The finding that only 11 cell lines are currently available instead of more than 70 led to a call for lifting the restrictions and development of new cell lines.
Writing in Friday's issue of the journal Science, National Institutes of Health Director Elias Zerhouni said his agency is giving a high priority to research using stem cells because of the potential for treatment of diseases such as diabetes and Parkinson's.
Zerhouni's report on the work supported by the NIH also shows that initial estimates that more than 70 stem cell lines were eligible for study were optimistic.
Donald Kennedy, editor in chief of the journal, contends in an accompanying editorial that development of new cell lines for research is necessary. "It is plainly not sound policy to retain the current restrictions on work" with human embryonic stem cells, he said.
These cells form very early in an embryo's development. They can develop into numerous types of cells to form organs and other parts of the body. Researchers hope to use these cells to repair damaged organs and cure diseases.
But the work is controversial because the cells are taken from days-old embryos, which then die. Opponents say this is unethical.
Sources of cells are excess embryos from fertility clinics. The American Society for Assisted Reproductive Technology reported Thursday that there are 396,526 frozen embryos in storage in the United States and that 88 percent are planned for use in helping families have children in the future.
Obtaining stem cells for research has led to studies into the potential of cloning embryos, a process also criticized by opponents.
The president has ordered that stem cell research can continue but scientists receiving federal funds can use only cell lines that were available on Aug. 9, 2001. The Health and Human Services Department reported at the time that more than 70 cell lines — continuously propagating cell colonies — were available.
But Zerhouni says in his paper that many of those cell lines were in the early stages of development and were not to the point where they could be distributed for use. To overcome this, he reports, the NIH provided grants to bring the cell lines to the point where they can be used.
"As a consequence of this support, the number of cell lines available for widespread distribution has grown from a single cell line in the spring of 2002 to 11 cell lines at present," Zerhouni wrote.
Kennedy contends that new lines are needed for research because all current ones were developed in the presence of mouse cells that provided needed growth factors, and thus may be contaminated with viruses or proteins from those cells.
Human embryonic stem cells can now be grown without the mouse cells, Kennedy says, thanks to new methods developed using private funds or in other countries not subject to the U.S. restrictions.
The low number of cell lines currently available has drawn complaints from scientists that the shortage was hampering research.
"The existing restrictions are keeping advances from being realized," Dr. George Daley of the Whitehead Institute at the Massachusetts Institute of Technology told a Senate Appropriations subcommittee last fall.
Gaining access to those limited cell lines has been difficult, several researchers said. They cited costs, problems negotiating agreements with the cells' owners and restrictions imposed by governments of foreign countries, where many of the cells are located.
Zerhouni reports in his paper that the NIH has negotiated agreements with several sources of stem cells for their use in NIH programs. The deals specify that the cells be made available to other researchers under terms no more stringent that those the agency agreed to.