The human body is a marvellous thing.
But like anything built by evolution, it has plenty of flaws.
Whereas sharks grow new teeth throughout their lives, adult humans get one set, which must last 60 years or more.
That is tricky.
A combination of poverty, sugar-rich diets and poor hygiene means 2.5bn people around the world suffer from tooth decay, in which acid produced by mouth-dwelling bacteria eats away at the hard enamel that coats the outside of a tooth.
That can open the door to painful infections, which cause further damage.
Once decay has set in, all a dentist can do is fill the gap with an artificial plug—a filling.
But in a paper published in Cell, Hannele Ruohola-Baker, a stem-cell biologist at the University of Washington, and her colleagues offer a possible alternative.
Stem cells are those that have the capacity to turn themselves into any other type of cell in the body.
It may soon be possible, the researchers argue, to use those protean cells to regrow a tooth’s enamel naturally.
The first step was to work out exactly how enamel is produced.
That is tricky, because enamel-making cells, known as ameloblasts, disappear soon after a person’s adult teeth have finished growing.
To get round that problem, the researchers turned to samples of tissue from human foetuses that had been aborted, either medically or naturally.
Such tissues contain plenty of functioning ameloblasts.
The researchers then checked to see which genes were especially active in the enamel-producing cells.
Tooth enamel is made mostly of calcium phosphate, and genes that code for proteins designed to bind to calcium were particularly busy.
They also assessed another type of cell called odontoblasts.
These express genes that produce dentine, another type of hard tissue that lies beneath the outer enamel.