Scientists want to have a variety of stem cell lines so that they can pick the best ones for their experiments. Some lines are better suited for becoming pancreatic cells, others for neurons (no one is sure why). Moreover, older cell lines are harder to grow, and many contain mutations and chromosomal abnormalities.
Some of the most exciting embryonic stem cell lines are those that could closely model human disease. A mechanic hoping to understand a jet engine would be unhappy given a helicopter to study, but scientists studying human diseases routinely resort to studying artificial, animal equivalents of disease. Experiments with appropriate human cells could reveal information that experiments with mice could not.
In 2003, scientists in the UK created stem cell lines using embryos from fertility clinics that would otherwise have been discarded because they carried mutations for genetic diseases. These lines, along with cell lines created elsewhere in Europe, carry mutations for Huntington's, cystic fibrosis and other genetic diseases. Scientists hope to grow these stem cells into tissues afflicted by the disease, the better to assess and perfect treatments. (As of April 2007, none of these lines are eligible for U.S. government funding.)
Most embryonic stem cell lines are not considered good enough for human trials, though at least one company, in Singapore, is working on lines for exactly this purpose. Several US companies believe they can make therapeutic-grade cells from older lines. In 2007, multiple researchers reported that both mouse and human cells could be converted to cells that behave like embryonic stem cells. Researchers believe these so-called induced pluripotent stem (iPS) cells will be incredibly valuable for studying disease and potentially even cell therapy. Several caveats remain however. As of mid-2008, these so-called induced pluripotent cells still need to be more extensively compared to embryonic stem cells, and the technique to make the cells requires permanently inserting multiple gene copies into the cells, making them more variable and less predictable.
Over a dozen countries have derived stem cell lines. Depending on the application they are intended for, some lines are better than others.
Every embryonic stem cell line is genetically distinct. Like the embryos they were made from, the lines are either male or female. Embryonic stem cell lines have also been created and grown under a variety of conditions. For all these reasons, stem cell lines behave differently in the lab. Some grow faster than others. A few lines are readily coaxed into heart cells, whereas other lines readily differentiate into neural stem cells and still others are particularly easy to maintain in culture.
In the United States, only research on human embryonic stem cell lines created before 2001 can receive federal funding. Of these, a dozen or less are in wide use. Many of these older lines have accumulated genetic flaws and don't grow as well as newer lines. Most researchers worry that the lines could be contaminated by the animal cells that they have been grown with; many researchers feel the lines don't grow well as newer lines. Moreover, at this time, the lines created for infertile couples reflect little racial or genetic diversity.