Mantle histology, histochemistry and ultrastructure of the pearl oyster

The edge and isthmus of the mantle of Pinctada margaritifera (Mollusc, Bivalvia) were studied using light and electron microscopy from a morphological and histochemical point of view. Six areas with several subareas were differentiated in the mantle edge. One of these areas is similar to the epithelium isthmus. Trials to elucidate the nature of the pigment responsible for the black colour of the shell and pearl indicate a melanin-like material. Possible functions of the distinctive epithelial areas in Pinctada margaritifera mantle are discussed according to available data on the secretion of organic matrix of the shell, protein selerotization and deposition of crystals in some pearl-forming molluscs. Three different epithelial areas in the outer mantle edge of Pinctada margaritifera are probably involved in the sequential processes of deposition of calcite prisms, aragonite fibres and aragonite rhombs.