There is a gap between biochemical findings and ultrastructural interpretation of “glycogen granules”. Biochemists have recognized that glycogen contains covalently bound proteins. These include enzymes involved in giycogen metabolism: glycogenin (protein primer responsible for initiation of glycogen synthesis), glycogen synthase and phosphorylase, and presumably other regulatory enzymes. The structures formed by the association of glycogen and protein have been called protein-glycogen complexes, considered as proteoglycans, or as dynamic cellular organelles, glycosomes.

The question arises as to why the biochemical recognition of a protein component in glycosomes has not been acknowledged in electron microscopy (EM)? This protein is visible in every section stained by uranium (U) and lead (Pb) salts where it appears as 20-30 nm granules (Fig. 1). However, these granules are commonly interpreted as glycogen, despite the fact that glycogen does not react with ionic compounds and therefore cannot be stained by U-Pb.