Embryonic stem cells
Embryonic stem cells are taken from a young embryo. They may be taken from the inner part of the embryo, also known as a blastocyst, or split off at the 4 or 8 cell stage before this, leaving a viable embryo. An embryo reaches the blastocyst stage about 4-5 days after fertilization. At that point they contain about 50-150 cells.
Embryonic stem cells can differentiate into the three "germ layers": ectoderm, endoderm and mesoderm. The 220 types of cells in humans are all based on these germ layers. Some scientists have shown effectively that embryonic stem cells have greater differentiation potential, and divide into greater numbers, than adult stem cells can.
In 2001 President Bush allowed federal funding for research performed only on the 60 human embryonic stem cell lines that were in existence at the time. Cell lines drift genetically and morphologically over time, and due largely to the ban on producing more embryonic stem cell lines with NIH funds, researchers focused on producing embryonic-like stem cells from adult (differentiated) cells; these cells are known and induced pluripotent stem cells (iPS cells). These cells have similar properties to embryonic stem cells but do no require the destruction of or any use of human embryos - humans donate their own cells (e.g. skin cells) and though slight genetic manipulation these cells, they can be reverted into essentially an undifferentiated state and gain the ability to become nearly any cell in the body (i.e. pluripotency). This major advance in the stem cell and medical fields came much sooner than most researchers expected; this can be credited largely to the Bush ban regarding human embryonic stem cells.
Despite the strong genetic similarities, the production of chimpanzee stem cell lines have been problematic, though stem cells have been produced for other non-human primates.
- Those who value human life from the point of conception, oppose embryonic stem cell research because the extraction of stem cells from this type of an embryo requires its destruction. In other words, it requires that a human life be killed.
Many liberals, on the other hand, claim that all the embryos used for stem cell research are already condemned to death because most of them are the byproduct of in vitro fertilization, a procedure that requires the production of a number of embryos far in excess of the number that are successfully implanted. Few parents choose to cryogenicaly store these excess embryos, and with no further use for these embryos, they are often destroyed. Thus, liberals claim that embryonic stem cell research does not result in the destruction of any additional embryos.  Conservatives argue that unused embryos can be adopted within a 10 year window of viability  and thus saved from destruction; however, embryo adoption is very rare and far too few prospective adoptive parents exist for all unused embryos to be adopted.