Alternative Stem Cell Sources
Embryonic Stem Cells
Embryonic stem cells are isolated early in embryonic development. Their derivation requires the use and destruction of the organism at the embryonic stage of development.
The interest in these cells is due to their ability to become any cell type in the individual. Mouse embryonic stem cells have been studied for decades, which have produced an enormous amount of information on how an organism develops and how to manipulate these cells to become different cell types.
However, it remains difficult to control exactly what cell types these stem cells can become. A related safety concern associated with the use of embryonic stem cells is that they sometimes form tumors (teratomas).
Somatic Cell Nuclear Transfer
Somatic cell nuclear transfer involves taking the cell from an individual (either a child or an adult donor) and transferring the nucleus, which contains the genetic information in the DNA and chromosomes, and inserting it into an egg. This egg is then stimulated, without fertilization, into developing into a living embryo.
It is possible, in principle, to transfer this embryo into a female’s body to initiate a pregnancy that will result in the live birth of an offspring that is nearly genetically identical to the cell donor.
This technique (sometimes called “reproductive cloning”) was used to clone Dolly the sheep several years ago. It has never been accomplished in primates.
Alternatively, the living cloned embryo can be destroyed to isolate the resulting embryonic stem cells. Such cells would be nearly genetically identical to the donor of the somatic cell. This is sometimes called “therapeutic cloning.” No cloned embryonic stem cells have ever been successfully isolated from human beings.
Induced Pluripotent Stem (iPS) Cells
Induced pluripotent stem cells are generated by taking cells, such as skin cells, from a person and then injecting a small number of specific genes or molecules into the cells, which converts the cells into stem cells.
Thus, these iPS cells do not result in the death of any embryos or require the use of unfertilized eggs. One concern with iPS cells is that one of the genes that are introduced into the cells is an oncogene (gene that can cause cancer).
Additionally, viruses are commonly used to introduce the genes into the cells and these viruses can cause mutations in the cells. Recent studies have reported the generation of iPS cells without using oncogenes or viruses.
Adult Stem Cells
Adult stem cells are found in most, if not all, tissues in all living organisms. While these adult stem cells cannot become any cell in the organism, they can potentially become any cell in the organ from where they originated. Because these cells are present in an individual’s body, there is very little chance that the regenerated tissue will be rejected by the immune system.
Additionally, these adult stem cells often appear to have the capacity to recognize the location of damage and the dying cell types, such that only those missing cells are regenerated. Thus, adult stem cells do not require the death of any embryos or use unfertilized eggs.