The first entire replacement human organ to be grown from stem cells in a mature host will be...

A heart?

A liver?

A pancreas?

Nope.

Chances are, it will be a tooth:

Regenerating a whole tooth is no less complicated than rebuilding a whole heart, says Songtao Shi of the University of Southern California, who heads a team working on creating such a tooth.

Not only do you have to create smart tissue (nerves), strong tissue (ligaments) and soft tissue (pulp), you've got to build enamel -- by far the hardest structural element in the body. And you have to have openings for blood vessels and nerves. And you have to make the whole thing stick together. And you have to anchor it in bone. And then you have to make the entire arrangement last a lifetime in the juicy stew of bacteria that is your mouth.

It's a nuisance, but researchers are closing in on it. In fact, they think the tooth will probably be the first complex organ to be completely regenerated from stem cells. In part this is because teeth are easily accessible -- say ahhhhh. So are adult stem cells, found abundantly in both wisdom and baby teeth -- no embryos required, and your immune system won't reject your own cells.

Nobody is predicting when the first whole tooth will be grown in a human, although five to 10 years is a common guess. "The whole tooth -- we've got a long way to go," says Shi.

But his team is pursuing what he believes is a practical and immediate result: growing important parts of teeth that he thinks people will want to use right away. They're working on creating a living root from scratch. "I think it will take a year," Shi says. "Depends on how lucky we are, and how good we are."

The only downside here is that many of us had our wisdom teeth removed in early adulthood as recommended by our dentists, so the stem cells needed to create a new tooth won't be available for us. We can only hope that there will be continued development of techniques for converting mature cells into stem cells. Plus, if regenerated teeth are on the horizon, the other organs mentioned above can't be far behind.

In fact, we've been tracking this progress for some time.

Remember the tooth!

[Update: Phil here. Please don't ask me what the photo and caption above mean. I have no idea. I believe an uncredited co-blogger has generously added them to this entry!]