WASHINGTON (AP) - Despite a papal condemnation of embryonic stem cell research, scientists say it cannot be abandoned because it could lead to such great medical benefits for millions of patients.

Pope John Paul II said Tuesday that any research that results in the death or injury of a human embryo is "not morally acceptable" even though the work may eventually offer important medical boons.

The Pope urged researchers, instead, to concentrate on stem cells from adults. Some studies have suggested adult stem cells can be as medically useful as the stem cells taken from embryos. The adult cell research does not involve the death of a human embryo.

But American researchers who are studying both the adult cells and the embryonic cells say that both paths must be followed if the medical miracles possible from stem cells are to be achieved.

Stem cells became an extremely active area of medical research after early studies in mice suggested the so-called "master cells" could restore damaged nerves, correct some brain diseases, strengthen ailing hearts and, perhaps, even cure diabetes.

Stem cells are biological building blocks. They are the ancestral cells for all the body's tissue. Stem cells differentiate, or evolve, into the heart, lung, skin and all the other organs in the body.

Researchers believe that by learning to direct or control the direction of change in stem cells they will eventually be able to grow new organs or to renew ailing body parts with injections of special stem cells.

There are three basic types of stem cells:

• Totipotent stem cells: The single cell formed when an egg is fertilized by sperm. This cell can develop into a complete individual.

• Pluripotent stem cells: Four days after fertilization, a totipotent cell develops a hollow sphere called a blastocyst. Within the blastocyst is a cell mass that includes pluripotent stem cells. Removing pluripotent stem cells kills the embryo. These cells can be grown in great numbers and directed to form cells that, in turn, make the tissues in the body, but not a complete individual.

• Multipotent, or "adult," stem cells. These develop from pluripotent stem cells and some types appear to be produced throughout life. These cells convert into specific cell types that form organs. Adult stem cells have been found for blood, nerves and other tissues.

The Pope and other abortion foes oppose research with the pluripotent stem cells because it requires the death of embryos. A U.S. federal law forbids government funding of research that results in the death of an embryo.

However, the National Institutes of Health recently published guidelines that would permit funding of embryonic stem cell research provided the funds were not used to kill the embryo. In effect, private researchers would extract the stem cells from surplus fertility clinic embryos and then pass the cells along to federally funded researchers.

Some recent studies in mice have shown that adults stem cells are more flexible than previously believed. Blood stem cells, for instance, have been converted into nerve, muscle and bone cells. The Pope and others believe that the promise of this adult stem cell research makes the embryonic stem cell studies unnecessary.

Most researchers disagree. "My work deals with adult stem cells," says Dr. Bryon Peterson, a University of Florida, Gainesville, researcher who has grown liver cells from mouse bone marrow stem cells. "I feel they will eventually be the most beneficial for patients. But right now we don't know if they will have the same plasticity (flexibility) as embryonic stem cells."

Until more is known, said Peterson, "we need to pursue all aspects" of stem cell research.

Dr. Ronald D.G. McKay, an adult stem cell researcher at NIH, said recent studies in mice show that embryonic stem cells can be converted to nerve or brain cells to treat Parkinson's disease, Alzheimer's disease and other central nervous system disorders.

"These experiments show that we can get cells of therapeutic interest from embryonic stem cells, but we don't know if we can get those cells from another source," he said. "In the future, we might be able to get these cells from other places, and we are looking - really looking."

Until then, said McKay, "it would be foolish to abandon" research that offers such great promise to relieve human misery. "We can't turn our backs" on embryonic stem cell research, he said. "It is just too important."

Dr. Johnny Huard, a University of Pittsburgh adult stem cell researcher, said that even if adult stem cells could be used for medical treatment, embryos may still be the preferred source of stem cells because they are "of much higher quality."

"We have compared the number of stem cells from adult mice and from newborn mice," he said. "The quality of the adult stem cells is not as good as the newborns and we find the number declines over time."

Also, said Huard, the adult stem cells are more difficult to keep alive and growing in the laboratory.

"If are going to use stem cells for therapy, we are going to need quality cells that can grow quickly," he said. "I don't think that cells we get from adults will be able to do what we need them to do."