Ultrastructure and Mineralization of the Shell of Lingula unguis Linne, (Inarticualte Brachiopod)

Ultrastructure of the shell of Lingula unguis Linne, an inarticulate brachiopod, was studied mainly by electron microscopy to investigate mineralization of the shell. Lingula has a chitino-phosphatic shell, which consists of the periostracum, alternations of organic and mineralized layers, and the punctae. The main part of the periostracum shows a striped pattern almost perpendicular to the shell surface and covered by a triple-layered membrane. Showing an elaborate reticulated pattern the organic layer is made up of fine chitinous fibrils. The mineralized layer thins out antero-posteriorly and laterally. Not uniformely mineralized, it shows different grading zones of crystallites at different parts of the shell, as distinguished in the following: amorphous particles or minute granular crystallites; acicular crystallites which are arranged subparallel or intersected at a low angle; coalescent acicular crystallites which intersect irregularly. The organic matrix of the mineralized layer consists of fine proteineous fibrils without periodicity. Cytoplasmic projections of the epithelial cells intrude the punctae, and the distal ends of them adjoin the inner wall of the punctae by a net of fine threads. X-ray diffraction analysis revealed that Lingula shell consisted of carbonate fluorapatite which was similar to francolite. Detected from an electron probe analysis were Ca, P, Mg, F, S, and Na. Amino acid analysis of the shell showed a large amount of alanine and glycine, as well as the characteristic presence of hydroxyproline. A histochemical test of specimens for an organic matrix showed a similarity to collagen, but the ultrastructure of it differed from the typical collagen fibrils.