Spicule Formation in the Invertebrates with Special Reference to the Gorgonian Leptogorgia virgulata

Many of the invertebrates possess calcium carbonate spicules.This paper is a review of the formation of these structuresin the Porifera, Coelenterata, Platyhelminthes, Mollusca, Echinodermataand Ascidiacea. Mature spicules appear to be extracellular structures.Sponge spicules initiate intercellularly then become extracellular.Alcyonarian, turbellarian, echinoid and ascidian spicule depositionbegins intracellularly and then becomes extracellular. The continuationof growth in the extracellular environment has not been documentedexcept for the echinoids. Placophoran spicules initiate andremain as extracellular structures. Early spicule growth seemsto occur from or within a single cell. However, cell aggregationand/or neighboring cells appear to be important to the processof spicule formation. The spicule forming cells, in general,are found in a collagenous medium which may be associated withspicule growth. The organic matrix from the spicules of the gorgonian Leptogorgiavirgulata is a glycoprotein. Autoradiography reveals that thismatrix is apparently synthesized in the rough endoplasmic reticulumand Golgi complexes and then transported to the spicule formingvacuole via Golgi vesicles. To gain information about the entryand transport of calcium ions, the effects of ouabain and vanadateon calcium uptake were examined. Ouabain had no effect on calciumuptake. Vanadate treatment increased the uptake of calcium inscleroblasts and epithelial tissue and decreased its uptakein spicules. This may suggest that vanadate sensitive ATPasesare involved in the pumping of calcium out of scleroblasts,out of epithelial cells into the mesoglea, and into scleroblastorganelles. Autoradiography using ⁴⁵Ca indicates that the majorityof these ions initially accumulate in the branch axis. The labelmoves through the axial epithelium to the mesoglea and reachesthe spiculeforming vacuoles in the scleroblasts via dense bodies