Fig. 5. Immunoelectron microscopic localisation of p40/50 in Balbiani rings (A), nucleoplasm (B) and cytoplasm (C) in salivary gland cells. Ultrathin cryosections of salivary glands were incubated with a p40/50 antibody, a gold-conjugated secondary antibody was added to visualise the labelled sites, and the sections were finally embedded in polyvinyl alcohol. Small arrows indicate the position of gold particles; the barred arrows denote specifically labelled BR particles. A schematic drawing outlining the assembly and transport of BR particles is shown below the electron micrographs. Large arrows relate the electron micrographs to the corresponding stage during mRNA biogenesis. Bar, 100 nm.