Embryology, Yolk Sac

Excerpt

The yolk sac, or umbilical vesicle, is a small membranous structure outside the embryo with various functions during embryonic development (see Image. Human Embryo at 14 Days With Yolk Sac). The yolk sac reduces in size, communicates ventrally with the developing embryo via the yolk stalk, and later regresses. The yolk stalk is a term that may be used interchangeably with the vitelline duct, omphaloenteric duct, or omphalomesenteric duct. The yolk stalk serves to connect the yolk sac to the midgut, which is an early derivative of the gastrointestinal system (see Image. The Digestive Apparatus). Both the midgut and yolk sac are endodermal in origin. See Image. Embryology.

Although it contains no yolk in humans, the yolk sac has several critical biological functions during early gestation, including primitive hematopoiesis and germ cell production. Clinically, the yolk sac can be visualized using transvaginal sonography by five weeks post-fertilization, and its visual examination serves as a significant predictor of adverse pregnancy outcomes until its disappearance on sonography by the second trimester.