Q. What major scientific obstacles still need to be overcome before the full promise of embryonic stem cell research is realized by patients?

A. Scientists expect it will take many years, and in some cases, many decades before the full potential of stem cells to treat patients is clear.

Why so long? Consider that:

• UCSF's Gail Martin co-discovered embryonic stem cells in mice in 1981.
  
• It took scientists 10 years to gain the necessary insights just to control the behavior of finicky mouse embryonic stem cells in the culture dish.
  
• Scientists suspect it may take at least as long to learn the subtle characteristics of human embryonic stem cells, which behave quite differently from mouse cells in culture.
  
• Scientists still need to identify and understand the genes and growth factors that direct stem cells to form specific cell types.
  
• In the case of adult stem cells, scientists need to determine where these cells reside in each tissue and the roles they naturally play there. Once scientists have accomplished this, they still need to determine whether these cells behave like embryonic stem cells and can be directed to generate specific cell types needed by patients.
  
• Whether from embryonic or adult stem cells, scientists need to determine how the body accepts and responds to stem cell-derived cells introduced into various tissues.
  
• The implications of links between stem cells and diseases such as cancer are only starting to be explored by many laboratories around the world.

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