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

Abstract

Orthorhombic aragonitic crystals, embedded with a granular lipo-protein matrix and surrounded by a trilaminar membrane, are localized in the apical cytoplasm of epidermal cells of Scleractinian corals. Adult specimens of Astrangia danae (Agassiz) and settled planulae of Porites porites (Pallas) contain crystals averaging 0.7 mu by 0.1 mu by 0.3 mu within Golgi-derived vesicles. Short-term labelling with 45Ca reveals distribution of radioactivity amont a basic tissue fraction (92%) an acid tissue fraction (5%) and a skeletal fraction (3%). Identification of the primordial crystal population within membrane-bound visicles provides overwhelming evidence for the intracellular mode of calcification in Scleractinia. Moreover, it permits development of a novel concept of cellular regulation over these dynamic events. The membrane-bound vesicel is a miniature crystal fabrication station and a vehicle responsible for transportation of seed crystals and an organic matrix material to sites of discharge from the cell. The vesicle membrane becomes a probable locus of active transport and enzymatic activity as well as a physical barrier to be penetrated for release of vesicle contents into the extracellular milieu. Contact between the vesicle membrane and the plasmalemma would result in exocytosis and the onset of skeletogenesis. Principles governing crystal growth would prevail from then on. The released crystal becomes a nucleation catalyst and the organic matrix, a supply of ionic calcium for self-limiting crystallization. Crystals are produced by the organism spontaneously and continuously from shortly after larval attachment throughout the life of the polyp. Therefore, these membrane-bound vesicles signal the dynamic process by which initiation, differentiation, growth and limitation of the coral skeleton is regulated.

Occurrence, distribution, and localisation of metals in cnidarians

“…If the Ca is associated with mucus granules of some sort and not with the 250 nm vesicles, as envisaged by Hayes and Goreau (1977), or with the small 50 nm vesicles as suggested by Johnston (1980), then the mucus and its associated Ca could still function in some similar manner to that proposed by Goreau (1959). Goreau suggested that Ca 2ϩ was "absorbed" on a polyanionic mucopolysaccharide (Goreau, 1956) in the organic matrix, where it subsequently combined with HCO 3 Ϫ to form calcium carbonate.…”

“…In support of this hypothesis Johnston refers to preliminary experiments on the histochemical detection of Ca in intercellular spaces in all cell layers by precipitation with sodium fluoride, except at the apical regions of the intercellular spaces in the calicoblastic ectoderm and in the subskeletal space. Hayes and Goreau (1977) and Goreau and Hayes (1977) found that the large vesicles in the calicoblastic ectodermal cells contained electron-lucent crystals. They argued that these were calcium carbonate crystals embedded in an organic matrix and suggested that these formed the initial sites for nucleation catalysis, calcium carbonate being subsequently precipitated on these primary crystals from the fluid in the subskeletal space to form crystals growing by epitaxy.…”

Calcification Rates in Corals

“…The high carbonic anhydrase levels predicted have been found (1,5,8). The increase of alkalinity stimulates supersaturation and catalyzes nucleation of calcium carbonate (7). Marshall suggests that carbonate ion removal by calcification releases CO2 for photosynthesis, which would make carbonic anhydrase superfluous and drive cellular pH acidic.…”

Aragonite crystallization in primary cell cultures of multicellular isolates from a hard coral,Pocillopora damicornis

“…The gastroendodermal cells of hermatypic hard corals host symbiotic zooxanthellae (unicellular algae evolved from free-living dinoflagellates of the gymnodinioid form, Symbiodinium sp.). In vivo histochemical and ultrastructural observations of calcifying apical branches of hard coral (13,14), and on radiochemical studies of 45 Ca 2ϩ incorporation into the skeleton (15), have shown that calcification takes place in the calicoblastic epidermis. Autotrophic zooxanthellae symbionts provide part of the coral nutrients and their activity facilitates calcification (16).…”