Concerning the nature of the cryosectioning process

Viral Infection at High Magnification: 3D Electron Microscopy Methods to Analyze the Architecture of Infected Cells

As obligate intracellular parasites, viruses need to hijack their cellular hosts and reprogram their machineries in order to replicate their genomes and produce new virions. For the direct visualization of the different steps of a viral life cycle (attachment, entry, replication, assembly and egress) electron microscopy (EM) methods are extremely helpful. While conventional EM has given important information about virus-host cell interactions, the development of three-dimensional EM (3D-EM) approaches provides unprecedented insights into how viruses remodel the intracellular architecture of the host cell. During the last years several 3D-EM methods have been developed. Here we will provide a description of the main approaches and examples of innovative applications.

EVA-enhanced embedding medium for histological analysis of 3D porous scaffold material

When sectioning a 3D porous scaffold made of a soft elastomeric material embedded in paraffin medium, it is not easy to obtain a section because of the different mechanical properties of the paraffin and tissue/scaffold. We describe a new embedding material for histological analysis of various biomaterials that is composed of paraffin and ethylene vinyl acetate (EVA) resin (0, 3, 7, and 13 wt.%). 3D porous poly(L-lactide-epsilon-caprolactone) (PLCL) and chitosan scaffolds were fabricated to test the sectioning efficiency of the paraffin/EVA embedding material. The new embedding material was characterized by rheological analysis and solvent solubility testing in xylene and n-hexane. The hydrophilicity of the new material was assessed by contact angle measurement and its surface roughness was measured using AFM analysis. The staining efficiency of sections embedded in a paraffin/EVA mixture was determined by eosin staining of the chitosan scaffold and chitosan/collagen hybrid scaffold using a fluorescently labeled collagen. Section roughness decreased with increasing EVA content. The softening temperature of the paraffin/EVA mixture was similar to that of paraffin (50-60 degrees C by rheometer). The paraffin/EVA mixture dissolved completely in xylene after 30min at 50 degrees C, and after 30min in n-hexane at 60 degrees C. Therefore, the new embedding medium can be used for histological analysis of various biomaterials and natural tissues.

Cryo-X-ray analysis-A novel tool to better understand the physicochemical reactions at the bioglass/biological fluid interface

The present study deals with the short-term physicochemical reactions at the interface between bioactive glass particles [55SiO(2)-20CaO-9P(2)O(5)-12Na(2)O-4MgO. mol%] and biological fluid (Dulbecco Modified Eagle's Medium (DMEM)). The physicochemical reactions within the interface are characterized by scanning transmission electron microscopy (TEM) (STEM) associated with Energy-dispersive X-ray spectroscopy (EDXS). Microanalysis of diffusible ions such as sodium, potassium, or oxygen requires a special care. In the present investigation the cryo-technique was adopted as a suitable tool for the specimen preparation and characterization. Cryosectioning is essential for preserving the native distribution of ions so that meaningful information about the local concentrations can be obtained by elemental microanalysis. The bioglass particles immersed in biological fluid for 24 h revealed five reaction stages: (i) dealkalization of the surface by cationic exchange (Na(+), Ca(2+) with H(+) or H(3)O(+)); (ii) loss of soluble silica in the form of Si(OH)(4) to the solution resulting from the breakdown of Si--O--Si bonds (iii); repolymerization of Si(OH)(4) leading to condensation of SiO(2)); (iv) migration of Ca(2+) and PO(4) (3-) to the surface through the SiO(2)-rich layer to form CaO-P(2)O(5) film; (v) crystallization of the amorphous CaO-P(2)O(5) by incorporating OH-- or CO(3) (2-) anions with the formation of three different surface layers on the bioactive glass periphery. The thickness of each layer is approximately 300 nm and from the inner part to the periphery they consist of Si--OH, which permits the diffusion of Ca(2+) and PO(4) (3-) ions and the formation of the middle Ca--P layer, and finally the outer layer composed of Na--O, which acts as an ion exchange layer between Na(+) ions and H(+) or H(3)O(+) from the solution.

Towards high-resolution three-dimensional imaging of native mammalian tissue: Electron tomography of frozen-hydrated rat liver sections

Cryo-electron tomography of frozen-hydrated specimens holds considerable promise for high-resolution three-dimensional imaging of organelles and macromolecular complexes in their native cellular environment. While the technique has been successfully used with small, plunge-frozen cells and organelles, application to bulk mammalian tissue has proven to be difficult. We report progress with cryo-electron tomography of frozen-hydrated sections of rat liver prepared by high-pressure freezing and cryo-ultramicrotomy. Improvements include identification of suitable grids for mounting sections for tomography, reduction of surface artifacts on the sections, improved image quality by the use of energy filtering, and more rapid tissue excision using a biopsy needle. Tomographic reconstructions of frozen-hydrated liver sections reveal the native structure of such cellular components as mitochondria, endoplasmic reticulum, and ribosomes, without the selective attenuation or enhancement of ultrastructural details associated with the osmication and post-staining used with freeze-substitution.

Electron tomographic analysis of frozen-hydrated tissue sections

Electron tomography of frozen-hydrated tissue sections enables analysis of the 3-D structure of cell organelles in situ and in a near-native state. In this study, 160-200-nm-thick sections were cut from high-pressure frozen rat liver, and improved methods were used for handling and mounting the sections. Automated data collection facilitated tilt-series recording at low electron dose (approximately 4000 e(-)/nm(2) at 400 keV). Higher doses (up to 10,000 e(-)/nm(2)) were found to increase contrast and smooth out surface defects, but caused section distortion and movement, with likely loss of high-resolution information. Tomographic reconstruction showed that knife marks were 10-40 nm deep and located on the "knife face" of the section, while crevices were 20-50 nm deep and found on the "block face." The interior of the section was normally free of defects, except for compression, and contained useful structural information. For example, the topology of mitochondrial membranes in tissue was found to be very similar to that in frozen-hydrated whole mounts of isolated mitochondria. In rare cases, a 15-nm banding pattern perpendicular to the cutting direction was observed in the interior of the section, most evident in the uniformly dense, protein-rich material of the mitochondrial matrix.

Thickness measurement of hydrated and dehydrated cryosections by EELS

Electron energy-loss spectroscopy (EELS) provides a useful method for determining the thickness of frozen-hydrated and dehydrated cryosections in terms of the inelastic mean free path. Cryosection thickness is an important parameter because plural inelastic scattering limits the sensitivity of elemental microanalysis based on core-loss EELS, and because overlapping structures can affect interpretation of microanalytical data as well as the quality of electron images. The purpose of this work was to establish the minimum practical thickness for cutting cryosections and to explain the measured values for hydrated and dehydrated specimens. Hydrated sections were typically found to be between 1.5-2.5 times thicker than expected from the nominal microtome setting; this difference can be largely explained by compression during cutting. Comparison of micrographs from hydrated and dehydrated cryosections of rapidly-frozen, vitrified liver revealed a lateral shrinkage of approximately 20% on drying. The measured compression and shrinkage factors are consistent with dark-field scanning transmission electron microscopy (STEM) mass measurements on freeze-dried sections. Freeze-dried cryosections, cut to a nominal thickness of 90 nm and supported on thin Formvar/carbon films, had a relative thickness t/lambda i in the range of 0.5 for cytoplasm to 0.9 for mitochondria when analyzed at 100 keV beam energy. Mass loss of approximately 30% occurring at high electron dose enabled useful core-loss spectra to be recorded even from high-mass compartments such as mitochondria without excessive plural scattering.

A model for cryosectioning based on the morphology of vitrified ultrathin sections

Electron microscopy of vitrified ultrathin sections allows cell ultrastructure to be studied in the hydrated state. Sectioning of the frozen material is, however, a limiting step, since the cutting forces cause severe mechanical deformation. In order to address this problem, we have investigated the surface of cryosections. It is shown that cryosections have two fundamentally different surfaces. One surface is rough, deformed by cutting-induced deformation lines which are orientated perpendicular to the cutting direction. The other surface, in comparison, is not affected by those deformation lines. Except for knife marks it is smooth. In order to explain the observations, the following model is proposed. The rough relief corresponds to the former block face. Its roughness originates from material that is squeezed out of the section plane when the section is compressed in the cutting direction and bent away from the specimen block. The smooth section surface is the surface in contact with the knife during the sectioning. This contact keeps the surface smooth while imprinting the knife marks.

Surfaces of cryosections: is cryosectioning 'cutting' or 'fracturing'?

In order to determine if cryosectioning involves 'fracturing' or 'cutting' we examined the surfaces obtained in cryosectioning by a metal-replicating procedure commonly used in freeze-fracture microscopy. Platinum-carbon replicas were made of the surfaces of both the sections and the complementary surfaces of the sample stubs from which the sections were cut. When samples of frozen red cells were sectioned at -120 degrees C with large knife advancements (1 micron), the chips produced did not resemble sections. Membrane fracture faces, produced by splitting of the lipid bilayer, were found in electron micrographs of replicas of the sample stubs. This demonstrates that a cryomicrotome can be used to produce large intact replicas. When dull knives were used with small knife advancements, both smooth and fractured regions were found. The sections produced with dull knives had a snowflake appearance in the light microscope. When sharp knives were used with small advancements (0.1 microns), replicas of the surfaces were free of fracture faces and the sections had a cellophane-like appearance in the light microscope. Therefore, in cryosectioning a different process other than 'fracturing' is responsible. This 'cutting' process may be micromelting of a superficial layer by the mechanism of melting-point depression from the pressure exerted by the sharp edge of the knife.

Cryo-electron microscopy of vitrified specimens

Cryo-electron microscopy of vitrified specimens was just emerging as a practical method when Richard Henderson proposed that we should teach an EMBO course on the new technique. The request seemed to come too early because at that moment the method looked more like a laboratory game than a useful tool. However, during the months which ellapsed before the start of the course, several of the major difficulties associated with electron microscopy of vitrified specimens found surprisingly elegant solutions or simply became non-existent. The course could therefore take place under favourable circumstances in the summer of 1983. It was repeated the following years and cryo-electron microscopy spread rapidly. Since that time, water, which was once the arch enemy of all electronmicroscopists, became what it always was in nature – an integral part of biological matter and a beautiful substance.

X-ray microanalysis of freeze-dried and frozen-hydrated cryosections

The elemental composition and the ultrastructure of biological cells were studied by scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray microanalysis. The preparation technique involves cryofixation, cryoultramicrotomy, cryotransfer, and freeze-drying of samples. Freeze-dried cryosections 100-nm thick appeared to be appropriate for measuring the distribution of diffusible elements and water in different compartments of the cells. The lateral analytical resolution was less than 50 nm, depending on ice crystal damage and section thickness. The detection limit was in the range of 10 mmol/kg dry weight for all elements with an atomic number higher than 12; for sodium and magnesium the detection limits were about 30 and 20 mmol/kg dry weight, respectively. The darkfield intensity in STEM is linearly related to the mass thickness. Thus, it becomes possible to measure the water content in intracellular compartments by using the darkfield signal of the dry mass remaining after freeze-drying. By combining the X-ray microanalytical data expressed as dry weight concentrations with the measurements of the water content, physiologically more meaningful wet weight concentrations of elements were determined. In comparison to freeze-dried cryosections frozen-hydrated sections showed poor contrast and were very sensitive against radiation damage, resulting in mass loss. The high electron exposure required for recording X-ray spectra made reproducible microanalysis of ultrathin (about 100-nm thick) frozen-hydrated sections impossible. The mass loss could be reduced by carbon coating; however, the improvement achieved thus far is still insufficient for applications in X-ray microanalysis. Therefore, at present only bulk specimens or at least 1-micron thick sections can be used for X-ray microanalysis of frozen-hydrated biological samples.

Frontiers in electron probe microanalysis: application to cell physiology

The application of electron probe microanalysis techniques, using X-ray and electron energy loss instruments, to problems in cell physiology is reviewed. The details of the special methodological requirements for the analysis of cryosections at high spatial resolution in an analytical electron microscope are discussed together with a comprehensive review of data obtained on major organ systems and cell types.

Ultrastructural localization of osteocalcin in rat tooth germs by immunogold staining

Osteocalcin was localized by indirect immunogold staining of thin frozen sections of rat tooth germs which had been fixed by different methods. Acrolein fixation proved to be satisfactory considering the preservation of fine structure and antigenicity. In odontoblasts, osteocalcin was found to be localized in the cisternae of the rough endoplasmic reticulum and Golgi apparatus. Few positive transport vesicles were found. Staining for osteocalcin in odontoblastic processes was only observed after strong fixation and was intense in odontoblasts engaged in early dentine formation. Predentine was slightly positive in the neighbourhood of positive processes. Matrix vesicles were negative and strong osteocalcin labeling of dentine seemed to appear after the onset of mineralization.

Transverse sectioning of plastic-embedded immunolabeled cryosections: morphology and permeability to protein A-colloidal gold complexes

In order to provide data for meaningful interpretation and quantitation of immunogold labeling on cryosections their morphology and permeability to protein A-gold were evaluated: We studied plastic sections of immunogold-labeled ultrathin and semithick cryosections cut perpendicular to the original cryosection plane. Various soluble and insoluble antigens in different specimens (hemoglobin and histone H5 in chicken erythrocytes, tubulin in Leishmania cells, and outer membrane protein OmpA in Escherichia coli) were fixed with glutaraldehyde-formaldehyde, formaldehyde, or periodate-lysine-paraformaldehyde and incubated with specific antibodies and protein A-gold of different sizes. The cryosection surface may be rough or smooth depending both on the sectioned material and on dehydration and drying artifacts or possibly on the cutting process itself. Well-preserved sections are capable of withstanding considerable deformation without showing clefts or cracks. If the sectioned specimen is sufficiently fixed, protein A-gold is not able to enter the IgG-labeled sections significantly but follows surface irregularities. However, gold particles can be detected within visibly damaged sections.

Endogenous elements in the prostate. An X-ray microanalytical study of freeze-dried frozen sections and histochemical localization of zinc by potassium pyroantimonate

Freeze-dried frozen sections prepared from unfixed rat lateral prostate were examined by X-ray microanalysis in an attempt to establish the in vivo distribution of endogenous ions. Poor morphological resolution was a limiting factor in the analysis of subcellular regions of the tissue. Glutaraldehyde fixation prior to cryo-sectioning resulted in considerable loss of elements. The results are discussed and compared with those obtained from ultrathin sections of tissue treated with potassium pyroantimonate. Using the latter method, it was possible to demonstrate a subcellular distribution pattern for the element zinc and to correlate the metal with specific organelles. It is considered that, unlike a number of other tissues, the rat prostate does not lend itself readily to cryoultramicrotomy as a preparative regime.

Sublimation rates of ice in a cryo-ultramicrotome

Sublimation rates of ice in the nitrogen-filled cryochamber of an ultramicrotome have been measured. The values are four to five orders of magnitude smaller than vacuum sublimation rates at the same temperature. This is a consequence of the much shorter mean free path length of water molecules in nitrogen at atmospheric pressure, as compared to freeze drying under vacuum. The partial pressure at the phase boundary and gas diffusion and convection determine the rate of freeze drying. This has implications for section handling and the design of transfer equipment. Generally, it means that the danger of even partial dehydration of semithick (0.5-1 micrometer) sections used mainly for X-ray microanalysis is less serious than commonly assumed.

The localization and assay of chemical elements by microprobe methods

The principle of the microprobe analysis of chemical elements is illustrated in Fig. i. Some kind of radiation is directed on to the specimen, generating signals characteristic of the elements present. Local analysis in situ is achieved in one of two ways. Most often the impinging beam is finely focused so that the signal at any moment comes only from a selected microregion. Alternatively, in some instruments, the impinging beam floods a larger region but the emergent signals characteristic of a particular element may be selected and focused to give an elemental ‘map’ or ‘image’.