He showed 60 Minutes an animation of what happened after he injected more than a million of those cells directly into the spinal cords of rats that were partially paralyzed.
"This is a video of an injured rat. The signal that's going from the brain down to the spinal cord controlling all of the muscles of the body are interrupted by a spinal cord injury," Keirstead explains.
The new cells he created traveled right to the damaged area of the spinal cord and wrapped themselves around the nerves there, enabling the signals to flow uninterrupted through the body. Six weeks later, the once paralyzed animals were able to lift their tails and they could walk.
What was the reaction in the lab when everybody saw that?
"We've been riding quite high for quite a long time now," Keirstead says, laughing. "It's tremendous in research to have something working so well."
Asked what would be considered success in humans, Dr. Keirstead says, "I think we could call this a dazzling success if we saw the smallest improvement in the ability of a human to do anything that they could not do. If they could move a single finger, I would call that a raving success. Let's hope it's a lot more."
Keirstead admits he is concerned about giving people false hope. "The fears of giving somebody false hope are real," he says. "We're not trying to come up with something to take you from zero capabilities to 100 percent. These are incremental advances. And it is experimental."
The clinical trial that Keirstead hopes to conduct will take place next year and would be in collaboration with Geron, the leading biotechnology company in the field. The trial would be only for people with newly injured spinal cords because his work shows they would be more likely to benefit.
He is also working to apply that research to people with long-term paralysis like Suzanne Short. While Keirstead has been widely praised for his work, he has also been criticized by some scientists for trying to rush his treatment into people.
Asked about the criticism that Keirstead is moving too fast, Short says, "I don't think he's moving fast enough. I wish there were 10 more of him to keep working. But, no, it'll happen some day. Hopefully I'll be around for it to happen."
Keirstead admits he is concerned about side effects when stem cells are put into humans.
"I'm very concerned and I'll be losing sleep, no doubt, when this first gets into humans," he says. "There is a potential for harm, this is a risky endeavor, like any clinical trial."
What's not known is whether embryonic stem cells will integrate and function in the human body as well as they have in animals, nor is it known what the side effects will be. Some research indicates that stem cells can form tumors or other abnormal cells. Still, most scientists have confidence in the enormous potential of stem cells to treat incurable diseases such as diabetes, Alzheimer's and, especially, heart disease, which is the No. 1 cause of death in the United States.
That's a trend Dr. Robert Robbins, chair of cardiothoracic surgery at Stanford University, is working to reverse.
The reason for his optimism can be found inside a Petri dish.
"These are the cells that go to make up the heart muscle cells," Robbins explains. "They all started out as cells from embryos. With the potential to develop into any type cell."
Robbins hopes to one day inject the cells, which actually beat like a heart, into someone whose heart has suffered some kind of damage.
In theory, those cells would then replace the damaged part of the heart.
Robbins and his team injected cells like those directly into the hearts of mice with severe cardiac disease. The researchers then tracked the cells and determined that they had stayed in the heart and that the cells were all beating in unison. After six weeks, the new heart cells had replaced the damaged ones, and heart function was restored to near normal.
Looking at the big picture, what's the significance of his findings?
"There's a lot of enthusiasm for this area of research," says Robbins. "There's great hope for patients that suffer from heart failure that may be too old or can't get a heart transplant. And a heart transplant is a big operation. You would much prefer having a therapy that you could go out of the hospital with just a band-aid on your leg versus having a big operation."
Robbins says he hears from a lot of heart patients inquiring about stem cell treatment. What does he tell those patients?
"Well, I would say that it's at least five to 10 years away before I think that we will have enough definitive data in animal studies that it will be safe to go forward with embryonic stem cells," says Robbins.