Currently, the only known way to obtain unlimited stem cells is by using days old embryos from fertility clinics that would otherwise be discarded. Though these embryos have never been in the uterus and have never become a fetus, some people are opposed to this research because they consider this embryo to be morally equivalent to a human being.
After stem cells are removed from the embryos, they continue to divide in a lab, creating more copies of themselves (also known as a stem cell line).
They then can be exposed to specific growth conditions in order to develop into the desired (tissue-specific) stem cells, such as muscle cells.
Somatic cell nuclear transfer (SCNT) involves transplanting a patient's genetic material from something as simple as a skin cell into an unfertilized egg in order to grow patient specific stem cells.
No sperm is involved, and therefore no fertilization occurs, in this procedure. Also, because the group of cells from which the stem cells are derived are not implanted in a uterus, no fetus is involved.
SCNT has promise for therapies because the stem cells have identical DNA to the patient, thereby avoiding problems of rejection by the patient's body. SCNT is also a critical tool to create disease-specific cell lines that can be studied in vitro (outside the body) to learn how complex diseases develop and to understand how certain drugs may affect the progression of that disease. The technique has been proven to work in animal cells but has not yet been proven in human cells.
There is a big difference between reproductive cloning and therapeutic cloning.
Reproductive cloning involves creating an embryo and then actually implanting it into a uterus. It is highly unlikely this could work for humans, and for ethical and scientific reasons, all but a handful of radical scientists are strongly opposed to attempting it. Therapeutic cloning creates an embryo but never implants this in a uterus as it intends to only create patient specific stem cells, not a person.
Many countries and some US states already have legislation that permits therapeutic cloning while continuing to prohibit reproductive cloning. The vast majority of scientists support the prohibitions against reproductive cloning.
Under administration policy, only stem cell lines created before August 9, 2001 can be studied using federal funds. Although it was originally claimed that more than 60 lines would meet this eligibility criterion, most researchers in the field consider this to be a big overestimate. NIH (National Institutes of Health) currently lists only 22 lines that are available for distribution to researchers. Though many of these these federally approved lines have proven useful for basic research, they are also limited in their utility for a variety of reasons, including lack of genetic diversity, abnormalities, and poor growth characteristics. Therefore, if the therapeutic potential of unlimited stem cells is to be realized, researchers must also work with new lines. This cannot be done using NIH funding (the main source of research funds for most academic labs in the US), and so researchers must rely on private philanthropic support.
Limited stem cell-based therapies are already in widespread clinical use, in the form of bone marrow and cord blood transplants. These procedures, which save many lives every year, demonstrate the validity of stem cell transplantation as a therapeutic concept. Similar successes should be possible with unlimited stem cells for many other diseases, but it is impossible to predict how soon this will happen.
Not yet. Although embryonic stem cell research has shown great potential, the field is still very new—human embryonic stem cells were first identified in 1998. Moreover, in the US and some other countries, progress has been slowed by funding restrictions and political controversies.
Probably not. This type of cell is present for only a limited time during the earliest stages of growth. After that, the process of development does not appear to require a cell that can form so many different types of cells and tissues and so, unlimited cells disappear. However, there have been a few very interesting reports of cells that have some of the features of unlimited stem cells. Sources here include the amniotic fluid that surrounds the fetus in the womb, rare cells in the blood system, certain types of fat cells, and even the small amount of pulp that remains in baby teeth after they are lost. While scientifically interesting, none of these other unlimited-like cells have proven capable of doing everything that true unlimited stem cells can do. As such, while they remain important to also study in the laboratory, they should not been seen as replacements for unlimited (embryonic) stem cells.
The umbilical cord and placenta are fully-developed organs and are a source of one or two types of limited (adult) stem cells, the blood producing stem cell and perhaps some connective tissue cells. Upon giving birth, it is possible to save or donate these stem cells for potential transplants to treat blood diseases in small children or donate them for research.