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What are stem cells?

By Ruth SoRelle, MPH

The cells that make up the human body are the basic functional units make life possible.

As Lawrence S.B. Goldstein, PhD, professor of cellular and molecular medicine at the University of California, San Diego, School of Medicine, describes it, "It's like a small business. Just as small businesses are specialized to make a specific product or perform a specific service, cells have different jobs. They are specialized or differentiated. Beta cells make insulin, neurons transmit electrical signal. Heart cells contract to pump blood."

When these cells can no longer perform their specialized functions, diseases result. In cases such as heart failure, the cells are alive but damaged. They no longer contract as well as they used to. In diseases such as diabetes, the beta cells are dead and can no longer produce insulin.

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Powerful tools
Stem cells are those cells in the body that either give rise to more cells like themselves or to differentiated cells that can perform specialized jobs in the body.

"That's why these cells are so remarkable," said Goldstein. "If we can learn to control them and grow them in large quantities, they are the raw material to replace damaged cells or organs. They may become powerful tools that allow us to better understand and combat disease."

"All of these are diseases we would like to treat but have inadequate therapies," said Goldstein. "There are bottlenecks to many treatments. If you need a bone marrow transplant for leukemia, if we can find a genetic match, you are in good shape. If not, you are in tough shape."

Islet cell transplants to treat diabetes face the same problems. There is an inadequate supply donor pancreas to treat type 1 diabetes.

Using human embryonic stem cells as therapy takes advantages of both of their activities. On one hand, scientists are looking at ways to grow large quantities of the human embryonic stem cells in culture.

"In this form, they are the raw materials to replace damaged cells and organs," said Goldstein.

Exposing them to specific signals in the form of molecules can cause them to become more specialized cell types that can perform the functions of individual organs

"The way to think about stem cell biology is not just as a one-trick pony," he said. "It's not just replacing damaged cells. You have to look at it as a broad enabling technology that is very powerful. It's as though you were working on automobiles with sticks and rocks and suddenly someone gave you a tool box."

National Institutes of Health image

Total potential
In nature, the totipotent cell or the zygote is the first in human development. This cell can make every single cell type in the embryo, fetus and adult. It is the beginning.

Totipotent cells have total potential. They specialize into pluripotent cells that can give rise to most, but not all, of the tissues necessary for fetal development. Totipotent means having unlimited capacity.

Blastocysts
The zygote gives rise to the blastocyst, which is unremarkable in appearance.

"It looks like a tennis ball with a collection of a few cells called the inner cell mass, which are pluripotent," said Goldstein. This can occur in the laboratory, when sperm and egg are mixed and resulting zygote allowed to develop.

This happens in in vitro fertilization. Not all such zygotes or blastocysts are implanted. If the unused blastocyst is donated for research, scientists can extract the pluripotent cells in the inner cell mass and grow them in a dish for a long time.

These embryonic stem cells are "pluripotent." They cannot make the placental cells required for the establishment of an embryo, but they can make every other kind of cell.

Scientists can also take a donated egg and remove the nuclear material, including the DNA. They then take a skin or other somatic (not an egg or sperm) cell from the body and remove its nucleus and DNA. They put that DNA into the donated egg and treat the hybrid cells with chemicals and electricity. This also gives rise to a blastocyst.

This is called nuclear cell transfer, and because it could someday be used in human cloning, it raises the hackles on many people. Used as "therapeutic cloning," however, it can prove valuable in the treatment of sporadic or hereditary diseases. It can also minimize the need to block the immune system when cells are transplanted.

Adult stem cells
Adult stem cells are those that are the progenitors that become bone marrow, liver, brain and the other tissues required for a whole human organism.

"There are vital, remarkable cells that can grow and divide throughout the adult life," said Goldstein. "As stem cells become more developed, they become more restricted in their abilities."

While opponents of stem cell research say work on embryonic stem cells is not necessary because of adult stem cells, Goldstein pointed out that adult stem cells have limited applications.

While they can cure people with cancers of blood and lymph systems, "there is limited understanding of the use of adult stem cells for other diseases," said Goldstein. "If you have type 1 diabetes, we don't have an adult stem therapy to offer you. Our best adult stem cell scientists all say that their work on adult stem cells does not allow you to conclude that adult stem cell research can replace embryonic stem cells. There is no strong scientific opinion to say you can do one to the exclusion of the other."

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