This lesson explores extraembryonic membranes in humans.
We’ll discuss some basics of embryo anatomy, define extraembryonic membranes and their types, and look at the function of each type in detail.
What Are Extraembryonic Membranes?
When you’re making eggs for breakfast, you’re probably not thinking about how an egg is one of the largest cells on Earth. That’s right, the chicken egg is a single cell, just like the tiny cells in our bodies! The chicken egg you made for breakfast wasn’t fertilized, though. If the chicken had mated with a rooster before laying the egg, an embryo would have grown inside the egg.The layers inside the egg around the embryo are called extraembryonic membranes, and they nourish and protect the embryo. The yolk that we eat is actually a food source for the embryo growing inside the egg, which is why it’s so rich in calories.
Although humans might look quite different from chickens, we share a common ancestor from millions of years ago. And as it turns out, our embryonic development parallels that of the chicken: Humans and chickens have similar extraembryonic membranes! Mammals and birds (and even reptiles) produce four different kinds of extraembryonic membranes to protect the embryo: amnion, yolk sac, allantois, and chorion. Let’s explore these in detail.
The amnion is the innermost membrane of the embryo.
It surrounds the embryo, creating a fluid-filled cavity. The amniotic fluid inside protects the embryo from mechanical stress and impact.Think of the amnion as the packing material inside a package. If you were to ship a fancy wine glass to your friend, the bubble wrap would prevent it from slamming around and breaking. The amnion has the same function, and as the embryo grows, the amnion grows with it. This enlarges the amniotic cavity, allowing for growth and movement of the fetus later in pregnancy.
In birds, reptiles, and egg-laying mammals, the yolk sac is a structure that develops on the ventral side (or belly side) of the embryo, providing nutrition for the growing embryo inside the egg.
The gooey, yellow inside of sunnyside-up eggs are actually the yolk sacs. As we mentioned earlier, they provide food for the chick embryos. However, in humans, the yolk sac no longer serves a nutritional purpose. Food is provided directly from the mother through the placenta, an organ outside the extraembryonic membranes that provides nutrients and removes waste for the embryo.Instead, the human yolk sac contributes cells that will become blood, sperm, and egg cells in the fetus.
During development, the yolk sac moves away from the embryo, morphing into a yolk stalk with the yolk sac appearing as a ball at the end. As cells migrate from the yolk sac up the yolk stalk to the embryo, the stalk eventually pulls away entirely and is no longer present in the fetal stage of development. So instead of providing food as in other animals, the yolk provides cells that actually become part of the fetus later in development.
The allantois is a growth from the hindgut of the embryo, or the region that will eventually become the lower digestive tract. In other species, the allantois is used for respiration and storage of waste. In humans, blood vessels develop from the allantois that become part of the umbilical cord, the tissue that attaches the fetus to the placenta, allowing it to get nutrients from the mother and eliminate waste.Think of the umbilical cord as a lifeline to a deep sea diver. The cord attaches the diver to the ship and provides oxygen for the diver. The umbilical cord does the same thing, acting as a lifeline attaching the embryo to the maternal tissue.
The chorion is the outermost extraembryonic membrane, which is the bridge between the embryonic membranes and the placenta.
As the chorion first develops in the second week of pregnancy, it forms projections called chorionic villi. The chorionic villi grow and interface with the maternal part of the uterus. As they grow, they form the placenta, which nourishes the embryo and removes waste. The embryo is attached to the placenta through the umbilical cord, which in turn attaches to the chorionic plate that encloses the chorionic cavity.
Extraembryonic membranes are the layers enclosing the embryo inside the uterus. There are four layers: the amnion, yolk sac, allantois, and chorion. The amnion is the innermost layer, enclosing the embryo in the amnion fluid, which protects it from mechanical stress. The yolk sac is used for nourishment in egg-laying animals, but it supplies cells that form parts of the fetus like eggs, sperm, and the gut in humans.The allantois is also used for a different purpose in humans, compared to other animals. It forms parts of the umbilical cord, which attaches the fetus to the placenta.
The chorion is the embryonic part of the placenta and grows chorionic villi to attach to the maternal side. The umbilical cord attaches the embryo to the chorionic plate, which encloses the chorionic cavity.