Anatomy & Physiology of Pregnancy

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  • 0:05 Extraembryonic Membranes
  • 0:44 Amnion & Amniotic Fluid
  • 2:08 Allantois & Yolk Sac
  • 3:02 Chorion
  • 3:45 Placenta & Umbilical Cord
  • 5:24 Lesson Summary
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Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

We know a woman's uterus can house a baby, but did you ever wonder what it looks like inside? Or, how the baby gets food? In this lesson, you'll learn about what the fetal environment inside the uterus actually looks like.

Extraembryonic Membranes

Did you know you started out as a single cell, but now you have hundreds, maybe even thousands of different types of cells in your body? This is because as the fertilized egg begins to develop, its cells are assigned different roles.

Cells inside the embryo become all the different cells in your body. But some cells are located outside the developing embryo and are called extraembryonic cells, or membranes. While this may look like a big word, if you break it down it's pretty easy to understand. ''Extra,'' meaning outside of and ''embryonic'' referring to the developing embryo, so extraembryonic means ''outside the embryo.''

Amnion and Amniotic Fluid

You can think of the extraembryonic membranes as a protective bubble that surrounds the developing fetus, insulating it from the outside environment. This ''bubble'' has 4 different parts to it. First, the layer closest to the embryo is called the amnion. The amnion surrounds the embryo and is filled with amniotic fluid.

You can think of it like a small tiny baby, about the size of a peanut. Now, put that little peanut into a balloon. The balloon is like the amnion, surrounding the baby, or in this case, the peanut. Then, fill the balloon up with water. The water is now surrounding the peanut, protecting it. The water is like the amniotic fluid. Together, the amnion and the amniotic fluid have a few different jobs:

  1. Protect the embryo from injury (like if mom's belly gets bumped).
  2. Provide a safe space for the embryo to move around, giving him or her plenty of free space to practice kicking mom's belly button.
  3. Help the fetus maintain a normal temperature and support proper lung development. Did you know when in the uterus, the fetus is actually breathing in amniotic fluid? The first breath of air doesn't come until after birth!
  4. Transfer waste from the fetus to the mom's bloodstream so her body can get rid of it. Did you know a fetus releases urine? Kind of makes you appreciate your mom that much more.

Allantois and Yolk Sac

Our next structure is called the allantois, which is the precursor to the umbilical cord. This would be like attaching a thread to the peanut and the other end of the thread to the balloon, kind of like an anchor on a boat. This structure actually doesn't stick around the whole time. It is only present during the beginning of fetal development because these cells form the precursor to the umbilical cord blood vessels. Once the umbilical blood vessels are fully formed, the allantois is no longer needed.

The yolk sac comes next. You know that yellow substance you see in the center of an egg? Well, in a fertilized egg, that yolk sac helps to nourish the developing embryo or fetus, at least in most mammals. In humans, however, we have something called the placenta to provide nourishment to the fetus, so our yolk sac is small and eventually becomes part of our digestive and respiratory systems.


And that brings us to the last layer, the one furthest away from the embryo --\ the chorion. This is like surrounding our balloon with another balloon so you have a double layer. The chorion is the outermost layer. It connects the amniotic sac to the placenta. This connection is essential to development because the fetus relies on the placenta for nourishment from the mother.

Not only does the chorion provide the connection between mother and fetus, it also secretes a hormone essential in helping the uterus to maintain pregnancy during the first trimester. This hormone, human chorionic gonadotropin (hCG), is what all those home pregnancy tests detect to let a woman know whether or not she is pregnant.

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