The influence of embedding media and fixation on the post-embedment ultrastructural demonstration of complex carbohydrates. II. Dialyzed iron staining.

The influence of fixation and the embedding medium on post-embedment dialyzed iron (DI) staining of acidic complex carbohydrates in mouse colon was studied at the ultrastructural level. DI staining of ultrathin sections from non-osmicated tissues embedded in epoxy resins was very weak, whereas DI staining of non-osmicated tissues embedded in non-epoxy resins such as polystyrene, polyester resins, methacrylates and newly developed embedding mixtures was strong. Brief exposure to OsO4 (5 min) abolished the DI staining in stored secretions of goblet and mucous cells, in apical cytoplasmic vesicles and at the microvillous surface of columnar absorptive cells and in the Golgi cisternae of all colonic cell types in epoxy embedded tissues, but only reduced slightly or had no effect on the DI reactivity observed in these sites in tissues embedded in non-epoxy resins. Prolonged exposure to OsO4 (60 min) prior to embedment in non-epoxy resins further reduced DI staining in all reactive sites and abolished the staining in Golgi cisternae of all colonic epithelial cells. Embedment of non-osmicated tissues in a styrene-Vestopal W mixture and of tissues briefly exposed to OsO4 after primary glutaraldehyde fixation in styrene-methacrylate is recommended for optimal post-embedment DI staining of the acidic groups of complex carbohydrates.