The Yolk Sac’s Essential Role in Embryonic Development

The yolk sac is a pouch that envelopes the yolk. In birds and reptiles, it is a large extraembryonic membrane throughout the embryonic period. It supports all stages of embryonic development by supplying the embryo with nutrients stored in the yolk. The yolk is absorbed by the embryo before it hatches, so that in birds, for example, it cannot be observed unless the egg is artificially cracked. In many mammalian species, the yolk sac is a temporary structure, which in humans, for example, regresses by about 15 weeks of gestation. For these reasons, the yolk sac may be considered a mere nutrient-filled sac in birds and an empty, vestigial sac in mammals and has been received less attention than other organs such as the placenta and liver. However, the yolk sac plays a crucial role in development as an extraembryonic organ that absorbs, metabolizes and distributes nutrients essential for embryonic development, contributes to early hematopoiesis and secretes proteins and growth factors necessary for embryonic growth. In this review, we summarize the studies to date and provide perspective on the function of the yolk sac, mainly focusing on avian and mammalian species.