INTRACELLULAR CRYSTAL-BEARING VESICLES IN THE EPIDERMIS OF SCLERACTINIAN CORALS,ASTRANGIA DANAE(AGASSIZ) ANDPORITES PORITES(PALLAS)

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Abstract
An EM and radiochemical investigation of stony corals, P. porites and A. danae (Cnidaria, Anthozoa, Scleractinia), was conducted to identify and localize the primordial population of CaCO3 crystals comprising the inorganic skeleton. An unresolved controversy exists as to whether the original site of mineralization in corals is intracellular or extracellular. Orthorhombic aragonitic crystals, embedded within a granular lipo-protein matrix and surrounded by a trilaminar membrane were found in epidermal cells of adult Astrangia and of settled larval Porites. These crystals averaged 0.7 .times. 0.1 .times. 0.3 .mu.m and were localized in Golgi-derived vesicles of the apical cytoplasm. Short-term labeling of Astrangia with 45Ca revealed distribution of radioactivity among a basic tissue fraction (92%), an acid tissue fraction (5%) and a skeletal fraction (3%). The acid tissue fraction included, at least in part, the crystalline Ca salt. Identification of the primordial aragonitic crystals within membrane-bound vesicles provided evidence for the intracellular mode of calcification in Scleractinia. A novel concept of cellular regulation over coral skeletogenesis was developed. The vesicle was a miniature crystal fabrication station as well as a vesicle for transportation of seed crystals and their organic matrices to sites of discharge from the cell. The plasmalemma and Golgi vesicle membranes were probable loci of transport and enzymatic activities for accumulation of Ca2+ and CO3-. Skeletogenesis was initiated by exocytosis of vesicle contents. The crystals released became nucleation catalysts and the organic matrix, a reserve source of Ca2+ for skeletogenesis. Crystal-bearing vesicles were produced continuously throughout life starting shortly after larval attachment. These vesicles signaled the dynamic process by which initiation, differentiation, growth and limitation of the coral skeleton was regulated.