How many kinds of stem cells are there?
The answer to this question is not known with certainty. Researchers in the field divide stem cells into four broad catagories: adult stem cells, fetal stem cells, embryonic stem cells, and more recently(2007), induced stem cells. These catagories refer to the source of the cells, but do not describe their nature. A new type of stem cell, nuclear transplant stem cells (“ovasomes”), takes advantage of new methods to create stem cells. Some of the methods are the same as those used for cloning animals, and thus, unfortunately, this new methodology has been highly controversial. Another new type of stem cell, parthenote stem cells from “activated eggs”, takes advantage of new methods to stimulate unfertilized eggs to divide. This new catagory has received less public debate because it is less common and less well understood. It is becoming clear, however, that parthenote stem cells may have important therapeutic potential.
