Chromatin and nuclear envelope of freeze-fractured, neuronal interphase nuclei, resolved by scanning electron microscopy.

Abstract

High-resolution scanning electron microscopy (SEM) has previously been used to study intracellular detail, including chromatin. It has, however, been commonly carried out either on cellular subfractions or following extraction methods to visualize detail. In the work presented here, intracellular detail of neurons of the dorsal root was visualized in situ by viewing freeze-fracture faces obtained after hypotonic expansion. This procedure permits the detailed resolution, by SEM, of juxtanuclear and intranuclear detail to a degree impossible without hypotonic dispersal. In agreement with work previously reported, nuclear chromatin of these interphase cells presents largely as 30-nm fibers, with a next higher hierarchical structure imparted by swelling in magnesium chloride. Detailed analyses showed that particles as small as 10-nm nucleosomes comprising the 30-nm chromatin fiber could be resolved, with "end-on" views of such fibers showing 5 nucleosomes per helical turn of the fiber. Chromatin fibers positioned subjacent to nuclear pores, or associated with "nuclear spaces" communicating with nuclear pores, were frequently found to be resolved as clusters, in an apparently more decondensed conformation, rather than tightly coiled into the 30-nm fiber. In addition, details of the nuclear envelope, including nuclear pores and perinuclear filaments as well as membranes of the endoplasmic reticulum, decorated with ribosomes, were clearly resolved.