Brachiopods (Biology) Research Papers - Academia.edu (original) (raw)

The fibrous calcite layer of modern brachiopod shells is a hybrid composite material and forms a substantial part of the hard tissue. We investigated how cells of the outer mantle epithelium (OME) secrete calcite material and generate the characteristic fibre morphology and composite microstructure of the shell. We employed AFM, FE-SEM, and TEM imaging of embedded/etched, chemically fixed/ decalcified and high-pressure frozen/freeze substituted samples. Calcite fibres are secreted by outer mantle epithelium (OME) cells. Biometric analysis of TEM micrographs indicates that about 50% of these cells are attached via hemidesmosomes to an extracellular organic membrane present at the proximal, convex surface of the fibres. At these sites, mineral secretion is not active. Instead, ion transport from OME cells to developing fibres occurs at regions of closest contact between cells and fibres, however only at sites where the extracellular membrane at the proximal fibre surface is not developed yet. Fibre formation requires the cooperation of several adjacent OME cells. It is a spatially and temporally changing process comprising of detachment of OME cells from the extracellular organic membrane, mineral secretion at detachment sites, termination of secretion with formation of the extracellular organic membrane, and attachment of cells via hemidesmosomes to this membrane. Brachiopods are extant shell-forming, marine, sessile organisms abundant throughout the Phanerozoic, particularly during the Paleozoic when they dominated the marine benthic ecosystem. They are of interest to modern and paleo-environment research, as they cover most of the geological record and live in a wide range of marine habitats (e.g. 1-18). Their shells consist mainly of low-Mg calcite, which is assumed to crystallize in equilibrium with seawater with only small or negligible "vital effects". Modern terebratulide and rhynchonellide brachiopod shells consist of up to three mineralized shell layers: the outer primary, the inner fibrous, and, where developed, an innermost columnar layer 19-22. In two-layered shells the fibrous layer forms an extensive part of the shell. The fibres are hundreds of micrometers long and are essentially single-crystalline mineral units 23,24. They have four surfaces: a proximal convex surface at their base, concave surfaces at their two lateral sides and a concave surface at their apical side. The shape of brachiopod fibres is unique and well developed in the Lower Cambrian, when the orders Protorthida, Orthida and Pentamerida of the class Rhynchonellata emerged with shells having fibrous microstructures 25,26. In recent brachiopods, the morphology and dimension of fibres are characteristic for a given brachiopod species and are evolutionarily adapted to the animal's habitat 27,28. Brachiopod shells are also of interest to material science, as these are important prototypes for bioinspired lightweight and energy-efficient hybrid materials. In these materials, advantageous mechanical properties of one component not only compensate for adverse properties of other's (e.g. 29-31), but additional gain is derived from the overall composite nature of the biological hard tissue (e.g. 32). The mineral component provides high elastic modulus and high compressive strength, while its inferior tensile strength and brittleness is compensated by the