The inner-shell film: An immediate structure participating in pearl oyster shell formation

In mollusks, the inner shell film is located in the shell-mantle zone and it is important in shell formation. In this study, we found that the film was composed of two individual films under certain states and some columnar structures were observed between the two individual films. The inner shell film was separated with the process of ethylenediaminetetraocetic acid (EDTA) treatment and the film proteins were extracted. Amino acid analysis showed that the film proteins may consist of shell framework proteins. The calcite crystallization experiment showed that the film proteins could inhibit the growth of calcite, while the CaCO3 precipitation experiment showed that the film proteins could accelerate the rate of CaCO3 precipitation. All these results suggested that the film plays an important role in shell formation. It may facilitate the aragonite formation by inhibiting the growth of calcite and accelerate the shell growth by promoting the precipitation of CaCO3 crystals.