An early embryonic membrane that provides nutrients and the first blood cells before the placenta takes over.
Structure
The yolk sac is a small spherical membrane visible on early ultrasound, suspended in the chorionic cavity beside the developing embryo.
Function
Although it contains almost no actual yolk in humans, it produces the embryo’s first hematopoietic stem cells and serves as the source of primordial germ cells that migrate to form the gonads.
Fate
After about week 12 of gestation, the yolk sac shrinks and is incorporated into the umbilical cord, eventually disappearing entirely.
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