Wednesday, December 16, 1998 Published at 16:12 GMT
Bite on this
Smile - carbonated calcium hydroxy-apatite
By BBC News Online Science Editor Dr David Whitehouse
The hardest known substance in animals is tooth enamel. Why it is so tough has always lacked a good explanation but now researchers at the University of Southern California's School of Dentistry are closing in on the secret.
They have identified tiny spheres that regulate the formation and organisation of tooth enamel by controlling the substance's crystalline growth.
They are called nanospheres because they are only 20 nanometres in diameter (A nanometre is a billionth of a metre, and a 20-nanometer- diameter sphere is roughly 1/500th the size of a red blood cell.)
They are formed by a naturally occurring family of tooth-specific proteins known as amelogenins.
"More than 98% of tooth enamel consists of carbonated calcium hydroxy-apatite," says professor A.G. Fincham. "Essentially, your teeth are made of rock."
The dentist's goal is that one day they may be able to replace mercury-based gold and silver fillings with material more like natural tooth enamel.
"Beyond that, the same principles that nature uses to make enamel might also be applied to create novel synthetic materials," Professor Fincham says.
Tooth enamel begins to form in the human embryo when a specialised layer of cells, called ameloblasts, in the embryonic tooth bud secretes amelogenin proteins. The amelogenins self- assemble to form the matrix within which the crystals of mineral start to form.
"The earliest enamel crystals form in extremely long, thin ribbons and are rather beautifully parallel," Fincham notes.
"Magnified in an electron microscope, they looked like tiny ping pong balls among the long ribbons of crystal."
A more powerful atomic force microscope recently revealed that the spheres are uniformly 18 to 20 nanometres in diameter.
Such crystals grown in the lab by traditional methods are about 100 times smaller than the crystals nature makes. They grow haphazardly, and the resulting material is considerably weaker than natural enamel.