Because there has been very little pre-clinical data published on human iPS cells.
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But over the past two years, he notes, scientists have discovered that they can generate viable animals from iPS cells.
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Thus far, iPS cells have been a less controversial and more acceptable source of stem cells for conservative politicians to promote.
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Scientific advances such as these put human iPS cells into the same spotlight that embryo-derived hES cells have been under since 2001.
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Except that researchers still couldn't prove that the heart, nerve, muscle and other cells they made from the iPS cells were exactly like the ones generated from the embryonic stem cells.
When the first iPS cells were developed two years ago in Japan and the United States, skin cells were reprogrammed to be able to have properties similar to a human embryonic stem cell.
But currently there remain too many unresolved problems with their generation, and embryonic stem cells are considered a necessary tool in the continued research on how to perfect iPS cells for potential therapeutic use.
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Plus, the iPS cells had the advantage that patients could generate their own stem cells and potentially grow new cells they might need to treat or avert diseases like diabetes, Alzheimer's or heart problems.
These iPS cells, like stem cells derived from embryos, can be turned into many different kinds of cells, and researchers believe they eventually could be used to regenerate tissue for organs and repair damage.
Now, the Stanford study has shown fat cells can be a player in the quickly evolving area of iPS stem cell research, not because they have their own stem cells but because the fat cells can be turned into iPS cells.
That technique for generating embryonic-like stem cells (called induced pluripotent stem cells, or iPS cells) bypassed the need for transferring the cells into eggs, as Wilmut had done, and also averted the ethical issues attached to extracting stem cells from embryos as Thomson had done.
Scientists at Stanford University School of Medicine have discovered that the millions of fat cells removed during liposuction can be easily and quickly turned into induced pluripotent stem cells, or iPS cells, more easily than the skin cells that researchers used when the first iPS cells were created in 2007.
Opponents of embryonic stem cell research, which involves destroying the donated embryos from which cell lines are derived, have long argued that the discovery of iPS cells in 2006 (for which Shinya Yamanaka was awarded the Nobel Prize in Medicine this past year) obviates the need for embryonic stem cell research.
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The literature on similarities and differences between ES cells and iPS cells, he said, is quite controversial and scientists still do not know whether differences between different iPS cell lines are just due to genetic differences between the adult cell lines that were used for reprogramming or whether genetic changes occurred during reprogramming.
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The iPS cells, which have many of the same basic properties, do not raise the same ethical questions as embryonic stem cells because they come from skin or now fat cells that have been reprogrammed to go back in time, so to speak, and have the ability to turn into any other kind of cell in the body.
While cloning stem cells is a technical breakthrough, there's already a method of deriving embryonic-like stem cells that doesn't require the use of embryos at all: induced pluripotent stem (IPS) cells, said Dr. George Daley, who is director of the Stem Cell Transplantation Program at Children's Hospital Boston and an international expert in stem cells.
One iPS cell line was particularly effective: 20 out of 151 embryos yielded adult mice, which were fully composed of iPS derived cells over 90% of the time.
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Since that time, more advanced cloning technologies have evolved, including the iPS system of induced pluripotent stem cells, which can be grown from nearly any skin cell.
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