Surface structures consisting of chromatin fibers in isolated barley (Hordeum vulgare) chromosomes revealed by helium ion microscopy

Advanced environmental scanning electron microscopy reveals natural surface nano-morphology of condensed mitotic chromosomes in their native state

The challenge of in-situ handling and high-resolution low-dose imaging of intact, sensitive and wet samples in their native state at nanometer scale, including live samples is met by Advanced Environmental Scanning Electron Microscopy (A-ESEM). This new generation of ESEM utilises machine learning-based optimization of thermodynamic conditions with respect to sample specifics to employ a low temperature method and an ionization secondary electron detector with an electrostatic separator. A modified electron microscope was used, equipped with temperature, humidity and gas pressure sensors for in-situ and real-time monitoring of the sample. A transparent ultra-thin film of ionic liquid is used to increase thermal and electrical conductivity of the samples and to minimize sample damage by free radicals. To validate the power of the new method, we analyze condensed mitotic metaphase chromosomes to reveal new structural features of their perichromosomal layer, and the organization of chromatin fibers, not observed before by any microscopic technique. The ability to resolve nano-structural details of chromosomes using A-ESEM is validated by measuring gold nanoparticles with achievable resolution in the lower nanometre units.

Localization and quantitative distribution of a chromatin structural protein Topoisomerase II on plant chromosome using HVTEM and UHVTEM

Topoisomerase II (TopoII) is an essential structural protein of the metaphase chromosome. It maintains the axial compaction of chromosomes during metaphase. It is localized at the axial region of chromosomes and accumulates at the centromeric region in metaphase chromosomes. However, little is known about TopoII localization and distribution in plant chromosomes, except for several publications. We used high voltage transmission electron microscopy (HVTEM) and ultra-high voltage transmission electron microscopy (UHVTEM) in conjunction with immunogold labeling and visualization techniques to detect TopoII and investigate its localization, alignment, and density on the barley chromosome at 1.4 nm scale. We found that HVTEM and UHVTEM combined with immunogold labeling is suitable for the detection of structural proteins, including a single molecule of TopoII. This is because the average size of the gold particles for TopoII visualization after silver enhancement is 8.9 ± 3.9 nm, which is well detected. We found that 31,005 TopoII molecules are distributed along the barley chromosomes in an unspecific pattern at the chromosome arms and accumulate specifically at the nucleolus organizer regions (NORs) and centromeric region. The TopoII density were 1.32-fold, 1.58-fold, and 1.36-fold at the terminal region, at the NORs, and the centromeric region, respectively. The findings of TopoII localization in this study support the multiple reported functions of TopoII in the barley metaphase chromosome.

Higher-order structure of barley chromosomes observed by electron tomography

The higher order structure of the metaphase chromosome has been an enigma for over a century and several different models have been presented based on results obtained by a variety of techniques. Some disagreements in the results between methods have possibly arisen from artifacts caused during sample preparation such as staining and dehydration. Therefore, we treated barley chromosomes with ionic liquid to minimize the effects of dehydration. We also observed chromosomes on a film with holes to keep pristine chromosome structure from being flattened as seen when placed on a continuous support film. A chromosome placed over a hole in a thin carbon film was mounted on a tomography holder, and its structure was observed in three dimensions (3D) using electron tomography. We found that there are periodic structures with 300-400 nm pitch along the axis in barley chromosomes. The pitch sizes are larger than those observed in human chromosomes.