Figure 5
TEM micrographs of chemically fixed and decalcified contact between epithelium and shell calcite in modern Magellania venosa. (a) Mineral formation ceases with the secretion of an organic membrane covering the proximal, convex part of the fibre (black arrows). At these sites, we see two membranes: (i) the apical cell membrane of the attaching OME cell (red arrows), and (ii) the organic membrane lining the basal portion of the fibre (black arrows in b). (b) Site of active fibre secretion, there is only one membrane present and visible, namely the apical membrane of the OME cell (red arrows in a and b), which is tightly attached to the calcite of the forming fibre. Neighbouring cells are connected to each other by belt desmosomes. Note the absence of tonofilaments in cells below those parts of the fibre that are actively secreting. rer: rough endoplasmatic reticulum, m: mitochondria.
