Quantitative X-ray mapping of biological cryosections

X-ray mapping in electron-beam instruments

This review traces the development of X-ray mapping from its beginning 50 years ago through current analysis procedures that can reveal otherwise obscure elemental distributions and associations. X-ray mapping or compositional imaging of elemental distributions is one of the major capabilities of electron beam microanalysis because it frees the operator from the necessity of making decisions about which image features contain elements of interest. Elements in unexpected locations, or in unexpected association with other elements, may be found easily without operator bias as to where to locate the electron probe for data collection. X-ray mapping in the SEM or EPMA may be applied to bulk specimens at a spatial resolution of about 1 microm. X-ray mapping of thin specimens in the TEM or STEM may be accomplished at a spatial resolution ranging from 2 to 100 nm, depending on specimen thickness and the microscope. Although mapping has traditionally been considered a qualitative technique, recent developments demonstrate the quantitative capabilities of X-ray mapping techniques. Moreover, the long-desired ability to collect and store an entire spectrum at every pixel is now a reality, and methods for mining these data are rapidly being developed.

Organization of interphase chromatin

The organization of interphase chromatin spans many topics, ranging in scale from the molecular level to the whole nucleus, and its study requires a concomitant range of experimental approaches. In this review, we examine these approaches, the results they have generated, and the interfaces between them. The greatest challenge appears to be the integration of information on whole nuclei obtained by light microscopy with data on nucleosome-nucleosome interactions and chromatin higher-order structures, obtained in vitro using biophysical characterization, atomic force microscopy, and electron microscopy. We consider strategies that may assist in the integration process, and we review emerging technologies that promise to reduce the "resolution gap."

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.

High resolution microanalysis and three-dimensional nucleosome structure associated with transcribing chromatin

The nucleosome is the ubiquitous and fundamental DNA-protein complex of the eukaryotic chromosome, participating in the packaging of DNA and in the regulation of gene expression. Biophysical studies have implicated changes in nucleosome structure from chromatin that is quiescent to active in transcription. Since DNA within the nucleosome contains a high concentration of phosphorus whereas histone proteins do not, the nucleosome structure is amenable to microanalytical electron energy loss mapping of phosphorus to delineate the DNA within the protein-nucleic acid particle. Nucleosomes associated with transcriptionally active genes were separated from nucleosomes associated with quiescent genes using mercury-affinity chromatography. The three-dimensional image reconstruction methods for the total nucleosome structure and for the 3D DNA-phosphorus distribution combined quaternion-assisted angular reconstitution of sets of single particles at random orientations and electron spectroscopic imaging. The structure of the active nucleosome has the conformation of an open clam-shell, C- or U-shaped in one view, elongated in another, and exhibits a protein asymmetry. A three-dimensional phosphorus map reveals a conformational change in nucleosomal DNA compared to DNA in the canonical nucleosome structure. It indicates an altered superhelicity and is consistent with unfolding of the particle. The results address conformational changes of the nucleosome and provide a direct structural linkage to biochemical and physiological changes which parallel gene expression.

High-resolution calcium mapping of the endoplasmic reticulum-Golgi-exocytic membrane system. Electron energy loss imaging analysis of quick frozen-freeze dried PC12 cells

The calcium pools segregated within the endoplasmic reticulum, Golgi complex, exocytic, and other organelles are believed to participate in the regulation of a variety of cell functions. Until now, however, the precise intracellular distribution of the element had not been established. Here, we report about the first high-resolution calcium mapping obtained in neurosecretory PC12 cells by the imaging mode of the electron energy loss spectroscopy technique. The preparation procedure used included quick freezing of cell monolayers, followed by freeze-drying, fixation with OSO4 vapors, resin embedding, and cutting of very thin sections. Conventional electron microscopy and high-resolution immunocytochemistry revealed a high degree of structural preservation, a condition in which inorganic elements are expected to maintain their native distribution. Within these cells, calcium signals of nucleus, cytosol, and most mitochondria remained below detection, whereas in other organelles specific patterns were identified. In the endoplasmic reticulum, the distribution was heterogeneous with strongly positive cisternae (more often the nuclear envelope and stacks of parallel elements that are frequent in quick frozen preparations) lying in the proximity of or even in direct continuity with other, apparently negative cisternae. The Golgi complexes were labeled strongly and uniformly in all cisternae and part of their vesicles, with no appreciable differences along the cis-trans axis. Weaker or negative signals were recorded from the trans-Golgi network elements and from scattered vesicles, whereas in contrast secretion granules were strongly positive for calcium. These results are discussed in relation to the existing knowledge about the mechanisms of calcium transport in the variations organelles, and about the processes and functions regulated by organelle lumenal calcium in eukaryotic cells.

Evaluation of several common standards for the X-ray microanalysis of thin biological specimens

The question of the best type of standard to use for X-ray microanalysis of thin biological specimens remains unanswered. Standards embedded in an organic matrix have the advantage that they resemble biological specimens, but their composition is generally not known exactly. We compared several standards and, surprisingly, inorganic binary salts sprayed onto a supporting film were the most suitable: they corresponded closely with several other methods using organic matrices; they were easily produced; and their composition is known. Glutaraldehyde-urea aminoplastic resin thin sections and thin films containing dissolved salts were problematic. The composition of the polymer appears to be variable, and the thin films did not correspond with any other standard tested. Chelex100 bio-standard beads and flakes loaded with accurately determined concentrations of ions, embedded in epoxy resin and thin sectioned, tended to correspond to the results obtained with the binary salts. However, the results from some bio-standards were inexplicably aberrant. An epoxy resin standard was used for bromine, and was found to agree closely with the binary standards.

Measurement of low calcium concentrations in cryosectioned cells by parallel-EELS mapping

Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope provides a high sensitivity for microanalysis of certain important biological elements such as calcium whose physiological concentrations in cells are rather low. Application of parallel-EELS mapping to the analysis of freeze-dried cryosections of rapidly frozen tissue provides a means of detecting small amounts of calcium in structures with diameter approximately 50 nm. Detector pattern noise due to channel gain variations can be reduced by acquiring difference spectra at each pixel. By segmenting nitrogen maps that reflect the structure through the protein distribution it is possible to sum spectra from specific compartments. These are then processed by fitting reference spectra for the Ca L23-edge and the carbon background. It has been found that useful data can be collected at 100 keV beam energy from freeze-dried cryosections of cerebellar cortex cut to nominal thickness of 100 nm. The analysis results in a sensitivity of +/- 0.4 mmol Ca/kg dry weight with a total acquisition time of 400 s, a significant improvement over that achievable with energy-dispersive X-ray spectroscopy.

Real-time quantitative elemental analysis and mapping: microchemical imaging in cell physiology

Recent advances in widely available microcomputers have made the acquisition and processing of digital quantitative X-ray maps of one to several cells readily feasible. Here we describe a system which uses a graphics-based microcomputer to acquire spectrally filtered X-ray elemental image maps that are fitted to standards, to display the image in real time, and to correct the post-acquisition image map with regard to specimen drift. Both high-resolution quantitative energy-dispersive X-ray images of freeze-dried cyrosections and low-dose quantitative bright-field images of frozen-hydrated sections can be acquired to obtain element and water content from the same intracellular regions. The software programs developed, together with the associated hardware, also allow static probe acquisition of data from selected cell regions with spectral processing and quantification performed on-line in real time. In addition, the unified design of the software program provides for off-line processing and analysing by several investigators at microcomputers remote from the microscope. The overall experimental strategy employs computer-aided imaging, combined with static probes, as an essential interactive tool of investigation for biological analysis. This type of microchemical microscopy facilitates studies in cell physiology and pathophysiology which focus on mechanisms of ionic (elemental) compartmentation, i.e. structure-function correlation at cellular and subcellular levels; it allows investigation of intracellular concentration gradients, of the heterogeneity of cell responses to stimuli, of certain fast physiological events in vivo at ultrastructural resolution, and of events occurring with low incidence or involving cell-to-cell interactions.

Personal-computer-based system for electron beam X-ray microanalysis of biological samples

A system based on a personal computer has been developed which provides a relatively inexpensive way to equip an electron microscopy laboratory for quantitative elemental analyses of cryosectioned biological samples. This system demonstrates the feasibility of making an X-ray analyser from a personal computer, together with commercially available hardware and software components. Hardware and software have been assembled to drive the beam in a scanning electron microscope, collect and analyse X-ray spectra, and save, retrieve, and analyse data. Our software provides a menu-controlled user interface to direct spectra acquisition and analysis. Spot analyses, video images, and quantitative elemental images may be obtained and results transferred in ASCII format to other computers. Wet weight, as well as dry weight, concentrations are calculated, if measurements were made of areas of the hydrated sample before it was freeze-dried. Grey-level copies of video and quantitative elemental images may be made on a laser printer.

Disruption of cellular elements and water in neurotoxicity: studies using electron probe X-ray microanalysis

Regulation of elements and water in nerve cells is a complex, multifaceted process which appears to be vulnerable to neurotoxic events. However, much of our knowledge concerning the potential role of elements in nerve cell injury is limited by the relatively gross level of corresponding analyses. If we are to confirm and understand the proposed role, more precise and detailed information is needed. As indicated in this commentary, research employing electron probe microanalysis and digital X-ray imaging has begun to provide this necessary information. Recent EPMA studies of nerve and glial cells in the peripheral and central nervous systems have shown that each cell type and their corresponding morphologic compartments exhibit unique distributions of elements and water. The use of microprobe analysis has allowed us to document precisely how elements and water redistribute in morphological compartments of damaged nerve cells. Accumulating evidence from EPMA studies suggests that, rather than being an epiphenomenon, intracellular changes in diffusible elements might mediate the functional and structural consequences of neurotoxic insult. It is also evident from this research that elements other than Ca might play a pertinent role in the injury response and that changes in intraneuronal elemental composition might develop according to a specific temporal pattern, e.g., transection-induced sequential alterations in axonal K, Na, Cl, and Ca. Therefore, rather than conducting end-point studies, longitudinal investigations are necessary to define the sequential pattern of elemental perturbation associated with a given neurotoxic event. Such research can also help identify the role of individual elements in the injury response. Future microprobe studies should be combined with measurements of ion levels (e.g., using fura-2 or ion selective electrodes) to provide a comprehensive and dynamic view of elemental deregulation. In addition, parallel biochemical studies should be performed to determine mechanisms of elemental disruption and possible biochemical and metabolic consequences of this disruption. Although evidence presented in this commentary suggests that each type of neurotoxic event produces a characteristic pattern of decompartmentalization, further work is necessary to confirm this possibility. Finally, based on a presumed involvement of elements in nerve injury, efforts are currently underway in several laboratories to develop appropriate pharmacological therapies for certain chemical- and trauma-induced neuropathological conditions (Dretchen et al., 1986; El-Fawal et al., 1989; Beattie et al., 1989).(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium measurements with electron probe X-ray and electron energy loss analysis

This paper presents a broad survey of the rationale for electron probe X-ray microanalysis (EPXMA) and the various methods for obtaining qualitative and quantitative information on the distribution and amount of elements, particularly calcium, in cryopreserved cells and tissues. Essential in an introductory consideration of microanalysis in biological cryosections is the physical basis for the instrumentation, fundamentals of X-ray spectrometry, and various analytical modes such as static probing and X-ray imaging. Some common artifacts are beam damage and contamination. Inherent pitfalls of energy dispersive X-ray systems include Si escape peaks, doublets, background, and detector calibration shifts. Quantitative calcium analysis of thin cryosections is carried out in real time using a multiple least squares fitting program on filtered X-ray spectra and normalizing the calcium peak to a portion of the continuum. Recent work includes the development of an X-ray imaging system where quantitative data can be retrieved off-line. The minimum detectable concentration of calcium in biological cryosections is approximately 300 mumole kg dry weight with a spatial resolution of approximately 100 A. The application of electron energy loss (EELS) techniques to the detection of calcium offers the potential for greater sensitivity and spatial resolution in measurement and imaging. Determination of mass thickness with EELS can facilitate accurate calculation of wet weight concentrations from frozen hydrated and freeze-dried specimens. Calcium has multiple effects on cell metabolism, membrane transport and permeability and, thus, on overall cell physiology or pathophysiology. Cells can be rapidly frozen for EPXMA during basal or altered functional conditions to delineate the location and amount of calcium within cells and the changes in location and concentration of cations or anions accompanying calcium redistribution. Recent experiments in our laboratory document that EPXMA in combination with other biochemical and electrophysiological techniques can be used to study, for example, sodium and calcium compartmentation in cultured cardiac cells. Such analyses can also be used to clarify the role of calcium in anoxic renal cell injury and to evaluate proposed ionic defects in cells of individuals with cystic fibrosis.

Correction for specimen movement after acquisition of element-specific electron microprobe images

Because a long time is generally required to generate X-ray maps of specific elements by electron beam methods, images are subject to a loss of resolution due to stage movement. Methods have been previously described for correcting stage drift during exposure by sensing the drift and deflecting the beam to follow the stage; but these methods require modifications of the equipment. When the drift is not excessive, it is possible to correct a series of images after the exposure series is finished. Here we demonstrate two methods for correcting the drift, one based on manual assignment of specimen position and one on the use of cross-correlation functions to determine objectively the misalignment of images in the series. The success of the methods is illustrated in calcium-specific images of a bone section that show the collagen periodicity after drift correction.

Quantitative microchemical imaging of calcium in Na-K pump inhibited heart cells

Quantitative electron probe X-ray imaging techniques have been utilized to determine simultaneously the element content within a single cultured embryonic chick heart cell and its intracellular compartments as well as the average elemental content of several heart cells within a population. These features of microchemical imaging have permitted establishment of data regarding: (1) the heterogeneity of calcium accumulation in mitochondrial, cytoplasmic and nuclear compartments under conditions which elevate total cell calcium without producing irreversible cell injury; and (2) the variability of calcium accumulation from cell to cell within the population sampled. The results indicate that during Na-K pump inhibition (K-free HT-BSS, 10(-4) M ouabain, 60 min) elevation of mitochondrial calcium, measured in situ by electron probe X-ray microanalysis, to levels more than 100 times greater than in the basal state, may not cause irreversible mitochondrial uncoupling and cell death.

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.

Capabilities and limitations of probe methods for the microanalysis of chemical elements in biology: a brief introduction

This article reviews some of the technical problems involved in achieving high-resolution artefact-free quantitative microprobe analyses of biological specimens, and points to some of the techniques now available to overcome these problems.