yolksac

The yolk sac, often written as yolksac, is a membranous structure that forms early in vertebrate development and is present in humans and many other mammals as a transient, supportive component of the embryo. In humans, it is relatively small and serves roles that give way to placental nutrition as gestation progresses.

Development and structure: The yolk sac originates soon after implantation from cells of the yolk sac endoderm and surrounding extraembryonic mesoderm. It is connected to the embryo by the vitelline or yolk stalk. In many descriptions, the initial cavity is the exocoelomic (primitive) cavity, and a secondary yolk sac forms as development continues. The yolk sac comprises an inner endodermal lining with an outer mesodermal layer, surrounding a fluid-filled space that communicates with the developing gut via the yolk stalk.

Functions: In early development the yolk sac serves as a site of primitive nutrient transfer and supports key developmental processes before placental circulation is established. It is also the initial site of hematopoiesis (blood cell formation) and contributes to the formation and migration of primordial germ cells. The yolk sac participates in early gut development by contributing to the formation of the primitive gut.

Clinical and comparative notes: In pregnancy, the yolk sac is often visible by ultrasound in the first trimester and can be a marker of viability; abnormal size or absence can be associated with adverse outcomes. In some cancers, yolk sac tumor (endodermal sinus tumor) arises from germ cells and resembles yolk sac tissue. Across vertebrates, yolk-containing structures have evolved to support embryonic nutrition and development, with variations in their prominence and function.