The measurement of water distribution in frozen specimens

There are three techniques to measure local water fractions in the cryomicroscope. First, water content may be measured by a direct analysis of oxygen in bulk samples using a windowless detector. Secondly, mass thickness may be estimated in frozen-hydrated, then frozen-dried sections. This technique offers unrivalled spatial resolution, especially if the radiation dose in the frozen-hydrated state is kept low by the use of electron scattering techniques instead of an X-ray microanalytical background determination. External water content standards can be used instead of frozen-hydrated sections and the whole analysis can even be performed exclusively on frozen-dried sections at room temperature. Thirdly, local water fractions can be evaluated from X-ray microanalytical measurements of element concentrations per mass in the frozen-hydrated and frozen-dried state. Corrections necessary for the other techniques cancel out. However, the high radiation dose required for a fully quantitative analysis excludes the use of these methods in thin or ultrathin sections.     

The preparation,examination and analysis of frozen hydrated tissue sections by scanning transmission electron microscopy and X-ray microanalysis

A method is reported for preparing, examining and analysing frozen hydrated tissue sections using transmission electron microscopy and X-ray microanalysis. Use of this method permits localization and measurement of water soluble or diffusible elements within the hydrated cell matrix. Since any change in total fresh weight of the specimen will affect the concentration of all components, great care has been taken to demonstrate that the mass neither increases nor decreases and to ensure that the tissue remains frozen-hydrated. Criteria for assessing whether or not the tissue remains frozen-hydrated are reported. After quench freezing, 1–2 μm thick sections of mouse liver were cut at 193°K and picked up on a specially designed annular specimen holder covered with an aluminium coated nylon film. Using a transfer device which prevents contamination of the tissue sections while maintaining them at a low temperature (below 143°K), the sections are transferred either to the vacuum evaporator cold stage or the scanning microscope cold stage. The tissue sections may be coated with an aluminium layer to improve electrical and thermal conductivity. The specimens are examined in the scanning transmission imaging mode and analysed using an energy dispersive X-ray analyser. Concentration of intra-nuclear and intracytoplasmic K, P, S and Cl are reported for mouse hepatocytes as ratios of the characteristic radiation to the continuum radiation used as a measure of mass. Ratios for all four elements were higher in the nucleus than the cytoplasm. Examples are given of this method as applied to plant and insect tissue.     

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-μm thick sections can be used for X-ray microanalysis of frozen-hydrated biological samples.     

Quantitative digital X-ray imaging using frozen hydrated and frozen dried tissue sections

Application of quantitative X-ray imaging to frozen hydrated tissue sections has presented a number of major problems including lack of a suitable algorithm which could deal effectively with mass loss due to radiation damage, problems of low characteristic X-ray signal to background ratios, and provide a means of analysis of the same location in both hydrated and dried states. This paper presents details of the application of our algorithm for analysis of frozen hydrated, then dried cryosections applied to quantitative X-ray imaging, which provides relatively high precision quantitative measurement of elemental content (related to both wet and dry weight) and water content of each pixel. This algorithm largely circumvents many of the problems of analysis of frozen hydrated tissue sections. Our algorithm for X-ray imaging obtains reasonably precise quantitative measurements coupled with morphological information by trading speed and image resolution.     

Agar standards for quantitative X-ray microanalysis of resin-embedded plant tissues

Calibration standards for quantitative X-ray microanalysis of resin-embedded plant tissue were prepared by adding 6600 mM KC1 to 5% agar. Agar blocks with an edge length of 1–2 mm were rapidly frozen, freeze-dried and embedded in styrene-methacrylate. Dry sections 1 μm thick were mounted on adhesive-coated grids. Apart from fine-scale inhomogeneities caused by ice crystal formation, the KC1 is evenly distributed in the agar blocks. The peak-to-continuum values of K and Cl were highly linearly correlated to the K and Cl contents over the whole concentration range.     

The application of EDXS to the biological sciences

The distribution of chemical elements in soft tissues may be faithfully preserved by very rapid freezing. Most often the material is then cryosectioned and the sections frozen-dried prior to analysis, but direct analysis in the hydrated state is an established alternative. For bulk specimens, the shape of the analysed volume is uncertain. But whichever current model is accepted, analytical spatial resolution must generally be limited to the order of 1 μm. Such specimens can be suitable for the specific analysis of cytoplasm, cell nuclei and large extracellular spaces but not for study on a finer scale. Analytical spatial resolution in the range 200–500 nm is obtainable with sections cut ～ 1 μm thick. In the frozen-hydrated state, small extracellular spaces can be analysed but multiple scattering obscures intracellular detail in the STEM image. The irradiation required for an EDXS analysis, approximately 50 nanoCoulomb (50 nanoAmpere seconds), need not produce intolerable radiation damage when spread over an area 200 nm or more in diameter. Finer structure, for example mitochondria and regions of rough or smooth endoplasmic reticulum, can be identified and analysed in frozen-dried cryosections cut ～ 100 nm thick. Recently such features have been visualized in 100 nm frozen-hydrated sections where the water is vitreous. This opens the prospect of analysing material where elemental distributions have been preserved on a very fine scale, since one might avoid even the ionic shifts from aqueous solution to supramolecular structures which must occur on freeze-drying. But radiation damage may be prohibitive when an irradiation of 50 nanoCoulomb is concentrated into a hydrated area less than 200 nm in diameter.     

X-ray microanalytical resolution in frozen-hydrated biological bulk samples

In coated frozen-hydrated gelatin gels the backscattered electron yield does not increase during electron irradiation as it does in uncoated samples. Neither is the backscattered electron yield greater from coated frozen-hydrated gels than that from more conductive organic samples. This is interpreted as indicating that a significant distortion of the electron interaction volume, due to the development of a space charge, does not occur in electron irradiated frozen-hydrated gelatine gels when they are coated with a conducting coat. The depth resolution as estimated from models of biological samples in the form of frozen-hydrated photographic film and frozen-hydrated sections of gelatine gel is consistent with that computed from X-ray depth distribution curves, i.e. close to 2.0 μm at 15 kV. Lateral resolution was estimated from photographic film to be close to 2.0 μm also, at 15 kV.     

Quantification for the X-ray microanalysis of cryosections

Some problems of the quantitative analysis of diffusible elements in cryosections are reviewed. The two prevalent methods for obtaining concentrations from X-ray data, one based on characteristic radiation alone and the other on continuum-normalization, are recapitulated. Both methods seem suitable at cellular level while the latter seems preferable at finer spatial resolution. Recourse to both methods together is desirable in the analysis of frozen-hydrated sections especially when there is no peripheral standard. Selective local contamination is a particular hazard in the analysis of chlorine. In the case of sodium, physical parameters set restrictive limits to the minimum concentration measurable by ‘energy-dispersive’ X-ray spectrometry (about 20 mm kg^?1) and to the spatial resolution attainable by diffractive X-ray spectrometry (～0·2 μm). One obvious danger to meaningful quantitative analysis is inadvertent redistribution of diffusible elements during the moments preceding the freeze-quenching of a tiny piece of tissue. Data are presented to show that concentration changes due to simple evaporation are a real hazard prior to the quenching of sub-millimetre size samples.     

Mass thickness determination by electron energy loss for quantitative X-ray microanalysis in biology

As is well known, electron energy loss spectroscopy can be used to determine the relative sample thickness in the electron microscope. This paper considers how such measurements can be applied to biological samples in order to obtain the mass thickness for quantitative X-ray microanalysis. The important quantity in estimating the mass thickness from an unknown sample is the total inelastic cross section per unit mass. Models for the cross section suggest that this quantity is constant to within ±20% for most biological compounds. This is comparable with the approximation made in the continuum method for measuring mass thickness. The linearity of the energy loss technique is established by some measurements on evaporated films and quantitation is demonstrated by measurements on thin calcium standards. A significant advantage of the method is that the energy loss spectrum can be recorded at very low dose, so that mass thickness determination can be made before even the most sensitive samples suffer damage resulting in mass loss. The energy loss measurements avoid the necessity to correct the continuum measurement for stray radiation produced in the vicinity of the sample holder. Unlike the continuum method the energy loss technique requires uniform mass thickness across the probe area, but this is not usually a problem when small probes (<100 nm diameter) are used.     

Analysis of directly frozen macromolecules and tissues in the field-emission STEM

A VG Microscopes HB501 field-emission high-resolution scanning transmission electron microscope (STEM) was used to image and analyse rapidly frozen, isolated macromolecules and small organelles in tissue cryosections. Dark-field images were obtained from frozen-hydrated microtubules demonstrating that sufficient contrast is available to reveal structural information. The samples were subsequently freeze-dried in the STEM and low-dose (? 10³ e/nm²) dark-field mass maps were recorded with single electron sensitivity. Elemental analysis of individual macromolecules was achievable at high dose using parallel-detection electron energy-loss spectroscopy, albeit with some structural degradation. Detection of copper (320 atoms) in di-decameric haemocyanin molecules was easily within the limits of sensitivity. Elemental analysis of hydrated cryosections is limited by radiation damage to a resolution of approximately 1 μm². For freeze-dried sections, however, the high probe current and stable cold stage of the HB501 STEM allow energy-dispersive X-ray (EDX) microanalysis of low elemental concentrations in highly localized subcellular volumes. EDX spectra from cryosections of cerebellar cortex show that a 100-s analysis time is sufficient to quantify the calcium content of 400-nm² regions within Purkinje cell dendrites with an uncertainity of ± 2 mmol/kg dry weight, equivalent to ± 12 atoms.     

Quantitative energy-filtered electron microscopy of biological molecules in ice.

The theoretical and experimental bases for quantitative electron microscopy of frozen-hydrated specimens are described, with special considerations of energy filtration to improve the images. The elastic and inelastic scattering from molecules in vacuum and in ice are calculated, and simple methods to approximate scattering are introduced. Multiple scattering calculations are used to describe the scattering from vitreous ice and to predict the characteristics of images of frozen-hydrated molecules as a function of ice thickness and accelerating voltage. Energy filtration is predicted to improve image contrast and signal-to-noise ratio. Experimental values for the inelastic scattering of ice, the energy spectrum of thick ice, and the contrast of biological specimens are determined. The principles of compensation for the contrast transfer function are presented. Tobacco mosaic virus is used to quantify the accuracy of interpreting image intensities to derive the absolute mass, mass per unit length, and internal mass densities of biological molecules. It is shown that compensation for the contrast transfer function is necessary and sufficient to convert the images into accurate representations of molecular density. At a resolution of 2 nm, the radial density reconstructions of tobacco mosaic virus are in quantitative agreement with the atomic model derived from X-ray results.     

Recrystallization and ice-crystal growth in a biological specimen,as shown by a simple freeze substitution method

The sensory hairs of the silkmoth, Bombyx mori, are suitable test objects to check for recrystallization and secondary freezing damage in a biological object, because cryofixation by immersion into propane (90 K) routinely yields well-preserved specimens without noticeable freezing damage. After rewarming the frozen specimens for 10 min to 230 K (boiling propane), the tissue preservation has not deteriorated, and even after 45 min at 230 K, ice-crystal ghosts rarely exceed 50 nm. Two minutes at 250 K (in deep freezer) produced moderate freezing damage with ice-crystal ghosts of 30–75 nm, whereas 90 min at 250 K resulted in severe damage with ice-crystal ghosts well over 100 nm. Secondary freezing damage by ice-crystal growth upon rewarming well-frozen biological specimens, therefore, is a relatively slow process, depending not only on the temperature, but also on the exposure time. Moreover, with some biological specimens, secondary ice-crystal growth starts at much higher temperatures than previously guessed, and with short exposure times rarely should become a hazard in fine structure work.     

Biological X-ray microanalysis.

By means of X-ray microanalysis it is now practical to detect approximately 10(-19) g of an element in a static-probe analysis within an ultrathin section, with analytical spatial resolution in the range 20--30 nm. The main difficulties for biological microanalysis are connected not with sensitivity but with specimen preparation and beam damage. Careful cryopreparation, beginning with the quench-freezing of a small block of tissue, is essential even for determining the storage sites, or sites of binding in vivo, of physiologically active elements. In frozen-dried or frozen-hydrated sections of quench-frozen tissue, it is now possible to measure local mass fractions of diffusible as well as of bound elements.     

X-ray microanalysis of biological material in the frozen-hydrated state by PIXE

Elemental microanalysis of biological material in the frozen-hydrated state using in-vacuum proton induced X-ray emission is described for the first time. For this purpose, a commercially available cryotransfer system was modified and coupled to the experimental chamber of the nuclear microprobe (NMP). The analyzed material was frozen in propane cooled by liquid nitrogen, fractured, carbon coated, and transferred onto the cold stage (100 K) of the nuclear microprobe chamber. Micro-PIXE and simultaneous proton backscattering was performed using a 3 MeV proton beam. Quantitative results were obtained by the standardless method, and tested using 20% gelatin standards. Monitoring of the gas composition inside the system by means of mass spectrometry performed before, during, and after proton bombardment showed good stability of the analyzed material for proton currents not exceeding 150 pA. Average concentrations of light elements (C, N, O, and indirectly H) were also obtained by the proton backscattering technique. No losses of elements measurable by particle-induced X-ray emission (PIXE) during proton irradiation were found during repetitive, short analyses of the same micro areas of gelatin standards. Measurements of thick sections of selected plant and animal material in the frozen-hydrated state-leaf sections of the plant Senecio anomalochrous Hilliard (Asteraceae) and larvae of Chysolina pardalina Fabricius (Chrysomelidae)-showed very good preservation of morphology and elemental distribution. Limits of detection of the order of a few micro g g(-1) were obtained for most elements.     

Progress in quantitative X-ray microanalysis of frozen-hydrated bulk biological samples

The analysis of bulk frozen-hydrated biological samples has developed now to a level where practical application of the technique is possible. Provided the sample is carefully coated with a conductive metal, the development of a space charge capable of causing a significant distortion of the electron diffusion volume does not seem to occur, and analytical resolution can be conveniently held to approximately 2 μm (both depth and lateral resolution). Two valid quantitative methods are available, and two methods of determining dry weight fractions are also available. An area where further research could lead to improvement in analysis of frozen-hydrated bulk samples is in the investigation of fracturing methods. If fracture planes that were flat and reproducible could be easily obtained, some of the difficulties of analysing frozen-hydrated bulk samples would be considerably reduced.     

Histological control of specimen selection for electron microscopy

This technique enables small specimens for electron microscopy to be obtained from specific zones of large pieces of tissue. The method depends on the use of frozen sections, for localization of histological features, but circumvents the risk of ice-crystal damage to the specimens selected.     

Microtubule structure studied by quick freezing: Cryo-electron microscopy and freeze fracture

Microtubules have been quickly frozen and examined by electron microscopy using several techniques: (1) freezing of a thin layer of solution by plunging into cryogen, followed by cryo-electron microscopy of the unstained vitrified samples; (2) freezing by the propane-jet method, followed by freeze fracturing and metal replication. The unstained frozen-hydrated microtubules show a structure in agreement with X-ray diffraction data; they differ from negatively stained particles mainly by the better preservation of cylindrical shape. Secondly, they reveal a supertwist of the profilaments that is not detected reliably by other methods. This allows a determination of the number of protofilaments and the polarity. The structural resolution of unstained microtubules is similar to that of stained ones (about 2–3 nm); it is limited by low contrast and lack of crystalline order. Propane-jet or cryo-block freezing followed by freeze fracturing reveals the structures of the inner and outer surfaces of the microtubule wall at a resolution of 4 nm or better. The outside is dominated by the longitudinal protofilaments whereas on the inside one observes tilted cross-striations. Although the freezing temperatures of the two methods are different (liquid nitrogen or helium) they yield similar results for the case of thin layers of protein solution.     

Electron microscope autoradiography of a diffusible intracellular constituent, using freeze-dried frozen sections of mammalian intestinal epithelium.

Autoradiograms were prepared from freeze-dried frozen sections of rat intestinal epithelium, previously incubated in vitro with 3H-galactose. Although the sections showed evidence of disruption by ice-crystal growth during freezing, they retained sufficient recognizable ultrastructure to facilitate the identification of several subcellular compartments. Statistical analysis of the grain distribution indicated that a significant non-uniform distribution of the labelled galactose occurs in the absorptive cells during transport, and is maintained in the sections throughout the various stages of autoradiography.     

Quantitative elemental X-ray imaging of frozen-hydrated biological samples

It is shown that quantitative X-ray imaging of planed, frozen-hydrated, biological bulk samples that have not been etched is possible. X-ray imaging represents a better alternative to static beam (selected area) analysis of fractured frozen-hydrated samples. This procedure avoids the undesirable necessity of etching planed frozen-hydrated samples to provide an interpretable electron image. Qualitative oxygen and carbon X-ray images, which can be acquired in a short time, can be used for distinguishing morphological features and remove the requirement for electron images. In test samples of frozen-hydrated albumin, containing salts, analyses by X-ray images compared well with static beam (selected area) analyses from the same samples. An example of an analysis of frozen-hydrated insect Malpighian tubules is given in which the response to ouabain treatment was analysed. In this example X-ray imaging showed that ouabain resulted in a significant increase in cytoplasmic and luminal Na and a significant decrease in cytoplasmic and luminal K. X-ray imaging also showed that there was a significant increase in cellular water content. The presence of a potassium gradient in soybean root nodules was also demonstrated. The use of standard deviation images for processing low count images increases analytical precision but results in underestimates of the true concentrations.