Stem cells are cells of the body (somatic cells) which can divide and become differentiated.
When an organism grows, stem cells specialize, and take specific functions. For instance, mature tissues like skin, muscle, blood, bone, liver, nerves, all have different types of cell. Because stem cells are not yet differentiated, they can change to become some kind of specialized cells. Organisms also use stem cells to replace damaged cells during lifetime.
Stem cells are found in most, if not all, plants and animals. They divide and differentiate into a range of cell types. Research in the stem cell field grew out of findings in the 1960s.
The two broad types of mammalian stem cells are: embryonic stem cells, and adult stem cells, which are found in adult tissues. In a developing embryo, stem cells can differentiate into all of the specialised embryonic tissues. In adult organisms, stem cells act as a repair system for the body, replenishing specialized cells, but also maintain the normal turnover of blood, skin, and intestinal tissues.
Stem cells can be grown in cell culture. In culture, they can be transformed into specialised cells, such as those of muscles or nerves. Highly plastic adult stem cells from a variety of sources, (umbilical cord, blood, bone marrow), are now routinely used in medical therapies. Researchers expect that stem cells will be used in many future therapies.
Embryonic stem cells (ES cells) are stem cells taken from the inner cell mass of the early stage embryo known as a blastocyst. Human embryos reach the blastocyst stage 4-5 days after fertilization. At that time, they are made up of between 50 and 150 cells.
The stem cells' state, and what the daughter cells turn into, is influenced by signals from other cells in the embryo.
Stem cell controversy
The stem cell controversy is the ethical debate centered only on research involving the creation, usage, and destruction of human embryos. Most commonly, this controversy focuses on embryonic stem cells. Not all stem cell research involves the creation, usage and destruction of human embryos. For example, adult stem cells or induced pluripotent stem cells, which do not involve creating, using or destroying human embryos, is not as highly controversial.
Embryonic stem cells have the potential to grow indefinitely in a laboratory environment and can differentiate into almost all types of bodily tissue. This makes embryonic stem cells a prospect for cellular therapies to treat a wide range of diseases.
The deliberate destruction of a human embryo is incompatible with Roman Catholic doctrine, according to which, Pontifical Academy for Life has stated that human blastocysts are inherently valuable and should not be voluntarily destroyed as they are "from the moment of the union of the gametes" human subjects with well defined identities.
Viability is another standard under which embryos and fetuses have been regarded as human lives. In the United States, the 1973 Supreme Court case of Roe v. Wade concluded that viability determined the permissibility of abortions performed for reasons other than the protection of the woman's health, defining viability as the point at which a fetus is "potentially able to live outside the mother's womb, albeit with artificial aid." The point of viability was 24 to 28 weeks when the case was decided and has since moved to about 22 weeks due to advancement in medical technology.
See also: http://en.wikipedia.org/wiki/Stem_cell_controversy
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)