Cellular Regulation of Biomineralization in Pinctada fucata

Abstract

Because it is widely recognized that the mantle tissue controls nacre precipitation, intensive studies have been conducted to unravel the regulatory mechanism underlying nacre formation. In this chapter, we will focus on the cellular regulation of shell formation in the pearl oyster Pinctada fucata. First, the morphology and proliferation rate in different parts of the mantle tissue have been investigated, and a proliferation hot spot located in the center of the thinnest mantle region was found. However, mantle tissues mainly composed of the mantle edge were used for primary cell culture, due to their high yield of migrated mantle cells. The primary mantle cell culture is used to study how the mantle cells regulate calcium carbonate precipitation. It was found that the physiological functions of the mantle cells were maintained in vitro. High expression of many shell matrix proteins, including ACCBP, Pif80, and nacrein, and high activities of carbonic anhydrase and alkaline phosphatase were detected. Numerous crystals were found inside the cultured cells by polar light microscopy and scanning electron microscopy, and FTIR and XRD analysis demonstrated that these particles were amorphous calcium carbonate (ACC). What’s more, the cultured mantle cells promoted and regulated calcium carbonate precipitation in the culture dishes. These results showed that mantle cells may directly participate in shell formation. In addition, we have studied the function of hemocytes in shell formation. Hemocytes were identified from P. fucata and found to be present in the extrapallial space (EPS). Many components involved in immunity and calcification were identified by proteomics analysis. Poststimulation of lipopolysaccharide and shell damage, most of the tested immune genes and calcification, was upregulated. Moreover, polar light microscopy, scanning electron microscopy, and energy spectrum showed that some hemocytes carried crystals of calcium carbonate, indicating they participated in both immunity and biomineralization. Our studies elucidated the vital roles of mantle cells and hemocytes in regulating shell growth and nacre formation, which would shed light on the improvement of the cultured pearl quality.