by Sally Pobojewski

The potential for all human complexity is locked inside 30 to 35 cells nestled within a five-day-old embryo called a blastocyst — the product of a union between one egg and one sperm. All 100 trillion cells in the human body are descended from these original cells. When removed from an embryo and grown in a culture dish under the right conditions, they continue to divide indefinitely. Scientists call them embryonic stem cells.

Stem cells are the black box of biomedical research. Scientists know what goes in and what comes out, but what happens in between — how we develop from a blastocyst small enough to fit on the point of a pin to a living, breathing, thinking human being — remains one of life’s most profound mysteries.

The mystery becomes even more intriguing when you consider that other stem cells, which scientists call somatic or adult stem cells, remain in many types of tissue throughout life. While embryonic stem cells are generalists, adult stem cells are specialists. They can make copies of themselves — a process called self-renewal — in addition to producing specialized cells, such as heart, skin or muscle cells. But unlike embryonic stem cells, adult stem cells usually have limits on what types of cells they can become.

Scientists at the University of Michigan Medical School are searching for answers to the many questions about adult and embryonic stem cells. These researchers come from different disciplines, have different goals and study different types of stem cells, but all have devoted their careers to the detailed, painstaking process of unlocking the stem cells’ secrets one by one. Research underway in U-M laboratories today could help answer fundamental questions about the development of life itself and lead to major advances in the future of medicine.

Michael Clarke Photo: D.C. Goings

“Stem cells are the key to everything,” says Michael Clarke, M.D., associate professor of internal medicine and one of the Medical School’s stem cell experts. “Their potential is huge, but an incredible amount of work remains to be done before we can begin to tap that potential.”

Embryonic stem cells and development
In the beginning is the stem cell...

Scientists say that embryonic stem cells are pluripotent, meaning they have the remarkable ability to become any of the more than 200 distinct cell types in the human body. But their unlimited potential doesn’t last for long.

Fourteen to 16 days after fertilization, stem cells in the human embryo begin to separate into three layers called the endoderm, mesoderm and ectoderm. Each layer must develop in a precise location within the embryo. Scientists call this complex process gastrulation and it is fundamental to all later development. If gastrulation doesn’t proceed normally, the embryo will die.

Deborah Gumucio Photo: Bill Wood

During the next few months of human development, a process of increasing specialization takes place. “Every step in the formation of each organ requires a precise orchestration of a number of processes to form the tissue, grow the tissue and differentiate the many different kinds of cells within the tissue,”says Deborah Gumucio, Ph.D., associate professor of cell and developmental biology and co-director of the U-M Center for Organogenesis.

“What tells stem cells to differentiate into the various organs? How do the organs, blood vessels and connective tissues become situated in the correct places? Once organs form, how do they know when to stop growing?” asks Gumucio. “We are beginning to learn tiny bits of information about individual signaling pathways, and some of these discoveries could translate into major medical advances. However, we are decades away from a complete understanding of these processes.”

Nothing in the embryo develops in isolation. Stem cells need a supportive matrix or scaffold to grow into a heart. The primitive heart sends molecular signals required for liver development. Organs can’t form without help from other stem cells, especially endothelial cells that form the walls of blood vessels and are active signaling and induction centers.

During early development, the embryo is a biochemical version of the Internet. Interactive messages fly back and forth, signaling genes to turn on or turn off and telling cells when to grow, when to change and when to die. The process is so complicated that it involves half of the 35,000 genes in the human genome.

Adult stem cells and tissue renewal
Mending broken hearts and shattered spines

Adult stem cells appear to have fewer options than their embryonic counterparts. Instead of being pluripotent, scientists say they are multipotent — meaning their potential for development is limited to a few specific types of cells.

Adult stem cell development is a gradual transition, which begins with production of intermediate cells called precursor or progenitor cells. Precursor cells then morph into a series of increasingly more specialized cells. Scientists used to believe a precursor cell could become just one specific kind of cell. Now they’re not so sure.
True adult stem cells are rare and separating them from the more-common precursor cells is not easy. In 1988, Irving Weissman, M.D., and his Stanford University research team became the first scientists to isolate adult stem cells. They were hematopoietic or blood-forming stem cells found in mouse bone marrow at a ratio of one stem cell for every 10,000 bone marrow cells.

Sean Morrison Photo: D.C. Goings

U-M stem cell researcher Sean Morrison, Ph.D., who was a graduate student in Weissman’s laboratory during the mid-1990s, refined the techniques scientists use to find mouse hematopoietic stem cells in different types of tissue and at different periods of development. Morrison is now an assistant professor of internal medicine and of cell and developmental biology in the U-M Medical School, and a Howard Hughes Medical Institute assistant investigator.

Since he joined the U-M faculty in 1999, Morrison has found stem cells in the peripheral nervous system of rats. Other scientists have identified adult stem cells in several areas of the brain and spinal cord, skeletal muscle, bone marrow, liver and skin of rats, mice and humans.

Scientists have known for 50 years that hematopoietic stem cells found in bone marrow, liver and the spleen can reconstitute every cell in the blood and immune systems. Cancer patients often receive transplants of these cells following intense radiation or chemo-therapy. Injected into the bloodstream, they somehow home directly to bone marrow and begin differentiating, replacing the red and white blood cells killed during treatment. Even though hematopoietic stem cells grow rapidly inside the body, scientists have yet to find a way to grow them outside the body in cultures where they can be studied.

Identified more recently than blood-forming stem cells, neural stem cells currently are under intense study, because some scientists believe they could replace dead or damaged neurons after spinal cord injuries or in Parkinson’s and Alzheimer’s disease. Scientists have shown that neural stem cells produce three different kinds of specialized cells in the central nervous systems of mice, rats and humans. Neural crest stem cells, which develop into the peripheral nervous system, also have been isolated in rats.

The discovery that stem cells exist in many types of tissue has led to new research on stem cell plasticity — the ability of adult stem cells or precursor cells to shift gears, change their differentiation pathway and become a completely different type of cell. Under certain conditions, scientists can force a developmental shift by treating cultured stem cells with specific proteins called growth factors or by inserting genes into their DNA.

Michael Long Photo: D.C. Goings

“Growth factors can trigger a series of changes inside the cell that cause differentiation,” says Michael Long, Ph.D., professor of pediatrics and another of the Medical School’s stem cell scientists. “But ultimately, genetics is everything. If you don’t have the right genes turned on or turned off, all the external factors in the world won’t make any difference.”

Scientists are interested in plasticity’s potential for regenerative medicine — growing new organs or tissues from stem cells to replace those dam-aged by accident or disease. “When I was in school, I learned that bone marrow, gut, blood and skin were the only tissues capable of regeneration,” Long says. “But recent experiments in mice show that bone marrow stem cells can regenerate the liver. Liver cells can develop into pancreas cells. Muscle cells can give rise to many types of tissue. Someday, we may be able to transplant bone marrow cells and treat diseases in ways we never thought possible before.”

Many obstacles remain before scientists can tap the full potential of adult stem cells, however. For one thing, most of the successful plasticity studies so far have involved stem cells from mice and rats. Scientists are all too familiar with the fact that just because something works in mice doesn’t mean it will work in people.

“There is limited evidence that adult stem cells may have broader potential than previously thought,” Morrison cautions, “but much of that evidence is still controversial and inconclusive.”

Cell death and stem cells
Live and let die

Even in the earliest stages of an embryo’s development, death is part of life. Some cells must die to make it possible for organs to grow into the proper size and shape. Scientists are intrigued by the role stem cells play in programmed cell death — a process they call apoptosis — because the ability of stem cells to regulate the balance between cell proliferation and cell death is directly relevant to cancer.

“Tissues like skin, blood and intestine have stem cells that proliferate constantly,” explains Deborah Gumucio. “For example, the entire lining of the gut is renewed every three to four days. A process of programmed cell death must accompany this proliferation to prevent the accumulation of too many cells. Cancer in these tissues is, in many cases, simply a mismanagement of this balance — too much proliferation, too little cell death, or both.”

U-M stem cell scientist Michael Clarke has been intrigued by the relationship between stem cells and cancer since — during his medical residency at the Indiana University Medical School — he had to watch helplessly as one of his patients, the girlfriend of basketball legend Larry Bird, died from leukemia. “I realized then that if we understood stem cells, we’d have a better chance at a cure,” Clarke says.

Now he studies self-renewal of adult hematopoietic stem cells and recently discovered an important gene involved in the process. “Self-renewal is a tightly regulated process under strict genetic control,” Clarke explains. “Only an absolute or fixed number of hematopoietic stem cells are allowed to exist in bone marrow. Otherwise every stem cell would be like a cancer.”

To clone or not to clone
The stem cell controversy

In a nationally televised address last August, President George W. Bush announced new regulations for scientists conducting federally funded research with human embryonic stem cells. The new rules restricted research to stem cell lines already in existence that were derived from “leftover” or excess human embryos created at fertility clinics. Since then, the National Institutes of Health have listed 72 cell lines worldwide that meet the approved criteria.

Intense media attention and public interest followed the President’s announcement — much of it focused on important ethical and religious questions about the morality of using human embryos for research. Most scientists appreciate that, for many, this is a controversial issue. But somewhere in the process, they say — between national opinion polls, opposing newspaper editorials, dueling expert television coverage and Congressional politics — facts about the science of stem cells are being lost, misrepresented or ignored.

Following the Bush decision on human embryonic stem cell research, scientists were flooded with calls from reporters wanting to know when the public could expect cures for diabetes, Parkinson’s disease or cancer. U-M researchers stress that we are a long way from knowing whether stem cell research will lead to cures or even more effective treatments for these and other diseases.

Marie Csete Photo: D.C. Goings

“The fundamentals of stem cell biology have been skipped in the excitement these cells generate,” says Marie Csete, M.D., Ph.D., assistant professor of cell and developmental biology, associate professor of anesthesiology, and one of several stem cell biologists in the U-M Medical School who emphasize the need for more basic research. “We are all likely to be sorry about skipping these ABCs down the road.”

Then there’s the fact that stem cell research often is portrayed as leading to cloning — or creating a genetically identical copy of an organism, especially a human being. “Stem cell research and cloning are two different things,” Morrison says. “The public needs much more education about the different types of stem cells and different types of cloning.”

Sue O’Shea Photo: D.C. Goings

“There’s a big difference between reproductive cloning — the technology used to create Dolly the sheep — and therapeutic cloning,” says U-M stem cell scientist Sue O’Shea, associate professor of cell and developmental biology. “No reputable scientist is interested in reproductive cloning of human beings and, clearly, it should be banned. But therapeutic cloning has great potential to help people who need to replace damaged or diseased tissue and organs.

“In therapeutic cloning, physicians take a small sample of your tissue and transplant the nucleus from some of these cells into a line of human embryonic stem cells,” O’Shea explains. “Under the right culture conditions, scientists could grow neurons with your specific antigens. Because they are genetically identical to you, there should be no immune transplant reaction.”

Richard Mortensen Photo: D.C. Goings

While O’Shea, Clarke, Csete, Long, Morrison and Richard Mortensen, M.D., Ph.D., the newest stem cell scientist to join the Medical School, have chosen to focus their research on these cells, there are many U-M scientists using stem cells in other scientific or clinical research initiatives. For example:

In a series of recent animal studies, U-M neurologist Jack M. Parent, M.D., has shown that neural progenitor cells in the brain respond to acute brain injury by moving to damaged areas and producing new neurons. Understanding this self-repair mechanism could help physicians limit brain damage from strokes or neurodegenerative diseases.

U-M pediatricians John E. Levine, M.D., and Gregory A. Yanik, M.D., recently performed the U-M’s first cord blood stem cell transplant for sickle cell anemia using stem cells from the umbilical cord of the patient’s infant sister.

James Ferrara, M.D., professor of pediatrics and internal medicine, hopes to learn how to prevent an immune reaction called graft-versus-host disease in cancer patients following bone marrow stem cell transplants. In recent studies with laboratory mice, Ferrara discovered that giving mice additional interleukin-18 during the procedure helped prevent this serious complication.

“We can see stem cells’ potential for developing new cell therapies for diabetes, cancer and neurodegenerative diseases,” says Mortensen, associate professor of internal medicine and physiology. “We have the expertise and the commitment. Working together, we hope to begin to tap that potential.”

To learn more about stem cells, visit:

www.nih.gov/news/stemcell
www.nature.com/nature/insights/6859.html
www.nytimes.com/science
www.whyfiles.org/127stem_cell/index.html

Also:

Michael Clarke

Sean Morrison

Michael Long

Marie Csete

Sue O’Shea

Richard Mortensen

A Stem Cell Glossary