Acid polysaccharides in the skeletal matrix and calicoblastic epithelium of the stony coral Mycetophyllia reesi

Like many corals the skeletal organic matrix and associated epithelium of M ycetophyllia reesi is physico-chemically unstable to preparative procedures for electron microscopy. Ethanol cryofracture of mineralized and demineral ized material is accompanied by delamination of tissue and skeleton. Filame ntous algae occur in the interface and account for some but not all of the separation artifact. Transmission microscopy accompanied by decalcification requires embedment in glycerol jelly to preserve the skeletal organic matr ix. Even then, the matrix is not fixed and is not retained within the gel u sing standard double fixation with or without tannic acid as an additive. R uthenium red, in combination with osmium, prevents the matrix from physical disruption, although positional artifacts relative to the calicoblastic ep ithelium are still evident. Inclusion of other glycan precipitating agents in the fixative sequence (Alcian blue, iron diamine or the detergent cetylp yridinium chloride) are more useful in preserving an acid polysaccharide-ri ch, fibrillar, extracellular matrix after demineralization. This material i s not observed in SEM preparations. The calicoblast cells appear to be the source of this extracellular material that also appears to contribute to th e composition of the mineralizing matrix. Moreover, a hyaluronan-like subst ance appears to play a significant role in matrix structure as suggested by its degradation by hyaluronidase. (C) 2001 Harcourt Publishers Ltd.