesenchymal cells that fill the space between Heuser's membrane and the trophoblast to form the extra-embryonic mesoderm.
    • The extra-embryonic mesoderm is divided into two layers: the extra-embryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primitive yolk sac; and the extra-embryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.

  • Digestive System Development

    • The result is that a piece of the yolk sac, an endoderm-lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut .
    • The yolk sac remains connected to the gut tube via the vitelline duct.

  • Development of Blood and Blood Vessels

    • In the yolk sac's blood islands, HPCs and EC lineages emerge from the extraembryonic mesoderm in near unison.
    • In 1917, Florence Sabin first observed that the development of blood vessels and red blood cells in the yolk sac of chick embryos occur in close proximity and time.

  • Blastocyst Formation

    • On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and quickly assumes the form of a small sac, called the yolk sac.

  • Sinusoid Development

    • The vitelline veins drain blood from the yolk sac.

  • Fourth Week of Development

    • At the end of the fourth week the yolk sac presents the appearance of a small pear-shaped vesicle (the umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct.

  • Development of Blood

    • In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac, called "blood islands. " As development progresses, blood formation occurs in the spleen, liver, and lymph nodes.
    • Structures called "blood islands" form in the yolk sac of an embryo by cellular differentiation of hemangioblasts into endothelial cells.

  • Early Pregnancy Tests

    • With obstetric ultrasonography the gestational sac sometimes can be visualized as early as four and a half weeks of gestation (approximately two and a half weeks after ovulation) and the yolk sac at about five weeks' gestation.