Ed Zimney, MD

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What exactly are embryonic stem cells?

Stem cells have been in the news a lot this week because of President Obama’s executive order lifting the Bush administration’s strict limitations on federal funding for embryonic stem cell research. But stem cells are quite confusing because they come in so many different forms, involve such complex terminology, and seem to require an advanced degree in science to understand. So let’s try to focus on some basic concepts that are relatively easy to understand, such as what stem cells are, why they are so important, and just what the differences are between embryonic and adult stem cells.

In order to understand stem cells, you first have to know a bit about cells in general. Cells are the basic functional units of life. There are countless organisms that consist of just one cell, such as bacteria. But most organisms, including humans, are made of many cells. In fact, the human body is composed of many trillions of cells (I’ve seen estimates varying from 10 to 100 trillion). So where did those trillions of cells come from? They came from one original cell that was created when a father’s sperm cell entered a mother’s egg cell, forming a fertilized ovum. That first cell divides into two cells which each divide again, forming four cells, then 16 cells, and so on.

That first cell is the ultimate stem cell, the one from which all future cells are derived. Actually, the first few dozen or so cells are equally capable of serving as this ultimate stem cell. But fairly soon an important process of differentiation begins in which cells start to become more and more specialized. Ultimately, after many divisions, most cells become fully specialized – they become heart cells or eye cells or skin cells, to name just a few. But in the beginning, the cells have the potential to eventually become any of these cells. These very early cells are called “totipotent” because they have the potential to be any type of cell.

Let’s get back to embryonic stem cells. There’s a structure called a blastocyst. This is an early-stage embryo, about 4 to 5 days old, which is composed of about 50 to 150 cells (around 6 to 7 cycles of division from the fertilized ovum). Even at this early stage some degree of differentiation has occurred. But the cells shown in blue are not very specialized yet and are therefore called “pluripotent” because they still have a lot of potential to give rise to all the different cells of the body. In fact, these are the embryonic stem cells that everyone is talking about.

Artificial Fertilization and Stem Cells
Of course what normally happens is that the fertilization of the egg by the sperm takes place inside the mother’s body and the blastocyst implants itself inside her uterus and the pluripotent or embryonic stem cells go on eventually to form a fetus, which develops into a baby. In this process, neither the blastocyst nor the embryonic stem cells can be removed from the mother and survive. These normally conceived embryonic stem cells are unavailable for any other use, either in research or for therapy. The only way to obtain embryonic stem cells is first to perform artificial fertilization of an egg that was removed from a woman’s body with sperm donated by a man, using the technique of in vitro fertilization.

When couples have fertility problems they may choose to undergo this process of in vitro fertilization, in which many eggs are removed and many are fertilized with the donated sperm. Eventually, only a few of these fertilized eggs are used in the attempt to impregnate the woman. The rest are either kept frozen, discarded, or donated to medical research. Of course, if they’re not successfully inserted into a woman’s uterus, these blastocysts are incapable of developing into fetuses or babies. If allowed to keep on dividing, the blastocyst would eventually die. But if the fertilized egg is donated for medical research, the pluripotent embryonic stem cells can be removed from the blastocyst and, when properly handled, can be coaxed into dividing over and over but not specializing. Instead they can remain as embryonic stem cells, but thousands and then millions of them can arise.

It’s important to fully understand that the intact blastocyst was at least theoretically capable of developing into a fetus if it had been implanted in a woman’s uterus. But once the decision is made to remove the embryonic stem cells, the blastocyst no longer exists and there is no longer any potential for it to develop into a fetus. This is the step where the embryo is “destroyed,” in that it is no longer an embryo. This is where some people object to embryonic stem cell researc,h because something that had the potential to form a fetus is denied that potential. The way that I look at it, however, is that this potential is hardly real. As I mentioned, these fertilized eggs are created in vitro and have no potential unless they are implanted in a woman. In all cases, way too many fertilized eggs are created that are never going to be used. They will only remain frozen forever, be discarded, or used in research. To my way of thinking, their potential is extremely limited and their only useful potential is for medical research.

I’ve spent a lot of time explaining embryonic stem cells. You can see that their importance to medical research lies in their unspecialized or undifferentiated state. They have the potential or ability, when handled properly, to eventually become any type of cell in the body. With extensive research we may be able to unlock many of their secrets and potentially use such stem cells in treating a myriad of diseases. But that’s a long way off and will require years and years of research. So what about adult stem cells? What are they, where do they come from and how are they different?

Adult Stem Cells
An adult stem cell is much more specialized or differentiated than an embryonic stem cell but not quite as differentiated or specialized as an end organ cell like a heart cell or a skin cell, which are at the end of the line in terms of specialization. Adult stem cells are found within a tissue or organ and they can develop into any of the specialized cells found in that tissue or organ. For many years, for example, we’ve known about stem cells in the blood and the bone marrow where blood cells are formed. As you know, there are many types of blood cells including red blood cells, white blood cells, and platelets. Blood stem cells are less differentiated and have the ability to form any of these types of cells. So if a person is about to undergo cancer chemotherapy, which will destroy many of their blood cells, they could first have some of their blood stem cells removed before treatment. They then would have these blood stem cells reinjected after chemotherapy so they can regrow their now depleted blood cell lines.

But while it’s now easy and routine to harvest blood stem cells, their utility is limited to forming new blood cells. They are already differentiated and can only form blood cells. In addition, while it’s easy to harvest blood stem cells, it’s not that simple to get at other types of adult stem cells. So the utility of adult stem cells is currently limited. There’s lots of research going on in this arena, and the utility of adult stem cells may well expand dramatically in the future.

Stem cell research is in its infancy (no pun intended). It will evolve on many fronts including both embryonic and adult stem cells. The lifting of the ban on federal funding for embryonic stem cell research means that this can now occur at a more rapid rate. But you should also understand that federal funding is still quite limited because of a law that only Congress can change. The President’s executive order allows federal funding for researchers to study embryonic stem cells but not to create them. Their creation must still be funded either privately or at the state level. That’s because there’s a law banning such funding and only Congress can change that law, not the President. Before President Obama’s order, there could be no federal funding of any embryonic stem cell research that wasn’t limited to certain existing stem cell lines. Now, if new cell lines are created through private or state funding, those new cell lines can be further studied using federal funds. This opens the door quite a bit, but, in my opinion, not nearly enough. There needs to be federal funding for embryonic stem cell creation too. When Congress tried to change this law, President Bush vetoed the legislation. It is likely that if Congress again votes to change the law that President Obama will sign it. I look forward to that day.

Last Updated: 3/12/2009
Posted in: healthy Living

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