Spicule formation in the calcareous sponge Sycon ciliatum

Abstract

The spicule primordium is formed in an intercellular cavity within a group of sclerocytes. This cavity contains organic material which ensheaths the growing spicule but does not appear to determine the nature of the mineral morph (magnesian calcite) or the crystallographic orientation of the spicule. The tip of each growing spicule ray is seated in a 'dense cup' in the cytoplasm of the sclerocyte concerned. Both ends of monaxons are initially inserted each into a dense cup. As rays elongate the sclerocyte membrane around the tip becomes invaginated and forms a system of 'converging spaces' that possibly indicate high secretory activity in that region. Spicule growth involves the displacement and expansion of the organic sheath by the enlarging spicule. Fully formed spicules which are exposed to the mesohyl become surrounded by collagen fibrils. However, these fibrils are in no way concerned with the process of mineral deposition and are never found within the spicule calcite.

Nano-cluster composite structure of calcitic sponge spicules—A case study of basic characteristics of biominerals

“…1(c)-(f)) requires strict control of the mineralization process, which seems improbable to be accomplished through the spatial arrangement of the spicule-forming cells alone (cf. Ledger and Jones [14]). Corresponding relations between morphology and crystallography have already been observed in spicules of different calcareous sponges [2,3,7], which is certainly not coincidental.…”

“…The spicules form in intercellular cavities lined by sclerocytes that control the calcite crystal nucleation and the fluid conditions for further CaCO 3 precipitation [12,14]. Spicule growth takes place by crystal elongation as rays, termed actines, in certain crystallographic directions, as well as by concentric thickening of the spicule actines [3,12,15].…”

Comparative study of spiculogenesis in demosponge and hexactinellid larvae

“…While the function of the proteinaceous axial filament in spicule formation is not known, an organic matrix appears to be required for silica deposition in all siliceous sponges, but not in deposition of silica in siliceous ameba or choanoflagellates (Harrison et al, 1981;Leadbeater, 1981). No organic matrix exists in the spicules in calcareous sponges (Ledger and Jones, 1977).…”

Extra-, Inter-, and Intracellular Mineralization in Invertebrates and Algae

“…One is the calcite spicules of calcareous sponges. The monaxon is formed within a narrow space between a pair of sclerocytes, and the triax arises in a intercellular space formed between each two of six aggregated sclerocytes (Jones, 1970;Ledger & Jones, 1977). One of the sites of spherule formation in the amphipod Orchestia is the intercellular space.…”

“…The overall shape of mineralized structure may be determined partly by the physical constraints of the vacuole membranes in some organisms, but it is unlikely that it exerts effects on detailed morphology. An exception may be the organic sheath's control over calcareous sponge spicule formation (Ledger & Jones, 1977).…”