Chemically, the mineral crystals in tooth enamel are a calcium hydroxy-apatite formed from calcium and phosphate ions, which are transported into the nanosphere matrix by ameloblast cells.

"At first," Fincham explains, "the elongated apatite crystals will grow solely on their end faces, becoming ever longer. With the nanospheres acting as spacers, these early crystals build a scaffold on which mature enamel can eventually form. After enzymes have broken down the amelogenin proteins, the crystals start to grow on all of their faces. They thicken, clump together and create mature enamel."

Apatite 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.

Four years ago, the CCMB researchers took the gene for an amelogenin protein from a mouse, placed it in a bacterial cell, and then used the bacterial reproductive process to produce an identical recombinant amelogenin protein. This recombinant amelogenin protein, which the researchers can now produce in quantity, has since been shown to self-assemble to make nanosphere structures identical to those seen in the mouse and other animals, including humans.

"The structure of the amelogenin enamel protein is virtually the same in all vertebrates, from wallabies to humans, suggesting it has a very specialized function," says Fincham. "That function is to spontaneously self-assemble into a matrix with nanospheres -- a matrix that controls the microarchitecture of the developing enamel, both the three-dimensional spacing between the initial mineral crystals and the later crystal growth."

Currently, CCMB researchers are growing apatite crystals within synthetic matrices made from recombinant amelogenin protein.

"We get very long, straight structures," Fincham reports. "The crystals grow on
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Contact: Bob Calverley
calverle@usc.edu
310-740-4750
University of Southern California
10-Dec-1998