Department of Earth and Environmental Sciences, LMU, 80333, München, Germany.
Central Facility for Electron Microscopy, University of Ulm, 89069, Ulm, Germany.
Sci Rep. 2019 Jan 24;9(1):598. doi: 10.1038/s41598-018-36959-z.
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.
现代腕足动物壳的纤维方解石层是一种混合复合材料，构成了硬组织的重要部分。我们研究了外套膜上皮（OME）细胞如何分泌方解石物质并产生壳的特征纤维形态和复合微观结构。我们采用 AFM、FE-SEM 和 TEM 成像技术，对嵌入式/蚀刻、化学固定/脱钙和高压冷冻/冷冻替代的样本进行了研究。方解石纤维由外套膜上皮（OME）细胞分泌。TEM 显微照片的生物计量分析表明，这些细胞中约有 50%通过半桥粒附着在纤维近端凸面存在的细胞外有机膜上。在这些部位，矿化分泌不活跃。相反，离子从 OME 细胞向发育中的纤维的运输发生在细胞和纤维之间的最紧密接触区域，但仅在细胞外膜在近端纤维表面尚未发育的部位发生。纤维的形成需要几个相邻的 OME 细胞的合作。这是一个空间和时间上不断变化的过程，包括 OME 细胞从细胞外有机膜上的分离、分离部位的矿化分泌、分泌终止和形成细胞外有机膜，以及通过半桥粒附着到该膜上的细胞。
