Oysters rely on a specialized kind of blood cell to deliver the calcium carbonate crystals they need to build their shells, according to a new report. The finding contradicts conventional wisdom about shell growth mechanisms and may hold valuable clues to improving methods for cultivating pearls.
Oysters and other mollusks surround themselves with a thin layer of organic material lining their protective shells. Scientists had assumed that this extracellular organic matrix was wholly responsible for shell growth: It was thought to directly nucleate calcium carbonate crystals from the surrounding solution, to regulate crystal growth and shape, and to terminate crystal growth.
But new work from adjunct assistant professor Andrew S. Mount and his colleagues at Clemson University in South Carolina suggests that this model of shell growth needs to be revised. Using scanning electron microscopy and X-ray microanalysis, Mount's team has shown that a class of immune cells in oysters' blood actually delivers the calcium carbonate crystals required for shell building [Science, 304, 297 (2004)]. In the electron micrograph shown, these crystal-carrying immune cells (green) are interacting with the crystalline structures (gold) at the mineralization front. (The organic matrix is shown in blue.) Mount's team has also observed the cells releasing their crystal cargo at these sites. There, the nascent crystals are remodeled into plates that make up new shell.
Cultured pearls likely grow using the same mechanism, Mount says. But whether immune cells play a similar role in other biomineralization processes remains to be seen, he acknowledges. Still, his findings may cause researchers to take another look.