A fine structural study of the calcareous opercular plate and associated cells of a polychaete annelid

The structure of the organic material and inorganic elements of the opercular plate and associated cells in the serpulid annelid, Pomatoceros lamarckii Quatrefages, have been described by transmission and scanning electron microscopy. After decalcification the organic material of the opercular plate was found to consist of three major structurally different components, an outer, thin, electron-dense layer, parallel rows of rectangular profiles partitioned into large units by cross-walls, and layers of orthogonally arranged fibres. The inorganic aragonite components were found, in contrast, to consist of two structurally different elements namely, highly ordered crystals with a prismatic-like morphology and smaller needle-like crystallites. Two morphologically distinct cell types, columnar opercular rim and cuboidal opercular plate cells, are responsible for the formation of the opercular rim and cuboidal opercular plate cells, are responsible for the formation of the opercular plate. Both possess membrane-bound bodies containing filamentous material. However, in addition, membrane-bound bodies, containing calcium carbonate crystals, are found in some cells. Such bodies are seen to be closely related to the Golgi system. Based on the cytoarchitecture of the cells, the mechanisms involved in the formation and calcification of the opercular plate are discussed.