The Promise of Stem Cells

We were all, once upon a time, a cluster of stem cells. From those we grew, and all other cells in the body came to be. And as our bodies rise triumphant from daily wear and tear, intact and ready to face the world anew, it is a stock of stem cells that replenishes tissues worn by age, or lost to injury and disease.

Researchers argue that stem cells show extraordinary potential for treating a broad variety of debilitating diseases--from Parkinson’s to diabetes. Today, policymakers are weighing the cells’ therapeutic promise against the ethical implications of their origin. Only last week, a presidential commission pondered whether research with stem cells derived from human embryos should be funded.

Yet, as the controversy unfolds, many people remain baffled over what these cells really are, and what their future applications might be.

“Even the people making policy decisions are a bit confused,” said John Gearhart of Johns Hopkins University, who leads one of the several laboratories that isolated human embryonic stem cells last fall.

Technically, a stem cell is a cell without a fixed identity that will produce, after dividing, two cells with different fates: One that remains a stem cell, and another that goes on to specialize into any one of the more than 200 different cell types in the body.

“The problem is that there are cells at various stages of development that [fit this definition],” said Terrence Deacon, a stem cell researcher at Boston University.

The term “stem cell” is generic, attributed to cells in the mature body but also cells at various phases of development, including those at the earliest stages of embryogenesis.

The most familiar of the stem cells are perhaps the embryonic stem cells, coveted for their remarkable potential to develop into every single type of cell in the organism.

“These are literally the mother of all stem cells, and will become the ancestors to the rest of the stem cells in the body,” Deacon said. Like a blank page, embryonic stem cells do not have any information as to their future identity, so they have endless possibilities.

It is these cells that lie at the core of the national debate over stem cell research, because they are only found during early development, after the fertilized egg has gone through a few divisions to form a clump of about 140 cells. To study the human version of these cells, scientists must harvest them from embryos, which has not come without ethical dilemmas because the embryos are destroyed in the process.

Less controversial and just as promising are the stem cells that come from the adult body.

Masters of restoration, these cells are the body’s reservoir of spare tissue. They cycle through our bloodstream or hide in many organs, waiting for body cues to produce specific cells. Halfway down the path to specialization, they can produce some but not all types of cells.

The so-called “mesenchymal” stem cells, for example, can produce bone or fat cells. They cannot, however, make brain cells, which is generally the job of the neural stem cells.

Whatever the type of stem cell, scientists hope to one day use them therapeutically for making specialized cells, like brain or heart muscle cells. Newly made cells would be injected back into a diseased organ, restocking the supply of functional tissue.

At present, the differences between the embryonic stem cells and the more specialized stem cells have gained special medical and legal relevance.

Some say that research with human embryonic stem cells might not be necessary, because the more specialized stem cells carry similar promise. Since the latter come from adults, the destruction of human embryos could be avoided, the argument goes.

Also, researchers can control the fate of the more mature stem cells, directing them to become a bone cell or a cartilage cell by simply adding different compounds. In contrast, scientists do not yet know how to make a specialized cell from an embryonic stem cell.

On the other hand, embryonic stem cells might be the only way to make spare cells for organs that naturally do not have a stem cell reservoir in the adult body.

“There are a fair number of tissues in the adult that do not have stem cells,” said James Thomson, whose group at the University of Wisconsin isolated human embryonic stem cells last November. For those tissues or organs--one example is the heart--embryonic stem cells are the best bet for making replacement tissue, he said.

So is there a way to tell which type of stem cell is better, and which should be the focus of research efforts? According to scientists, not yet.

“All of this stem cell work is so early in the game that you can’t say which one is preferred and which one is not preferred. We just got to let this play out, at least a little further,” Gearhart said.

But Gearhart and many colleagues say the debate about stem cells has been further confused by several incorrect perceptions. Some of the misconceptions could be raising false hopes for people.

“We have had a lot of requests for organs and body parts. We are not growing organs, we are just growing cells,” he said.

While stem cells may one day be used to repopulate diseased tissues or organs, an organ is a very complex structure and, for now, it is impossible to direct a stem cell into forming a whole organ.

People also worry about the potential of the cells to develop into an entire human being, and about their link to the controversial issue of cloning.

Embryonic stem cells, however, are incapable of forming a human embryo on their own because they have been stripped from the membranes and cells that provide the environment an embryo needs to grow. And while it is possible to create a cloned embryo from which stem cells could be harvested, research with embryonic stem cells can be done without using any cloning procedures, Thomson stressed.

As the political debate continues, researchers focus on more basic questions, such as finding the perfect conditions for stem cells to thrive, since the cells have been very difficult to grow in laboratory conditions.

The answers could be just around the corner. Recently, for example, scientists at the University of Washington, the Seattle Biomedical Research Institute and the Veterans Affairs Puget Sound Health Care System announced that they had finally succeeded at “mass producing” blood-making stem cells from mice.

Other immediate goals include figuring out what makes a stem cell stay unspecified, or which molecular switches tell stem cells to begin to specialize.

“We are electricians right now, finding out which switches need to be thrown,” said Byron Petersen, a stem cell scientist at the University of Pittsburgh, of the challenges that lie ahead for stem cell research.

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Regenerating Life

As they divide, stem cells produce more stem cells, as well as cells that specialize into more than 200 other cell types in the body.

STEM CELLS FROM EMBRYOS

After a fertilized egg has gone through a few divisions, it forms a cluster of about 140 cells known as a blastocyst. It is only at this point that embryonic stem cells can be extracted and used for research.

Fertilized egg, Blastocyst, Embryonic stem cell

As they receive proper cues, embryonic stem cells go on to make all tissue types... Heart muscle tissue, Brain tissue, Muscle tissue

Scientists are hoping to discover how embryonic stem cells become specialized so they can develop techniques for repairing diseased tissues and organs.

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STEM CELLS FROM ADULTS

The adult body has a reservoir of stem cells that are more specialized than embryonic stem cells but still have relatively broad potential to be converted into other types of cells.

Stem cells are extracted from the blood of tissues of an adult... then converted in the lab to a variety of cells.

Source: Dorling Kindersley Ultimate Visual Dictionary; The Human Body