Fig. 5.

Ultrathin sections of bacteria, virus and cell cultures after rapid embedding in LR White (2.5 μl accelerator per ml monomer). A, B. Longitudinal sections through a dividing bacterial cell of Escherichia coli. The cytoplasm appears granular and dense except for the centre where it shows fine strands, probably indicating localization of the DNA (A). The cell wall shows two membranes, the outer (om) and the inner membrane (im), typical for gram-negative bacteria (B). Note that the sample was neither post-fixed with osmium tetroxide nor treated by en bloc staining with uranyl acetate. C, D. Sections through a pellet of vaccina virus (cell culture supernatant mixed with gold colloids). C. Overview of the pellet in cross section. Virus particles (arrows) and cellular debris (^⁎) are surrounded by gold particles (black dots of about 10 nm size). D. Sections through virus particles in different orientations. The side view (arrow) shows the typical dumbbell shape of the core. The different membranes of the virus particles are well preserved. E, F. Retrovirus-infected L20B cells (post-fixed in 1% osmium tetroxide). Virus particles are localized in membrane-bound compartments (^⁎) of the cytoplasm that are continuous with the endoplasmic reticulum (E). Virus particles are formed by budding at the compartment membrane into the lumen (arrows in F). Particles consist of a membrane and an inner ring-like structure giving in a double ring-appearance typical for type-A retrovirus particles (F). (mi) mitochondrion; (n) nucleus. Bar in A, C, E = 0.5 μm and in B, D, F = 100 nm.