Enzymatic, analytical and structural aspects of electron-dense granules in cells of Ucides cordatus (Crustacea, Decapoda) hepatopancreas.

Electron-dense granules (EDGs) are singular structures found in the tissues of several vertebrate and invertebrate organisms. Two types of EDGs were observed in hepatopancreatic cells of the crab Ucides cordatus: (1) a non-mineralized EDG, found mainly inside vacuoles, which reacted positively to acid phosphatase and D-amino acid oxidase, possibly formed by degradation of lipid membranes, and (2) a mineralized EDG surrounded by endoplasmic reticulum membranes that gave a positive reaction for glucose-6-phosphatase. In this study we show the fine structure and composition of the mineralized EDGs using cytochemistry, analytical transmission electron microscopy and field-emission scanning electron microscopy. They are formed of microvesicle-like structures that are arranged in concentric spherical layers in the most mineralized portions of the granule. Analytical microscopy of mineralized EDGs indicated that they are composed of amorphous calcium-magnesium phosphate. Isolated EDGs treated with NaOCl lose several elements, including P, when compared with EDGs treated with deionized water. Removal of the organic matrix by NaOCl induced marked changes in the mineralized EDGs, showing that the organic matrix plays an important role in its elemental composition and structure.