Abstract
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.
Highlights
Brachiopods are extant shell-forming, marine, sessile organisms abundant throughout the Phanerozoic, during the Paleozoic when they dominated the marine benthic ecosystem
We demonstrate for the terebratulide species Magellania venosa, (i) the very close spatial relationship of the outer mantle epithelium with the calcite fibres, (ii) the tight control of the outer mantle epithelium cells on fibre secretion, and, (iii) describe the sequence of processes that take place with brachiopod fibre formation
Our results show that fibre secretion and fibre shape formation in the modern brachiopod Magellania venosa is a dynamic process
Summary
Brachiopods are extant shell-forming, marine, sessile organisms abundant throughout the Phanerozoic, 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). Brachiopod shells are of interest to material science, as these are important prototypes for bioinspired light-weight and energy-efficient hybrid materials In these materials, advantageous mechanical properties of one component 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). Aragonite or calcite fibres are embedded in a pliant biopolymer matrix (e.g.34,37–45), the latter being always cross-linked within the hard tissue (e.g.29–31). Biopolymer matrices are plasticized with water[30,31], whereas, when the matrix is a mineral, the latter is always softer relative to the hardness of the constituting fibres[46]
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