Figure 4
FE-SEM micrographs of polished surfaces of chemically fixed (a), high-pressure frozen and freeze-substituted (b–f) and etched (d,f) shell pieces of Magellania venosa. Samples in (b,d,f) were etched at a pH of 9, for 40 seconds with a 0.1 molar HEPES and 2,5% glutaraldehyde solution and critical point dried. Samples in (c,e) were polished but not etched nor critical point dried. Micrographs were recorded using secondary electron (at 4 kV; a) and converted backscattered electron (at 4 kV; d–f) signals, respectively. (a–f) Outer mantle epithelium (OME) cells are always in very close contact to the calcite of the fibres. It is well visible that at some cell – calcite interfaces the extracellular biopolymer lining of the fibre is not developed (red arrows in c,e,d,f). These are the sites where fibre formation is still in progress. In contrast, at sites where the extracellular biopolymer membrane along the proximal, convex surface of a fibre is well observable (yellow arrows in b,d,f), fibre mineralization is complete. At their basal side, epithelial cells are connected to the connective tissue by basal hemidesmosomes (red dots in a). Samples (c,d) are high-pressure frozen, freeze-substituted, embedded in EPON resin and polished with a diamond knife; samples (d,f) are, in addition, etched for possible detection and visualization of amorphous calcium carbonate. G: Golgi apparatus, m: mitochondria, t: tonofilaments, v- vesicles.
