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Stem Cell Maker
Stem Cell Maker

Answers to a tough ethical dilemma in research may come in machines like this.

Ancient Greeks believed the heart was the seat of emotion, intellect and spirit. But in the 1620s, English physician William Harvey proved by careful observations that the heart is a rather prosaic (albeit absolutely essential) pump that pushes blood throughout the body.

Doctors now know more about the heart than Harvey could ever have dreamed possible. But one thing they never figured out is how to rebuild living heart tissue once a part of the heart has died.

About a half-million Americans survive heart attacks every year, each with a little less heart muscle than they once had. That lost part of the heart is dead and gone forever.

Or so doctors once thought.

That belief began to change in 1999, when researchers showed that stem cells from mouse bone marrow could actually turn into new heart cells. Within a couple of years, researchers at a number of labs around the world were rebuilding heart—attack-damaged mouse hearts with injections of stem cells. Promising experimental results with humans soon followed.

Today there are several dozen clinical trials in the U.S. either underway or in the works to study how stem cells might improve the quality of life for cardiac patients.

But these and other promising medical developments occur against a now famous backdrop of controversy—mostly in the U.S.—over stem cell research.

The debate revolves around stem cells extracted from human embryos—the mass of fast-growing cells that (in about eight weeks) becomes a fetus. These embryonic stem cells possess the amazing ability to give rise to all the hundreds of types of cells that make up a complete human being. This unique ability immediately suggests these cells' enormous potential for use in repairing and replacing a variety of organs lost to injury and disease.

Trouble is, these cells come from embryos three to five days after fertilization, and when they're harvested, the embryos are destroyed. Thus, people who see eventual treatments (if not cures) for a number of debilitating diseases are pitted against those who see a slippery ethical slope in allowing embryos to be killed, even if the goal is to relieve suffering or save lives.

The morality stalemate led President George W. Bush in 2001 to restrict federal funding for embryonic stem cell research. It also played a contentious, highly visible role in recent national elections. Politicians and their supporters on both sides of the highly charged debate hold strong beliefs, so the matter isn't likely to be settled anytime soon.

But the good news—not yet well understood by the public—is that not all human stem cells are found in embryos. Even after birth, a person needs to continually create new cells to maintain and repair the body, and that job falls to adult stem cells (which, despite the name, are found in children as well as adults). These cells, located in various organs throughout the body, are not nearly as useful medically as embryonic stem cells, scientists say, because for the most part, they only generate the types of cells that make up their organ of origin.

Recent research, however, reveals that at least some types of adult stem cells show greater flexibility than previously thought. Stem cells that give rise to bone and cartilage, for example, can also create heart muscle cells, liver cells and nerve cells.

Although they lack embryonic stem cells' unique ability to generate any type of body cell, adult stem cells do offer some advantages for medical uses—chief among them that adult cells avoid the ethical problems of harvesting cells from embryos.

Still, there are drawbacks—mainly the difficulty of gathering enough cells for research use (and eventually for clinical applications). Adult stem cells are rare in the body, numbering on average only about one in 10,000 cells in bone marrow, and even fewer in some tissues. In addition, the cells are often difficult to find. And once finally isolated, they are a challenge to grow in the lab, where they tend to change spontaneously into specialized cell types, losing their stem cell potential and spoiling their value for research and therapeutic use.

"It's not a problem to just get a larger quantity of cells," said Teng Ma, an associate professor of chemical engineering at the Florida A&M University-FSU College of Engineering whose research centers on stem cells. "The problem is getting a larger quantity of cells that have the same properties as the starting population."

Most researchers grow stem cells by traditional culture methods—essentially they use flat Petri dishes. But increasingly, scientists are using bioreactors, vessels specially designed to grow cells of any kind. Most of these are severely limited by the length of time that they allow a batch of cells to continue dividing—usually no longer than a week.

Ma has taken an engineering and scientific approach to create a new type of bioreactor, a stem cell growing machine that surpasses similar technology currently available. Ma's device combines all major steps of the process into one modularized system that provides an ideal environment for stem cells to proliferate for 40 days—at least five times longer than most other reactors now being used.

Growing Stem Cells

Over the past five years, Ma has developed an adult stem cell bioreactor that can start with a small number of cells taken from an individual (say, 100,000 cells or so) and, in a matter of days, expand that population into the millions. Ma's machine is specifically designed to grow a type of adult stem cell found in bone marrow called mesenchymal (mes-IN-ky-mal) stem cells. These cells have long been known as progenitors of bone, cartilage and connective tissue cells, and have recently been shown to create other types of cells as well, including muscle, skin and nerve cells. This plasticity makes these cells promising candidates for treating a number of conditions.

"In several clinical trials, people have shown that you can use these cells in repairing bone defects, in repairing cartilage, and also recently in repairing heart muscles," Ma said.

The adult stem cells Ma uses as starting populations in his machine all come from Tulane University's Center for Gene Therapy. In a procedure similar to the one used for bone marrow transplants, donors have a small amount of marrow removed from a pelvic bone, and the stem cells are extracted from the marrow.

An Environment Just Like Home

In designing his machine, Ma reasoned that the best way to encourage the stem cells to multiply was to reproduce the conditions they were used to in the body. Using engineering principles and incorporating the latest discoveries about stem cells, Ma set about creating an environment like the one mesenchymal stem cells are used to in living bone marrow.

Ma first needed something to support the cells. He rejected traditional flat media such as Petri dishes in favor of a three-dimensional scaffold that would more closely resemble the stem cells' natural home. Each scaffold is a porous disk slightly smaller than a dime, made from compressed plastic fibers pitted with microscopic pores roomy enough for cells to set up homes.

"The pores inside the matrix are seven times larger than the cells," Ma said. "So for a cell the pores are actually pretty big—you're basically talking about a basketball in a small room." Also, the pores create a large surface area for the cells to adhere to without crowding.

The plastic matrices sit inside a small flow chamber that lets Ma precisely control the flow of a protein-rich fluid past the growing cells. The fluid delivers nutrients and essential chemicals to the growing cells just as circulating blood does in bone marrow.

Several of these flow chambers (with three matrices in each) can be placed inside a larger closed system consisting of the pumps, tubes, flasks and other essential equipment needed to keep the fluid flowing and the cells growing.

"In the system we have in the lab, we have four of these chambers stacked in series, a module for each chamber," Ma said. "And whenever you need one, you just take one out of system and harvest the cells while the other chambers are still running in the system."

Ma calls this modular design one of the main features of the system because it permits multiple groups of cells from the same donor to grow at once, while allowing them to be easily harvested at different times—a definite plus for eventual use with patients in a clinical setting.

Once Ma assembled the prototype machine, he had to learn how to operate it effectively. For example, the system has to be kept sterile, and it's no easy task to keep bacteria and other organisms out for a long period of time. But eventually Ma nailed down most of the basics.

"We know how to put cells in, how to grow them, how to expand them in large quantity, how to harvest them," Ma said.

And he also knows his machine does what it's supposed to do—it creates millions of new stem cells that, like the original seed cells he started with, have the ability to differentiate into various kinds of specialized cells.

"We have a set of experiments that allows us to determine the properties of the cells that come out of the system," Ma said. "From those results, we can say they are actually the same as the population we put in."

The fact that Ma's device can run continuously for up to 40 days makes it a prime candidate for eventual use in clinics and hospitals. Essentially, once in practice, the machine could run all the time, creating stem cells for an endless stream of patients.

Tweaking the Process

Figuring out how to make best use of the machine continues. Ma's machine is still very much a developmental project.

"It's not therapeutic," Ma said. "We're not saying this technology can be used to cure any disease yet—it's not at that stage."

Now in progress are tests to determine the precise scientific and engineering parameters that offer the best yield of cells, Ma said.

For example, mesenchymal stem cells are usually grown outside the body in an environment containing about 20 percent oxygen, the same as ambient air. But in the bone marrow microenvironment, the stem cells' natural home, the oxygen concentration is only about 2 to 5 percent. Reproducing that low-oxygen environment in Ma's machine dramatically increases the stem cells' growth rate, producing 20 times more cells over a six-week period.

Physical forces exerted by the circulating fluid are another important factor Ma is studying. Shear stress, for example, stimulates mesenchymal stem cells to become bone cells, an outcome that may be desirable in some cases but undesirable in others.

But with three patents for the device, Ma and his colleagues are encouraged. And while it might not be ready for the operating room yet, it's clearly one of the most effective tools yet developed in the worldwide quest to harness the astonishing power of stem cells while avoiding most of the political and cultural minefields that continue to surround stem cell research.

"Our approach is to design a system that can be automated for clinical use," Ma said, "so that a hospital can gather a patient's cells, put them in there and in the end get some desired cell population for therapeutic purposes."