yolk sac (redirected from inverted yolk sac)

yolk sac

n.

A membranous sac attached to an embryo, enclosing food yolk. Also called umbilical vesicle.

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yolk sac

one of the extraembryonic fetal membranes that balloons out from the fetal midgut. It helps to form a primitive placenta and promotes the development of the vitelline circulation. The vestigial yolk sac can be found about halfway along the small intestine of birds. The yolk sac membrane produces lymphoid stem cells that subsequently colonize the thymus and bursa of Fabricius. The stalk of the yolk sac is sometimes retained as Meckel's diverticulum of the small intestine.

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yolk sac infection

see avian omphalitis.

inverted yolk sac

a form of placentation in which fetal splanchnic mesoderm in direct contact with maternal uterine tissue; occurs in laboratory rodents. Called also yolk sac placenta.

yolk sac placenta

see inverted yolk sac (above).

yolk sac tumor

shows varying stages of differentiation into tissues of multiple germ layers. Called also endodermal sinus tumor.