Calcite fibre formation in modern brachiopod shells

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.

Single molecule protein stabilisation translates to macromolecular mechanics of a protein network

“…cell cytoskeletons) with properties such as structural support and repetitive energy dissipation. [8][9][10][11][12][13] For example, a-helical lamin protein domains that define the lattice-like network of the cell's nucleus, provide both crucial structural support, as well as aiding in the coupling of mechanical signals to complex biochemical processes in the cell. 14 The staggered architecture in a collagen microfibril permits energy dissipation through molecular sliding, rather than snapping and can withstand GPa of pressure.…”

Microstructure and crystallography of the wall plates of the giant barnacleAustromegabalanus psittacus: a material organized by crystal growth

“…Accordingly, the corresponding extrapallial space must be relatively wide and variable in thickness. This is unlike bivalves [18] and rhynchonelliform brachiopods [19], in which some microstructures are produced across extremely thin (100 nm or less) extrapallial spaces. Accordingly, the crystals can protrude only by a matter of tens of nanometres.…”

Supplemental Material: Lithium isotope composition of modern and fossilized Cenozoic brachiopods

“…Through many studies of brachiopod shell structure, Williams proposed that the different layers of the shell are produced by a single cell via a conveyor belt system (Williams, 1953(Williams, , 1966(Williams, , 1968a(Williams, , 1968bWilliams and Rowell, 1965). In contrast, a recent study proposed that more than one cell participates in the precipitation of a single calcite fiber/crystal, suggesting that calcite fibers are formed via ion transport of calcium (along with trace/minor elements) and carbonate rather than forming elsewhere in the epithelium and being transported and secreted by cell vacuoles (Simonet Roda et al, 2019). Additionally, while Simonet Roda et al (2019) did not find evidence of a metastable, amorphous calcium carbonate (ACC) precursor to brachiopod calcite, ACC has been observed in one study of a rhychonelliform brachiopod (Griesshaber et al, 2009), while other researchers found that brachiopod shell fibers are composed of calcite nanospheres or granules.…”