Preparation of isolated nuclei from K 562 haemopoietic cell line for high resolution scanning electron microscopy

Macromolecular substructure in nuclear pore complexes by in-lens field-emission scanning electron microscopy

Scanning electron microscopy (SEM) has produced a wealth of novel images that have significantly complemented our perception of biological structure and function, derived initially from transmission electron microscopy (TEM) information. SEM is a surface imaging technology, and its impact at the subcellular level has been restricted by reduced resolution in comparison with TEM. Recently, SEM resolution has been considerably improved by the advent of high-brightness sources used in field-emission instruments (FEISEM) which have produced resolution of around 1 nm, virtually equivalent to TEM "working resolution." Here we review our findings in the use of FEISEM in the imaging of nuclear envelopes and their associated structures, such as nuclear pore complexes, and the relationships of structure and function. FEISEM allows the structurally orientated cell biologist to visualise, directly and in three dimensions, subcellular structure and its modulation with a view to understanding its functional significance.

Imaging of cytoskeletal elements by low-temperature high-resolution scanning electron microscopy

Actin filaments and microtubules, both in situ and in vitro, were imaged using high-resolution scanning electron microscopy (HRSEM) at low temperature. For visualization of cytoskeletal elements in situ, fibroblasts were first extracted and fixed; for cytoskeletal elements in vitro, purified proteins were polymerized and fixed. Both types of specimen were then subjected to plunge freezing, controlled freeze-drying, cryo-sputter coating with a thin chromium layer, cryo-transferring and cryo-observation in an FESEM. The three-dimensional architecture of the cytoskeleton was well preserved, permitting examination of the structural relationships among cytoskeletal elements. Actin filaments and microtubules were identified by their characteristic helical features. Two periodicities of actin filaments, the short pitch of the left-handed helix measured at 5.5 nm and the 37-nm-long pitch helix, were revealed. Individual protofilaments were seen in microtubules as well as the characteristic 4-nm repeat of tubulin subunits along the protofilament. Clathrin cages were also observed. This technique provides a powerful approach for direct imaging of macromolecular structures with high contrast and high signal-to-noise ratio at a resolution of 2-3 nm.

Structural and functional organization of the nuclear envelope

The double-membrane nuclear envelope is punctuated by pores where the two membranes are joined. These pores are stabilized by the elaborate nuclear pore complex, which is anchored to the inner membrane by the nuclear lamina, as well as to other nuclear and cytoskeletal structures. Recent experiments have identified proteins involved in the stability of this organization as well as in the function of the nuclear pore complex, which we relate here to newer aspects of nuclear envelope structure.