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Amniotic Fluid, The Amnion, and the Yolk Sac

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Lesson Transcript
Instructor: Joshua Anderson
From a chick developing inside an egg to a human baby growing in its mother's womb, all developing vertebrates rely on a support system to protect them, feed them, supply them with oxygen, remove wastes, and do much, much more to ensure that the embryo grows and develops into a fully functional organism. In this lesson, you'll learn about the different parts of that support system and some of the things that they do.

Extraembryonic Membranes in Egg-Laying Vertebrates

Illustration of the four extraembryonic membranes of egg-laying vertebrates
Extraembryonic Membranes in Egg-Laying Vertebrates

The placenta is clearly the most important extraembryonic structure for placental mammals; however, there are other extraembryonic structures that also contribute to the development of mammals and other vertebrates as well. First, let's talk about the functions of these structures during the development of a vertebrate that lays eggs, like a bird, since egg-laying mammals evolved before placental mammals.

Birds and other egg-laying vertebrates have four different extraembryonic membranes: the chorion, the allantois, the yolk sac, and the amnion.

In vertebrate eggs, the chorion is the outermost membrane and lines the inside of the eggshell. You may remember, though, that in mammals, the chorion forms the embryonic portion of the placenta.

The allantois is the sac-like extraembryonic membrane that removes waste from the embryo. As the embryo grows, so does the allantois, eventually pressing against the chorion. This allows the allantois to exchange oxygen and carbon dioxide with the air outside the eggshell. In addition, the allantois also serves as a disposal site for uric acid. The yolk sac is the extraembryonic membrane that surrounds the egg yolk. The yolk sac has a well-developed vascular system that transports nutrients from the egg yolk to the developing embryo. And finally, the amnion is the extraembryonic membrane that surrounds the developing embryo. The amnion is filled with fluid, and its main job is to serve as a shock absorber to protect the embryo against any jarring impacts or movements.

Illustration of the chorion in placental mammals
Chorion Illustration

As you can see, the four different extraembryonic membranes each have a specific function in vertebrates that lay eggs. However, when mammals began using the placenta to nourish their unborn offspring, the needs of the embryos changed and some of the extraembryonic membranes were adapted to serve other purposes. Please note that for the purposes of this lesson, when I talk about mammals, I am referring to placental mammals.

The development of egg-laying mammals, like the duck-billed platypus and the echidna, more closely resemble the development of other egg-laying vertebrates, like birds and reptiles.

Extraembryonic Membranes in Placental Mammals

As you may remember, in mammals, the chorion forms the embryonic portion of the placenta. Before implantation, the mammalian and bird chorions are quite similar in structure; however, as the mammalian embryo implants and the placenta develops, it becomes far more complex and serves a very different function from the chorion of birds. The allantois also has to be greatly modified to serve the purposes of placental mammals. Instead of forming a sac-like structure to dispose of wastes, the allantois of mammals forms part of the umbilical cord. It is interesting to note that in this case, evolution conserved the waste disposal function of the allantois, but the structure of the allantois was greatly modified to form an umbilical cord that carries the waste away from the embryo and towards the mother's blood vessels. Placental mammals have no yolk, so the yolk sac contains only fluid and does not play a major role in nourishing the embryo. However, the mammalian yolk sac does maintain its well-developed vascular structure and contributes to the vascular system of the developing embryo.

Examples of placental mammals
Placental Mammals

Of all four extraembryonic membranes, the only one that maintains the same form and function in birds and mammals is the amnion. Just like in birds, the amnion of mammals forms a fluid-filled sac around the embryo and serves as a shock absorber. One could make the argument that since mammalian embryos go wherever their mothers do, the function of the amnion is more crucial in mammals than it is in egg-laying vertebrates whose eggs may not move between the time that they are laid and when the offspring hatch.

In any case, the fluid contained inside the amnion is called amniotic fluid and is also necessary for normal development of the lungs and limbs.

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