Citing the flood of stories in the media about the anticipated benefits and ethical quandaries posed by stem cell research, the National Institutes of Health complained in a recent report that "the terminology used to describe stem cells in the lay literature is often confusing or misapplied. . . . Even among biomedical researchers there is a lack of consistency in common terms to describe what stem cells are and how they behave in the research laboratory."

In order for a fertilized egg to grow into an adult human body, nature plays a kind of cell game. One master cell ultimately begets 100 trillion (that's 100,000,000,000,000), divided into more than 250 cell families that create life and sustain it day after day, replacing themselves as needed, some noticeably--as with cells of the skin, hair, bone marrow and intestines--and others subtly, as with cells in the brain.

Stem cells are the fire hydrants that stop the system from burning itself up. Paradoxically, they do not divide often and are stored throughout the body in tiny niches next to specialized cells, called nurse cells, that provide the growth factors and other chemical signals that allow stem cells to keep us alive.

"Stem cells are capable of self-renewal, which means that when they divide, they give rise to themselves, kind of like weeds," said Douglas Melton, chairman of Harvard University's department of molecular and cellular biology.

"Most other cells divide once and specialize into something like nerve or muscle cells. Stem cells first ensure their survival by replicating themselves. We biologists say that makes them immortal.

"Second, when they divide they also have the potential to specialize into other kinds of cells, so that a daughter cell can become whatever's needed--a cartilage cell, say, or a blood cell."

Human stem cells, like those of other mammals, come in three main types: totipotent, pluripotent and multipotent, each representing a stage of development.

The fertilized egg is considered a totipotent stem cell. It has total potency, the potential to become an entire individual. In the first hours and days after fertilization, this potential begins to divide into identical totipotent stem cells.

Then, four days after fertilization, the totipotent stem cells begin to specialize, forming what is called a blastocyst--a hollow ball of cells whose inner layer holds tissue-specific embryonic stem cells that will go on to form most of the cells, tissues and organs of the body.

These early stem cells are pluripotent, not totipotent. Their destinies are preprogrammed. They lack the ability to develop into an entire organism. When scientists talk about embryonic stem cells, these are the ones they mean, and these are the ones they want to study.

The only known sources of pluripotent stem cells are those isolated and cultured from early human embryos and from fetal tissue that was destined to be part of the gonads.

Adult cells from mature tissue

Adult stem cells--also called multipotent stem cells--come from mature tissues such as bone marrow, blood, the cornea and retina, skeletal muscle, dental pulp, liver, the skin, nerves, etc. Bone marrow stem cells are the most studied by scientists and have been saving patients from deadly diseases for a long time.

But these multipotent stem cells are rare in humans. They are difficult to identify, isolate and purify. They're considered less robust than embryonic stem cells, biologists say, and their immortality has yet to be proved. "They seem to have a more restricted potential," Melton said.

Moreover, adult stem cells may not exist for every cell type in the body or may be in regions, such as the brain, that are inaccessible.

Scientists hope that's not true. According to Stanford University's Irving Weissman: "There isn't one of us who doesn't wish we didn't need stem cells from embryos. But the adult stem cell story is based mostly on hope."

In stem cell research, an early-stage embryo is killed when the pluripotent stem cells are extracted, although in cellular terms, they just keep dividing. Life goes on. It merely switches locations.

"Nevertheless, we can't pretend that scientists who do stem cell research are in no way complicit in the destruction of embryos," said University of Pennsylvania ethicist Glenn McGee. "That's just wrong, a smoke-and-mirrors game played by proponents, including the NIH.

"It would be much better to take on the issue directly by making the argument that destroying embryos in this way is morally justified. If we can't make that claim, we shouldn't do this research."