Shell matrices of Recent rhynchonelliform brachiopods: microstructures and glycosylation studies

Abstract

Like most metazoan biomineralisations, the brachiopod shell is the end product of a biologically controlled calcification process. The main agent of the control is the extracellular matrix, which is secreted by the outer mantle epithelium. This matrix mediates the calcification process by allowing crystal nucleation and elongation in specific orientations and finally, by stopping crystal growth. The proteinaceous moiety of brachiopod shell matrices has been extensively studied. Less known are the post-translational modifications that occur in these matrices, in particular glycosylations. In this comparison of five species of Recent articulated brachiopods, the ratio of soluble to insoluble organic matrix varies between the species. Polydisperse macromolecular materials occur in each of these species with discrete proteins of 50 kDa in Notosaria nigricans, Calloria inconspicua and Neothyris lenticularis, 37 kDa in Terebratulina retusa and Gryphus vitreus and 20–25 kDa in N. nigricans. Protein mixtures from all five species respond differently to anionic stains (Stains-All and Alcian Blue). PAS staining results in a positive smear in C. inconspicua and T. retusa and highlights low molecular weight glycoproteins in C. inconspicua. The polysaccharide composition of the soluble matrix of T. retusa is different from the others due to high proportions of arabinose and low proportions of fucose. In all cases, polysaccharide composition of the insoluble matrix is dominated by glucose and glucosamine. Insoluble matrices have more glucose and xylose and less galactosamine and glucosamine than the corresponding soluble matrix. Relatively high amounts of glucosamine may suggest the presence of chitin in the shell matrix of rhynchonelliform brachiopods.