The structure and organization of, and the relationship between, the inorganic crystals and the organic matrix of the prismatic region of Mytilus edulis

Abstract

Electron microscope and electron diffraction studies of the prismatic region of the young blue mussel, Mytilus edulis, have revealed the structure and organization of, and the relationship between the inorganic crystals and the organic matrix. The inorganic crystals within the prisms are predominantly calcite, but also evident are impurities of celestite (SrSO4), strontianite (SrCO3), barite (BaSO4), and witherite (BaCO3). The crystals are approximately 700 long, 330 wide, and 140 thick. They are roughly rectangular in shape, have the habit of plates, are structurally organized within the uniquely anvil-like shaped prisms, and they show a well defined three-dimensional morphological orientation. Their morphological long dimensions parallel the width, their wide dimensions parallel the length, and their thick dimensions parallel the thickness of the prisms in which they are located. This three-dimensional morphological orientation of the crystals coincides with a well defined three-dimensional crystallographic orientation. Decalcified sections have revealed that the organic matrix is a striking fingerprint of the mineralized matrix. Like the mineralized matrix, it is structurally organized into prisms, which are delineated from one another by prism sheaths, and an intraprismatic matrix. The intraprismatic organic matrix when viewed in any two perpendicular profiles is remarkably similar in general orientation and organization to the fully mineralized tissue. It is structurally organized into closely packed sheet-like compartments and subcompartments in which the inorganic crystals are deposited. Like enamel, the prismatic organic matrix of Mytilus beautifully exemplifies the architectural principal of organization within mineralized tissues which permits large amounts of mineral (in this case, 100% of the crystalline phase) to be deposited in the highly ordered organic matrix without disruption of its structural organization. Moreover, the sheet-like compartments exert intimate restricting influences over the growing crystals by limiting their habit to rectangular-shaped plates, their size and more specifically, their highly ordered, three-dimensional morphological orientation, which coincides with a well defined three-dimensional crystallographic orientation within the prisms.