Stem cells are a special form of human life: they are alive and contain human DNA. They have a unique feature in that they can be coaxed into developing into some or all of the 220 cell types found in the human body. Eventually, stem cells may be routinely used by doctors to generate new organs or new replacement body parts for people: They might become a new pancreas to cure a person with diabetes, or new nerve cells to cure a paralized person, etc.
There are three types of stem cells:
Embryonic stem cells: A pre-embryo -- a just fertilized embryo called a blastocyst -- consists of many undifferentiated stem cells surrounded by a covering. Embryonic stem cells are usually obtained by a destructive process that extracts the cells while killing the blastocyst itself.
Main advantage: Embryonic stem cells can theoretically become any of the 220 cell types found in the human body. Thus they can theoretically be made into any organ or body part.
Main disadvantage: Many religious and social conservatives who are also pro-life believe that a pre-embryo from which embryonic stem cells are removed is an actual human person perhaps one with a soul. The process of extracting the cells is then considered to have intentionally murdered that person. Others disagree, and believe that human personhood starts much later, sometime during the embryonic, fetal or newborn stage. No scientific test has ever proven the existence of a soul.
Adult stem cells are obtained from a person's bone marrow, from lipid cells from adipose tissue, or blood -- including blood the umbilical cord of a newborn.
Main advantages: Obtaining adult stem cells is a non-destructive process. The DNA in the stem cells is identical to the DNA of the person from whom the cells are removed. Thus if the stem cells are used to fabricate a new organ or body part, there is no possibility of rejection. Also, research has been underway for about 25 years more than embryonic stem cells, so that they are now routinely used in medical procedures. In comparison, embryonic stem cell reasearch are still at the initial experimental trial state.
Main disadvantage: They are less useful in that they have already started to specialize and can only become one of a few cell types.
Induced pluripotent stem cells (iPS) are specially treated ordinary adult cells that can be processed to behave somewhat like embryonic stem cells. This is a recent development that offers great promise. Shinya Yamanaka discovered a method of doing this; he won a Nobel Prize for his discovery. This technique avoids the need to remove cells from a human blatocyst, a living, fertilized ovum, which has major ethical problems for many people.
"To realize the promise of novel cell-based therapies for such pervasive and debilitating diseases, scientists must be able to manipulate stem cells so that they possess the necessary characteristics for successful differentiation, transplantation, and engraftment. The following is a list of steps in successful cell-based treatments that scientists will have to learn to control to bring such treatments to the clinic. To be useful for transplant purposes, stem cells must be reproducibly made to:
Proliferate extensively and generate sufficient quantities of tissue.
Differentiate into the desired cell type(s).
Survive in the recipient after transplant.
Integrate into the surrounding tissue after transplant.
Function appropriately for the duration of the recipient's life.
Avoid harming the recipient in any way.
Also, to avoid the problem of immune rejection, scientists are experimenting with different research strategies to generate tissues that will not be rejected.
To summarize, stem cells offer exciting promise for future therapies, but significant technical hurdles remain that will only be overcome through years of intensive research."