Embryo Implantation and Placenta Formation

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  • 0:06 Zona Pellucida…
  • 1:09 Implantation
  • 1:57 Trophoblast Migration
  • 3:38 Embryonic Circulation
  • 5:20 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
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.

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 occurs when the blastocyst buries itself within the uterine wall
embryo 2

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.

Trophoblast Migration

As the blastocyst implants, the trophoblasts that make up the outer layer of cells multiply in number and invade deeper into the uterine wall. They maintain a protective barrier between the maternal cells and the developing embryo. As you might remember, the trophoblasts are the cells that will eventually form the chorion, or the embryonic-derived portion of the placenta. As the trophoblasts multiply in number and extend further into the wall, 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.

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

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