First announced in 1998, human embryonic stem cells are usually grown in the lab on a "feeder layer" of mouse cells. Feeder cells send as yet unknown signals to the primitive human ES cells, preventing them from turning into more "grown-up" cell types, such as bone, fat, or brain cells.
The Johns Hopkins team found that human marrow stromal cells can also act as feeder cells for human ES cells, letting them divide without differentiating. Tests show that the human ES cells, obtained from the University of Wisconsin, retain their primitive nature when grown on the stromal cells, the scientists report in the March issue of the journal Stem Cells.
"After eight months of dividing under these conditions, the human embryonic stem cells still look and act just like the originals," says Linzhao Cheng, Ph.D., assistant professor of oncology at the Johns Hopkins Kimmel Cancer Center. "Marrow stromal cells co-exist with blood-forming stem cells in bone marrow and support their growth in the lab, but we were surprised they could fully replace mouse cells in supporting more primitive ES cells."
Marrow stromal cells are also known as mesenchymal stem cells, which are capable naturally of becoming fat, cartilage and bone. Some animal experiments have suggested that mesenchymal stem cells can be manipulated to form other types of cells in laboratory dishes, but that is still being evaluated.
One concern with using mouse cells as the feeder layer for human ES cells is that an animal virus might be passed to people if the human ES cells someday are used to treat patients. Although such applications are in the fairly distant future, if human cells can act as feeders, new lines of human ES cells could be created without exposing them to mouse cells.
"When it is time to create new stem cell lines, this issue will be very important," says Cheng. "Our results together with others' show that it should be possible to establish new stem cell lines without using mouse cells or proteins."
Late last year, scientists in Singapore reported using human cells -- one set from fetuses, one from the reproductive tract of women -- as feeder cells to grow human ES cells in the lab. But the cells from adult bone marrow have some distinct advantages, says Cheng.
"Marrow stromal cells don't carry the ethical baggage of the abortion debate, they are routinely and easily obtained, and scientists already know a lot about them, including how to dramatically expand their numbers in the lab," says Cheng. "Scientifically, using a variety of human cells to support the growth of human embryonic stem cells could speed the hunt for the signals that keep the primitive cells from changing. If that happens, we might not need any feeder cells at all."
The researchers found that, at first, the human embryonic stem cells didn't divide as rapidly on the human marrow stromal cells as on the mouse cells. After one week, however, the cells adapted, grew well, and still had the right shape, behavior and protein markers (Oct-4 and SSEA-4, etc.) characteristic of human ES cells, says Cheng.
Numerous hurdles remain before embryonic stem cells -- or cells derived from them -- could be useful in treating conditions like Parkinson's disease or diabetes. In fact, basic understanding of these cells is still quite limited, including what makes them able to turn into any of the body's cell types, and what keeps them from doing so at the drop of a hat, notes Cheng.
"Mouse embryonic stem cells have been studied for more than 20 years," he says. "We know what signals keep them from differentiating, we know the whole pathway. But human embryonic stem cells have only been studied for the last five years, and only in the last year has the investigation expanded to multiple labs in the United States and elsewhere. What we've learned so far shows human and mouse ES cells to be quite different. The more ways we have to study human ES cells, the faster we can learn."
The research was funded by the W.W. Smith Charitable Trust and the National Institutes of Health. The human embryonic stem cell line used in this experiment was line H1 from the University of Wisconsin in Madison, also known as WA01 in the National Institutes of Health registry.
Authors on the study are Cheng, Holly Hammond, Zhaohui Ye, Xiangcan Shan, and Gautam Dravid, all of the Johns Hopkins School of Medicine.
Stem Cells, the journal
Background information about stem cells from the National Institutes of Health
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