When your mother’s egg cell was fertilized by your father’s sperm cell, the result was a single cell, called a zygote. That cell had all the information necessary to develop into the person you are today. In other words, it could produce everything necessary to build you. So that single cell had the capability of developing into any human cell. We call such cells totipotent cells. Of course, in order to make all those cells, the zygote had to start reproducing, resulting in an embryo.
As this cell (and its progeny) reproduced, the number of cells in the embryo grew. When that reproduction had produced about 12 cells, you were in the morula stage of your development, and on a microscopic level, you resembled a mulberry. As your cells continued to reproduce, they formed a hollow sphere called a blastocyst. At one end of the hollow sphere, there was a bunch of cells called the inner cell mass, which is represented by the blue cells in the illustration above. That inner cell mass developed into all the organs and tissues that make up your body.1
Interestingly enough, however, the cells in that inner cell mass were no longer totipotent. They could not, for example, form the kind of cells that make up the outer layer of the blastocyst, which are shown in yellow in the illustration above. However, they could end up becoming any of the cells in any of the organs or tissues of your body. As a result, they are called pluripotent cells. As they continued to reproduce, they started “choosing” what kind of cell they would become. Some of those pluripotent cells, for example, became skin cells. Once they did that, we say that the cells had differentiated. This means they lost their pluripotency, and would no longer be able to become some other type of cell. As a result, they would end up doing the same job for the rest of their lives.
Pluripotent cells are often called stem cells, and they have a lot of potential in medicine. After all, if someone suffers from severe organ damage, I could theoretically get his or her body to rebuild that damaged organ if I supplied it with enough stem cells. The stem cells could then differentiate into whatever cells are needed to replace those that died when the organ was damaged. While this sounds wonderful, there is a problem. The most ready source of pluripotent cells come from the blastocyst stage of an embryo’s development. If I remove those pluripotent cells from the blastocyst, I have embryonic stem cells, but unfortunately, the embryo dies.
Are embryos the only source of pluripotent stem cells? No. There are adult stem cells that can be found in any fully-developed person, but there is a problem with them as well. It is hard to find pluripotent adult stem cells. Most adult stem cells have partially differentiated, so the kinds of cells they can develop into are limited. For example, you have adult stem cells in your bone marrow. They can develop into any type of blood cell (red blood cell, macrophage, neutrophil, etc.), but they cannot develop into skin cells, nerve cells, muscle cells, etc. As a result, we call them multipotent. They still have choices as to what kind of cell they can become, but those choices are restricted compared to those of a pluripotent cell.
So if you want pluripotent stem cells, embryos are the easiest way to get them. Of course, it is also the unethical way to get them, because you end up killing a human being. So people who are concerned about ethics have been trying to find another way to get pluripotent stem cells. Back in 2007, three separate research groups found a way to take an adult skin cell and reprogram it to become pluripotent again.2 While those three groups (and others) have shown that such cells, called induced pluripotent stem cells, can develop into many different types of cells, one important question remains: Are they truly as pluripotent as embryonic stem cells?
Well, we are one step closer to answering this question, thanks to researchers at the University of Wisconsin-Madison. In a study published online September 11, they compared four embryonic stem cells to four induced pluripotent stem cells. Specifically, they compared the proteins that the cells produced. After all, a skin cell produces a set of proteins that is quite distinct from a muscle cell, so the set of proteins that a cell makes is a good probe of what the cell can actually do.
What was the result? They found that the embryonic stem cells and the induced pluripotent stem cells are at least 99% the same when it comes to the proteins they produce!3 In fact, in some measurements, they found that one embryonic stem cell produced proteins that were more similar to one of the induced pluripotent stem cells than another embryonic stem cell. As a result, it seems that from a protein standpoint, there is very little difference between embryonic stem cells and induced pluripotent stem cells.
This is great news for those of us who want to see the medical advances that pluripotent stem cells can produce without the murder that accompanies embryonic stem cell use!
1. Rod R. Seely, Trent D. Stephens, and Philip Tate, Anatomy and Physiology, Fifth Edition, 2000, pp. 962-963
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2. Cyranoski D, “Simple switch turns cells embryonic”. Nature 447: 618–619, 2007.
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