Yolk sac

The yolk sac is an extraembryonic membrane found in many vertebrate animals, including humans, and serves primarily to provide nourishment to the developing embryo. Its appearance and function vary across species.

In mammals, the yolk sac does not contain yolk, unlike in birds and reptiles where it is a primary source of nutrients. Instead, in early mammalian development, the yolk sac is involved in hematopoiesis (the formation of blood cells) and angiogenesis (the formation of blood vessels). These blood cells contribute to the early circulatory system, transporting nutrients from the mother to the embryo via the placenta. It also plays a role in the formation of primordial germ cells, which eventually migrate to the developing gonads and differentiate into sperm or eggs.

As development progresses, the yolk sac's role diminishes. In humans, it is typically visible via ultrasound in early pregnancy. By the end of the first trimester, its function is largely superseded by the placenta. The yolk sac eventually regresses, but remnants may persist and are occasionally visible postnatally.

The structure of the yolk sac consists of two layers: the splanchnopleure (composed of endoderm and mesoderm). The endoderm lining the yolk sac plays a crucial role in nutrient transport and the secretion of proteins.