An electron microscope study on the mineralization of the skeleton of the staghorn coral Acropora hebes

The skeleton formation in the reef-building coral Acropora hebes (Dana) was ultrastructurally investigated by observing the skeletogenic tissue and the skeleton in the apical portion of the branches. The skeletogenic tissue was made of a layer of tall calicoblastic cells which displayed high exocytotic activities. A number of hollow spherules and tiny vesicles were found in the sub-epithelial space between the calicoblastic cell layer and the skeletal plate. These organic materials appeared to occur in the perinuclear Golgi vesicles in the cells. The energy-dispersive X-ray analysis revealed a calcium element in the osmiophilic granules of the calicoblastic cells, but not in any other cell organelles. As the granules showed no diffraction pattern, it was suggested that they could be a Ca-reservoir in the cells. Crystalline particles were found to deposit on the periphery of the sub-epithelial spherules. The spherules developed to spherular crystals by depositing granulated crystalline particles. Two of the spherules appeared to fuse with each other to form a spindle-shaped crystal. The spherular and spindle-shaped crystals accumulated on the thecal ridge and the lateral side surface of the thecal plate, and seemed to contribute to the elongation and thickening of the thecal plate. The thecal plate exhibited a porous structure which probably originated from an aggregation of the central cores of these crystals. On the surface of the thecal plate more than about 5 m thick, scale-like structures composed of spherular crystal substructures were observed. These observations suggest that mineralization in A. hebes occurs in the extracellular space by elabolating the spherular and spindle-shaped precursor structures and that growth of the corallite is brought about by an aggregation and coalescence of these crystals.