Yolk Sac in Humans: Function, Definition & Measurement

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  • 0:00 What Are Human Yolk Sacs?
  • 0:49 Yolk Sac Functions
  • 1:22 Yolk Sac Measurements
  • 2:12 Lesson Summary
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Lesson Transcript
Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

When you hear the term 'yolk sac', you probably think of the eggs you eat, but humans have yolk sacs too! Read this lesson to learn why and how a yolk sac is used by human embryos.

?!!!What Are Human Yolk Sacs?

What do you think of when you hear the term yolk sack? If you're like a lot of people, you might immediately think of traditional chicken eggs. But chickens aren't the only ones using this evolutionary goldmine. Turns out, human embryos have yolk sacs too. In humans, the yolk sac attaches outside the developing embryo and is connected to the umbilical cord by a yolk stalk. This yolk sac acts as the preliminary circulatory system and is eventually absorbed into the gut of the embryo.

The yolk sac is lined by extra-embryonic endoderm and mesoderm. It was originally thought that the human yolk sac was a vestigial organ, no longer of use to the embryo, but research over the past decade or so has brought new insights to the use of the yolk sac by the embryo.

Yolk Sac Functions

The primary purpose of the yolk sac is to provide nourishment for the embryo at the earliest stages of development. This makes sense, because that is also the purpose of yolk sacs in other organisms, like the chicken eggs we originally talked about. The yolk sac is also responsible for the initial circulation and is in charge of delivering nutrients, via a primitive aorta, to the developing embryo through a process called vitelline circulation. The embryo usually develops a circulatory system by week 12, which is when the yolk sac ceases its own circulatory functions.

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