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?
How do adult stem cells evolve during development and how are they maintained in the adult? Are they “leftover” embryonic stem cells, or do they arise in some other way?
Why do stem cells remain in an undifferentiated state when all the cells around them have differentiated?
What are the characteristics of their “niche” that controls their behavior?
Do adult stem cells have the capacity to transdifferentiate, and is it possible to control this process to improve its reliability and efficiency?
If the beneficial effect of adult stem cell transplantation is a trophic effect, what are the mechanisms? Is donor cell-recipient cell contact required, secretion of factors by the donor cell, or both?
What are the factors that control adult stem cell proliferation and differentiation?
What are the factors that stimulate stem cells to relocate to sites of injury or damage, and how can this process be enhanced for better healing?