Despite extensive debate about stem cells and their use, there has been considerable confusion about what they are exactly, and what they do. Here are some basics.

Stem cells are the building blocks of tissues and organs in the body. The unspecified cells can become any tissue, muscle, organ or bone in the body, given the right conditions. Stem cells can be frozen, and once thawed will continue to renew themselves.

The term “stem cell” relates to the cells’ special kind of division, said Dr. Peter Bryant, professor of developmental and cell biology at the University of California at Irvine. “One product of the division remains the original stem cell, and the other product goes on to become something else differentiated cell division.”

In other words, stem cells can be maintained and different cell types budded off like clippings of one plant that could grow into different plants if their environment were changed.

Stem cell research, which goes as far back as 1950s, involves both human and animal stem cells. Mice and worm cells have been analyzed in great detail, and scientists are trying to determine to what extent human stem cells can be told what to become. Human embryonic stem cells weren’t isolated until the late 1990s.

The excitement and promise of stem cell research lies in the hope that these cells can be harnessed to regenerate damaged organs or help cure certain diseases.

“There are some definite targets,” said Dr. Phil Schwartz, director of the Human Embryonic Stem Cell Culture Training Course at Children’s Hospital of Orange County Research Institute. “Like diabetes, heart disease, the real killers. But which one we’ll reach first is really up in the air right now.”

There are two types of stem cells: adult and embryonic. The adult variety doesn’t necessarily have to come from grown-ups they’re just the cells that have already been programmed to create particular organs or tissues: blood cells, heart tissue, liver or muscle cells, for example. Adult stem cells are present in newborn babies and adults, especially in tissues that need to rejuvenate themselves frequently. Therapies created with adult stem cells are already in use to treat diseases like leukemia and lymphoma.

Embryonic stem cells are only present in the very early stages after fertilization. They are part of a ball of cells formed as the egg divides, called a blastocyst. “Those cells are so immature that they still have the potential to form any type of tissue,” Schwartz said.

Embryonic mice stem cells can divide continuously in a petri dish, and they retain the ability to generate any cell type when exposed to the right environment. Scientists are studying these cells in humans to determine if the same can be done. Adult stem cells can be obtained by various means from living donors, tissue removed during surgery, the umbilical cord of a newborn baby, an aborted fetus or even a cadaver within two days of death.

But human embryonic stem cells can only come from very early embryos grown in cultured petri dishes at in-vitro fertilization clinics. When the stem cells are removed from the embryos, the embryos lose their viability. Extraction of stem cells from embryos has created the most controversy.

Research on both adult and embryonic stem cells is legal. But because embryonic stem cell research kills the donor embryo, there has been opposition from several fronts. Many religious organizations oppose the idea of creating human embryos solely for research purposes.

President Bush in 2001 set up criteria for which types of embryonic stem cell research can be federally funded, restricting funding to the 60 cell lines then in existence. Only 23 of those lines were healthy enough to be used in research.