Stems Cells in general are cells that are naturally in the body but have not completely taken on a terminal function. The process of a cell specializing in function is termed differentiation. There are many levels to which cells can differentiate and therefore provides a classification of stem cells:

1. Totipotent- can differentiate into any cell type in the body plus the placenta, which nourishes the embryo. These are cells within the first 4 divisions of the fertilized egg.

2. Pluripotent- can differentiate into any cell type except the placenta, such as embryonic stem cells specifically the inner cell mass of a blastocyst.

3. Multipotent- can differentiate into cells down a specific lineage, such as hematopoietic stem cells or neural stem cells.

There are two major sources of stem cells, embryonic stem cells and adult stem cells. Embryonic stem cells are pluripotent and are recovered from the inner cell mass of a blastocyst during in vitro fertilization. The advantages of using embryonic stem cells are that they have the ability to become any type of cell in the body and have the capabilty to grow infinitely in culture. However, the drawbacks to embryonic stem cells are they can invoke an immune response form the receiving patient and have ethical controversies about their use. The immune response exists since the cells are taken from another person and not compatible wiht patients immune system. One solution to overcome this limitation is to take immune suppressors, however in some cases this can do more harm. The reason for the ethical controversy is that embryonic stem cells are taken from a feritilized embryo which has the potential to become an adult but is aborted in the procedure. Many people feel that despite all the good that can come from embryonic stem cell therapies it is not justified to end its growth. Although embryonic stem cells are taken from discarded embryos from in vitro fertilization, where the decision that a life will not be made is already made, the government still does not fund utilizing this as a source. Therefore much research has been put into adult stem cells which do not have an ethical controversy.

Adult stem cells are obtained from the patient directly or from another adult person. Since no life is being "taken", there is not ethical issues, making this source of stem cells more desireable in one sense. However, there are major drawbacks in comparison to embryonic stem cells. Adult stem cells are not able to be grown as greatly in culture since they are already in a much more differentiated state. Also, in mose cases, large enough numbers of adult stem cells are more difficult to attain from the person without risking other complications. Therefore the ability of adult stemc cells to be mass produced is significantly inhibited in comparison to embryonic stem cells.

The hope for stem cells, embryonic or adult, is to replace abnormally functioning or degenerative cells. This procedure is considered to be cell therapy as opposed to gene therapy where DNA is direclty altered to invoke a desired phenotype. The reason many people favor cell therapy over gene therapy is that for many diseases, function only needs to be restored and not created. Utilizing cells that are programmed to perform the desired task is much simpler in theory than introducing genes that program the task to the cell.