Stem cells and differentiation | From cells to organisms | High school biology | Khan Academy

To me, one of the most fascinating ideas in biology is that we all started as a fertilized egg. So, that is a cell right over there. And then, through many, many divisions, all of a sudden—I wouldn't say all of a sudden; it takes many months to develop even into a newborn baby—but eventually, you have a grown human being that has, depending on size, 30 to 40 trillion trillion cells. And these cells are not all the same.

For example, if we are thinking about the brain, you have a bunch of neurons in here. So, that is one type of cell. If you look at the bones, so let's look at this bone right over here; that is another type of cell—bone cells. And there are more technical terms for these, but if you look at this individual's, let's pick an organ, let's say their heart, this will be cardiac cells. So we could even say heart cells. I’m just scratching the surface; there are so many different—not only are there so many different cells, but there are so many different types of cells that if you were to look at each of them under a microscope and look at their function, they all seem to be very different.

But they all came from that original fertilized cell. So it's a big question: How did we get from one to 30 to 40 trillion cells that are very different from each other—in many cases, very different from each other? And how did these cells know to become that way? Well, it all boils down to stem cells, which have the ability to differentiate into specialized cells but can also have self-renewal; they can divide many times while remaining unspecialized.

The reason why it's called stem cells—you can imagine a plant. So, this is a plant right over here; the stem is the thing that is connected to the roots. And from that, you have all the branches of everything else. So, you can imagine stem cells can branch off and differentiate into all of the different types of cells you need in the body.

And so, that first fertilized egg is a stem cell, and it is a stem cell that can turn into any other type of cell. Because of that, we call it totipotent. Let me write that down: totipotent. This idea of potency—think of the word potential. We're talking about what does it have the potential to become.

Potency describes the ability of a stem cell to differentiate into specialized cells. Totipotent means it can get differentiated eventually into anything. But what's going to happen is that cell—that fertilized egg—is just going to keep dividing from one to two, and then from two to four, two to four, and it's just going to keep going; it's going to keep going. I'll do some dots—dot dots here.

So, after a bunch of divisions, now you have a bunch of cells here. We could call this the embryonic stage, and there is tissue around the embryo that also came from that first totipotent cell. So maybe we show some like that. But if we're looking at the embryos right over here, those are also stem cells, and they're not quite totipotent; they're pluripotent. PL-potent—you might have heard of embryonic stem cells. That's what the types of stem cells that people are most often referring to.

And these can differentiate into all of the body’s cell types, except for the tissues surrounding the embryo. Embryonic stem cells are pluripotent. But then you keep going, and I'm going to keep going—dot dot dot. So, you're going to have some of these that start going to create tissues of certain, say, organs. Maybe these are going to be heart tissue, but they haven't differentiated into the special type of heart tissue yet. Maybe these right over here are differentiating into liver tissue, and there are going to be many more cells than what I'm drawing here, like liver tissue, but they haven't differentiated into the specific type of liver tissue.

So, these we would call multipotent—multi-potent. So, they still have the ability; they have the potential to become other, more differentiated, more specialized types of cells, but they’re going to stick to that tissue type. Sometimes you might hear of adult stem cells. These can only become the cell types within a specific tissue.

For example, intestinal stem cells can only differentiate to replace cells in the lining of the intestines. And eventually, you're just going to keep dividing, and these cells are going to become more and more differentiated to the point that you eventually become this human being here, who still will have some of these adult stem cells that can help repair, as I just said, some of the tissue. But you're not going to have that embryonic stem cell anymore.

Now, we could talk more about that, but one of the really amazing things about this is how do you get all of this difference from that original cell? It turns out that every cell in your body, except for a few exceptions like red blood cells, they all have DNA, and they have the exact same DNA.

So, for example, if this is a section of a given chromosome, and let's say that these are the genes on that section of the chromosome—so how does one cell become, say, a neuron, and another one becomes a bone cell, especially if they have the same DNA?

So, what turns out is—every cell has the same DNA that has the same genes on it, but they express different combinations of those genes. So, for example, if this top one over here is a nerve cell, maybe it expresses this gene, this gene, and this gene, while if this is a bone cell right over here, it might express this gene, this gene, and this gene. And that's why they are different.

Now, it's a whole area of research on how does a cell know to differentiate in a certain way and how they can send signals to each other. But I will stop there because I think I might have blown your mind enough that this all happened to create us, and it somehow all works.