This wall Once the blastocyst reaches the

This lesson will cover the journey the mammalian embryo first makes after it has been fertilized. Learn how maternal blood moves through the embryo, and how the placenta is formed.

Zona Pellucida Prevents Implantation

The early embryo develops as it travels down the fallopian tube
embryo 1b

While the early mammalian embryo develops on its way down the fallopian tube, it is still surrounded by the zona pellucida which is a layer of extracellular matrix surrounding the mature oocyte, fertilized zygote and pre-implantation blastocyst.

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The zona pellucida surrounds the developing embryo during its entire journey down the fallopian tube and prevents the embryo from attaching prematurely to the wall of the fallopian tube. If the embryo implants in the fallopian tube, it will grow larger and cause an ectopic pregnancy outside of the uterus, which is the mammal-specific organ where prenatal development occurs. The fallopian tube isn’t big enough to handle a developing embryo, so a prematurely implanted embryo will outgrow the tube. Without intervention, the tube will burst, which can be fatal to the mother. But the blastocyst does eventually need to implant in the uterus, so how does it do this if it’s surrounded by the zona pellucida?

Implantation

Implantation occurs when the blastocyst buries itself within the uterine wall
A network of sinuses forms and are filled with maternal blood
embryo 3

Even though the blood comes from the mother’s circulatory system, flows through the sinuses, and then re-enters the mother’s blood vessels, it’s important to note that the sinuses are not blood vessels because they do not have a lining of endothelial cells to contain the blood. Instead, the blood comes into direct contact with the embryonic trophoblast cells.

These trophoblasts, interspersed with the blood-filled sinuses, surround the entire developing embryo. Eventually, the blood-filled sinuses expand and the trophoblasts form fingerlike structures called chorionic villi. These fingerlike structures of the placenta composed of embryo-derived trophoblasts become surrounded by maternal blood in what is now called the intervillous space.

Embryonic Circulation

In the meantime, the embryo itself continues to grow and develop. As it does, one of the very first organ systems to develop is the circulatory system. At only three weeks after implantation, the embryo already has a basic circulatory system, which in addition to growing within the embryo itself, also grows and extends into the chorionic villi as well.

These embryonic blood vessels grow very close to the intervillous space, which allows nutrients to diffuse from the maternal blood, through the trophoblasts and into the embryonic bloodstream. While nutrients are entering the embryonic blood stream, waste products diffuse out of the embryonic blood vessels into the trophoblasts, and finally, into the maternal blood, which carries it away from the embryo.

The placenta sits on one side of the embryo
embryo 4

Once the embryonic circulatory system has been established, the continuous supply of nutrients allows the embryo to begin to actually grow in size and mass. As it grows and expands, it pushes the chorionic villi furthest away from the maternal blood supply into the uterine cavity. These villi that end up in the cavity eventually degenerate, and the maternal blood supply retreats from these areas as well.

This leaves functioning chorionic villi and maternal blood on only one side of the embryo. This composite structure of embryonic and maternal tissues that supplies nutrients to the developing embryo is called the placenta.

Lesson Summary

Let’s review. While the early mammalian embryo develops on its way down the fallopian tube, it is still surrounded by the zona pellucida, which is a layer of extracellular matrix surrounding the mature oocyte, fertilized zygote and pre-implantation blastocyst. The zona pellucida surrounds the developing embryo during its entire journey through the fallopian tube and prevents the embryo from attaching prematurely to the wall of the fallopian tube.

Once the blastocyst reaches the uterus, it uses a protein digesting enzyme, or protease, to make a hole in the surrounding zona pellucida. It then squeezes through the hole to hatch out of the zona pellucida. Proteins on the outer surface of the blastocyst bind to the extracellular matrix of the uterine wall and allow the blastocyst to attach.

Once attached, the blastocyst secretes a number of different proteases that digest the extracellular matrix of the uterus and allow the blastocyst to bury itself in the uterine wall. This process by which the blastocyst binds to and then buries itself within the uterine wall is known as implantation.As the blastocyst implants, the trophoblasts that make up the outer layer of cells multiply in number and invade deeper into the uterine wall.

As they do this, they encounter and surround maternal blood vessels that already exist within the maternal tissue of the uterus. These maternal blood vessels then fuse with spaces between the trophoblasts to form a network of sinuses that are filled with maternal blood.Eventually, the blood-filled sinuses expand and the trophoblasts form fingerlike structures called chorionic villi. These fingerlike structures of the placenta composed of embryo-derived trophoblasts become surrounded by maternal blood in what is now called the intervillous space.

Embryonic blood vessels grow into the chorionic villi, very close to the intervillous space, which allows nutrients to diffuse from the maternal blood, through the trophoblasts and into the embryonic bloodstream. This composite structure of embryonic and maternal tissues that supplies nutrients to the developing embryo is called the placenta. It is the placenta that supplies the nutrients to the developing embryo, and allows it to grow larger.

Learning Outcomes

After watching this lesson, you should be able to:

  • Define the different areas a mammalian embryo goes to after being fertilized
  • Explain how implantation occurs
  • Understand the importance of the placenta
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