What are the key questions about adult stem cells?

Many important questions about adult stem cells remain to be answered. They include:

• How many kinds of adult stem cells exist, and in which tissues do they exist?
• What are the sources of adult stem cells in the body? Are they "leftover" embryonic stem cells, or do they arise in some other way? Why do they remain in an undifferentiated state when all the cells around them have differentiated?
• Do adult stem cells normally exhibit plasticity, or do they only transdifferentiate when scientists manipulate them experimentally? What are the signals that regulate the proliferation and differentiation of stem cells that demonstrate plasticity?
• Is it possible to manipulate adult stem cells to enhance their proliferation so that sufficient tissue for transplants can be produced?
• Does a single type of stem cell exist—possibly in the bone marrow or circulating in the blood—that can generate the cells of any organ or tissue?
• What are the factors that stimulate stem cells to relocate to sites of injury or damage?

http://stemcells.nih.gov/info/basics/basics4.asp