In this Oct. 22, 2008 photo, research associate Crystal Pacutin views pluripotent human embryonic stem cells grown on a mouse cell layer under a microscope at the University of Michigan Center for Human Embryonic Stem Cell Research Laboratory in Ann Arbor, Mich. (Paul Sancya/Associated Press)

Since being named "Breakthrough of the Year" by Science magazine in 1999, stem cells have captivated researchers and sparked debate — all while falling short of delivering on promises of new treatments for old diseases.

Researchers say major breakthroughs in dealing with disease are likely still years away.

There has also been intense debate over the use of stem cells. Scientists say embryonic stem cells are the most useful type because they have the potential to become any type of cell within the body. However, they are harvested from embryos grown in the lab.

Opponents argue that any embryo — whether it is naturally conceived or created in a lab — has the potential to develop into a mature human.

The timeline below illustrates some key dates and developments in stem cell research.