For ultrastructural analysis, FB samples were embedded in Durcupan ACM epoxy resin, as follows: the samples were pre-fixed with 3% glutaraldehyde in 0.1 M cacodilate buffer, pH 7.2, for 3 h at room temperature; after washing in the same buffer, they were post-fixed with buffered 1% osmium tetroxide for 2 h at 4 °C, dehydrated in an ethanol series, and embedded in the resin.