Transmission electron micrograph of nucleus showing the characteristic features of glutaraldehyde fixation introduced as a primary fixative in 1962 and since used routinely. Chromatin appears differentiated into coarse, darkly stained heterochromatin that accumulates near the nuclear periphery and the nucleolus and less darkly staining euchromatin. In contrast, chromatin appears uniformly stained with osmium tetroxide as primary fixative (for example, see CIL image 10974, in this image group). Figure 113 from Chapter 4 (Nucleus) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the corresponding chapter is available on the ASCB's BioEDUCATE website.