This lesson will explain how a zygote develops into an embryo.

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Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today.

We are going to be discussing the process that occurs from the time a zygote develops to the time it is implanted in the uterus.

A zygote is a fertilized egg that's made up of one cell. So it's an individual's first cell, and this zygote will divide repeatedly in order to eventually develop into a human being.

Implantation generally will occur six to seven days after conception. There's various steps that happen from the time of conception to the time of implantation, and we're going to discuss those today.

A zygote, as I mentioned, which is that fertilized egg, has to undergo various cell divisions before it can implant in the endometrium of the uterus.

Fertilization usually occurs in the ova duct, so that zygote will then start to undergo these cell divisions as it moves through the ova duct on its way to the endometrium.

We're going to take a look at this diagram right here, and discuss the processes that happen as that zygote is dividing, moving through the ova duct, and towards the uterus.

OK. So the first thing that happens-- so we start with our zygote which is our one cell. And eventually that cell will divide into two cells.

We call this here the cleavage furrow.

Cleavage is basically just that process of starting with one cell, and building up this ball of cells that will eventually become a human.

And then these little things right here are the polar bodies. And those will eventually disintegrate, so we're not going to spend much time today discussing their structure and function.

This is around day one or two.

We started with our one cell. That cell has divided by the process of cleavage, and we now have two cells.

Now each of those cells, is then going to divide, as well. So then we're going to end up with four cells. A ball of four cells.

Each cell is dividing by the process of mitosis. We're starting with one cell. Dividing into two. Each of those is dividing, and then we four cells.

Each new cell that has formed is called a blastomere.

A blastomere is each new cell that's formed during this process of cleavage.

Each blastomere is going to end up with a little bit of the egg's cytoplasm. And the bit of cytoplasm that it ends up with is going to determine the developmental path of that cell.

So we have these four cells here. Those are then going to divide, and eventually we're going to end up with this ball of 16 cells. I'm not going to draw them all right now, but we're going to end up with this ball of 16 cells.

And this ball of 16 cells is called a morula. Oops. I can't spell today. M o r u l a.

So this is called a morula.

Eventually, as this morula moves towards the uterus, it is going to develop into a blastocyst.

As the morula reaches the uterus, around day five, it starts to develop this fluid-filled cavity. It develops into a blastocyst.

This blastocyst is made up of different layers, as well. We have the trophoblast.

Trophoblast, which is the surface layer of cells, and then in here we have our inner cell mass.

The inner cell mass is what is actually going to develop into an embryo.

So the embryo develops from this inner cell mass.

And then this blastocyst eventually will, as it moves towards the uterus, will implant into the uterus. And then, as I mentioned, the embryo will develop then from the inner cell mass of the blastocyst.

This lesson has been an overview on how a zygote will eventually implant into the endometrium of a uterus.

  • Zygote

    The fertilized egg which is the first cell of an individual.

  • Blastocyst

    After the morula forms it will implant on the endometrium of the uterus; the blastocyst contains an inner of layers of cells that become the embryo while the outer layer of cells form the placenta.

  • Inner Cell Mass

    A layer of the blastocyst which will develop into an embryo.

  • Embryo

    A stage of development of an individual formed from the inner cell mass of a blastocyst after it has implanted in the uterus.