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

X-ray elemental mapping techniques for elucidating the ecophysiology of hyperaccumulator plants

Contents Summary 432 I. Introduction 433 II. Preparation of plant samples for X-ray micro-analysis 433 III. X-ray elemental mapping techniques 438 IV. X-ray data analysis 442 V. Case studies 443 VI. Conclusions 446 Acknowledgements 449 Author contributions 449 References 449 SUMMARY: Hyperaccumulators are attractive models for studying metal(loid) homeostasis, and probing the spatial distribution and coordination chemistry of metal(loid)s in their tissues is important for advancing our understanding of their ecophysiology. X-ray elemental mapping techniques are unique in providing in situ information, and with appropriate sample preparation offer results true to biological conditions of the living plant. The common platform of these techniques is a reliance on characteristic X-rays of elements present in a sample, excited either by electrons (scanning/transmission electron microscopy), protons (proton-induced X-ray emission) or X-rays (X-ray fluorescence microscopy). Elucidating the cellular and tissue-level distribution of metal(loid)s is inherently challenging and accurate X-ray analysis places strict demands on sample collection, preparation and analytical conditions, to avoid elemental redistribution, chemical modification or ultrastructural alterations. We compare the merits and limitations of the individual techniques, and focus on the optimal field of applications for inferring ecophysiological processes in hyperaccumulator plants. X-ray elemental mapping techniques can play a key role in answering questions at every level of metal(loid) homeostasis in plants, from the rhizosphere interface, to uptake pathways in the roots and shoots. Further improvements in technological capabilities offer exciting perspectives for the study of hyperaccumulator plants into the future.

Shrinkage of freeze-dried cryosections of cells: Investigations by EFTEM and cryo-CLEM

Freeze-drying of cryosections of cells or tissues is considered to be the most efficient preparation for microanalysis purpose related to transmission electron microscopy. It allows the measurements of ions and water contents at the ultrastructural level. However an important drawback is associated to freeze-drying: the shrinkage of the cryosections. The aim of this paper is the investigation of this phenomenon by means of three different methods applied to both hydrated and dehydrated cryosections: direct distance measurements on fiducial points, thickness measurements by energy filtered transmission microscopy (EFTEM) and cryo-correlative light electron microscopy (cryo-CLEM). Measurements in our experimental conditions reveal a lateral shrinkage around 10% but the most important result concerns the lack of differential shrinkage between most of the cellular compartments.

Electron probe X-ray microanalysis for the study of cell physiology

Of the analytical electron microscopy techniques available, electron probe X-ray microanalysis has been most widely used for the study of biological specimens. This technique is able to identify, localize, and quantify elements both at the whole cell and at the intracellular level. The use SEM or TEM to analyze individual whole cells gives a simple and rapid method to study changes in ion transport after stimulation, whereas the analysis of thin sections of cryoprepared cell sections, although technically more difficult, allows details about ionic content in intracellular compartments, such as mitochondria, ER, and lysosomes, to be obtained. In this chapter the principles underlying X-ray emission are briefly outlined, step-by-step methods for specimen preparation of whole cells and cell sections for microanalysis are given, as are the methods used for deriving quantitative information from spectra. Areas where problems might occur have been highlighted. The different areas in which X-ray microanalysis is being used in the study of cell physiology are briefly reviewed.

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.

Botanical X-ray microanalysis in cryoscanning electron microscopy

Modern microscopy in plant sciences has evolved in the direction of providing ultra-structural and analytical information simultaneously. Energy-dispersive X-ray microanalysis (EDX) is a powerful technique that allows the qualitative and quantitative measurement of many elements of physiological interest at the cellular and subcellular level. The most significant advance has been the development of freezing techniques to study cells in plant tissues by EDX in the cryoscanning electron microscopy. Cryofixation is fast enough to retain the original distributions of inorganic elements of tissue electrolytes sufficiently for microanalytical studies. This approach may have broad application for various types of localizations of relevance to plant physiology, environmental pollution, and plant-microbe interactions. In this chapter, the experimental procedure of analytical cryoscanning electron microscopy applied to botanical samples is outlined.

The distribution of light elements in biological cells measured by electron probe X-ray microanalysis of cryosections

The intracellular distribution of the elements carbon, nitrogen, and oxygen was measured in cultured rat hepatocytes by energy dispersive electron probe X-ray microanalysis of 100-nm-thick freeze-dried cryosections. Electron irradiation with a dose up to 106 e/nm2 caused no or merely negligible mass loss in mitochondria and in cytoplasm. Cell nuclei lost carbon, nitrogen, and-to a clearly higher extent-oxygen with increasing electron irradiation. Therefore, electron doses less than 3 x 105 e/nm2 were used to measure the subcellular compartmentation of carbon, nitrogen, and oxygen in cytoplasm, mitochondria, and nuclei of the cells. The subcellular distribution of carbon, nitrogen, and oxygen reflects the intracellular compartmentation of various biomolecules. Cells exposed to inorganic mercury before cryofixation showed an increase of oxygen in nuclei and cytoplasm. Concomitantly the phosphorus/nitrogen ratio decreased in mitochondria. The data suggest mercury-induced production of ribonucleic acid (RNA) and decrease of adenosine triphosphate (ATP). Although biomolecules cannot be identified by X-ray microanalysis, measurements of the whole element spectrum including the light elements carbon, nitrogen, and oxygen can be useful to study specific biomolecular activity in cellular compartments depending on the functional state of the cell.

Effects of bioactive glass particles and their ionic products on intracellular concentrations

Numerous studies have described the bioactive properties of glass particles in the SiO(2)-CaO-Na(2)O-P(2)O(5) system. This kind of material is capable of developing a direct contact with bone through dissolution and physicochemical reactions. We have investigated the influence of bioactive particles, and ionic products from the same particles, on the intracellular concentrations in monocyte cells, which are among the first cells to colonize implantation sites. The only way to access these concentrations and particularly diffusible ionic concentrations (potassium, sodium, and chlorine) is to use cryomethods coupled to electron probe microanalysis. We have paid particular attention to the potassium:sodium ratio, the most sensitive criterion of viability. We have cultured cells with bioactive glass particles and in a conditioned medium obtained from the dissolution of the glass particles in the standard medium. Our study demonstrates that cells cultured in a conditioned medium are more active than cells cultured in a standard medium, or cells exposed to bioactive particles, and particles are more toxic for cells than are ionic products.

Subcellular localization of boron in cultured melanoma cells by electron energy-loss spectroscopy of freeze-dried cryosections

Boron neutron capture therapy (BNCT) is based on the ability of the non-radioactive isotope 10B to capture thermal neutrons and to disintegrate instantaneously. This reaction opens a way to selectively destroy tumour cells after specific uptake of 10B. In this paper, a method based on electron energy-loss spectroscopy is presented for detecting and quantifying boron in freeze-dried cryosections of human melanoma cells. A practical detection limit of around 6 mmol kg-1 in 0.1- micro m2 areas is estimated using specimens prepared from standard boron solutions. Preliminary results of boron mapping in the spectrum-imaging acquisition mode reveal boron penetration and probably spot-like accumulation within melanoma cells when exposed to culture medium containing sodium borocaptate.

Effects of the ionophores valinomycin, ionomycin and gramicidin A on the element compartmentation in cultured rat hepatocytes

The element compartmentation in cultured rat hepatocytes was studied by electron probe X-ray microanalysis of freeze-dried cryosections after exposure of the cells to the ionophores valinomycin, ionomycin or gramicidin A. The most striking effect of these ionophores is the decrease of the intracellular potassium/sodium ratio from values of approximately 10 under control conditions to values below 1 after application of the ionophores. Changes of sodium, potassium and chloride are similar in cytoplasm and nucleus. However, elemental changes are delayed or impeded in mitochondria with respect to the surrounding cytoplasm. The water portion of cytoplasm and mitochondria slightly increases. Besides that, each ionophore has specific effects on the intracellular ion distribution. As compared to gramicidin A and ionomycin, valinomycin does not change the intracellular chloride content. Ionomycin induces calcium accumulation in mitochondria. The cytotoxic effects of the studied ionophores on the intracellular element distribution are more complex than supposed from their ion selective properties in membranes.

Calcium stores in differentiated Dictyostelium discoideum: prespore cells sequester calcium more efficiently than prestalk cells

Dictyostelium discoideum pseudoplasmodia exhibit a gradient of the cytosolic free Ca2+-concentration ([Ca2+]i) along their anterior-posterior axis involved in cell-type specific differentiation. [Ca2+]i is high in prestalk and low in prespore cells. We determined the content and localization of calcium and other elements in cryosectioned cells of pseudoplasmodia and fruiting bodies by X-ray microanalysis. Granular stores rich in Ca, Mg and P were identified. Average Ca was higher in prespore than prestalk granules (225vs 111 mmol/kg dry weight). Total Ca stored in granules was also higher in prespore than prestalk cells. The amount of P and S in granules differed between the two cell types indicating different store composition. In spores mean granular Ca was 120 mmol/kg dry weight. Stalk cells had smaller granules with 360 mmol Ca/kg dry weight. Complementary to microanalysis, vesicular Ca2+-fluxes were studied in fractionated cell homogenates. The rate of Ca2+-uptake was higher in pellet fractions of prespore than prestalk amoebae (4.7 vs 3.4 nmol/min x mg). Ca2+-release was greater in supernatant fractions from prestalk than prespore cells (16.5vs 7.7 nmol/10(8)cells). In summary, prestalk and prespore cells possess qualitatively different, high-capacity stores containing distinct amounts of Ca and probably being involved in regulation of the anterior-posterior [Ca2+]i-gradient.

Heavy metal cytotoxicity studied by electron probe X-ray microanalysis of cultured rat hepatocytes

Cytotoxicity of the heavy metals gold, mercury, thallium and lead was studied by measuring the intracellular element distribution of cultured rat hepatocytes by energy dispersive electron probe X-ray microanalysis of freeze-dried cryosections in a scanning transmission electron microscope. Exposure of the cells to aqueous solutions containing heavy metal ions in concentrations reaching a critical concentration caused increase of intracellular sodium and chloride content accompanied or followed by decrease of intracellular potassium content. Thus, the intracellular potassium/sodium ratio drastically decreased from control values of approximately 10 to values below 1 before changes of cell morphology became visible. In experiments with gold or mercury the decrease of the potassium/sodium ratio was preceded by transient cytoplasmic increase of sulfur and phosphorus. Heavy metal concentrations exceeding the critical concentration also caused an increase of cytoplasmic calcium concentration and finally decay of the cell structure. Cytotoxicity of heavy metals was found to increase in the order Pb, Au, Tl, Hg. Cytotoxic effects by Au, Tl or Hg in moderate concentrations were reduced by simultaneous addition of Zn or Pb to the culture medium. The results obtained prove electron probe X-ray microanalysis of cryosections as a sensitive probe of cell viability.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

The formation of type-I concretions in Drosophila Malpighian tubules studied by electron microscopy and X-ray microanalysis

There are two types of concretions in Drosophila Malpighian tubules: Type-I concretions originate in the distal segments of the anterior tubules, type-II concretions in the adjacent transitional segment between the apical microvilli. Type-I concretions are formed with the aid of carbonic anhydrase within intracellular vesicles, which migrate to the apical cell membrane where they are discharged into the lumen by exocytosis. The carbonic anhydrase inhibitors acetazolamide or hydrochlorothiazide prevent the formation of concretions by interruption of bicarbonate supply. In addition, the formation of concretions can be reduced by feeding with sodium cellulose phosphate.

Involvement of a local fenton reaction in the reciprocal modulation by O2 of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase gene and the insulin-dependent activation of the glucokinase gene in rat hepatocytes

H2O2 mimicked the action of periportal pO2 in the modulation by O2 of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase (PCK) gene and the insulin-dependent activation of the glucokinase (GK) gene. H2O2 can be converted in the presence of Fe2+ in a Fenton reaction into hydroxyl anions and hydroxyl radicals (.OH). The hydroxyl radicals are highly reactive and might interfere locally with transcription factors. It was the aim of the present study to investigate the role of and to localize such a Fenton reaction. Hepatocytes cultured for 24 h were treated under conditions mimicking periportal or perivenous pO2 with glucagon or insulin plus the iron chelator desferrioxamine (DSF) or the hydroxyl radical scavenger dimethylthiourea (DMTU) to inhibit the Fenton reaction. PCK mRNA was induced by glucagon maximally under conditions of periportal pO2 and half-maximally under venous pO2. GK mRNA was induced by insulin with reciprocal modulation by O2. DSF and DMTU reduced the induction of PCK mRNA to about half-maximal and increased the induction of GK mRNA to maximal under both O2 tensions. Hydroxyl radical formation was maximal under arterial pO2. Perivenous pO2, DSF and DMTU each decreased the formation of .OH to about 70% of control. The Fenton reaction could be localized in a perinuclear space by confocal laser microscopy and three-dimensional reconstruction techniques. In the same compartment, iron could be detected by electron-probe X-ray microanalysis. Thus a local Fenton reaction is involved in the O2 signalling, which modulated the glucagon- and insulin-dependent PCK gene and GK gene activation.

A depolarizing chloride current contributes to chemoelectrical transduction in olfactory sensory neurons in situ

Recent biophysical investigations of vertebrate olfactory signal transduction have revealed that Ca2+-gated Cl- channels are activated during odorant detection in the chemosensory membrane of olfactory sensory neurons (OSNs). To understand the role of these channels in chemoelectrical signal transduction, it is necessary to know the Cl--equilibrium potential that determines direction and size of Cl- fluxes across the chemosensory membrane. We have measured Cl-, Na+, and K+ concentrations in ultrathin cryosections of rat olfactory epithelium, as well as relative element contents in isolated microsamples of olfactory mucus, using energy-dispersive x-ray microanalysis. Determination of the Cl- concentrations in dendritic knobs and olfactory mucus yielded an estimate of the Cl--equilibrium potential ECl in situ. With Cl- concentrations of 69 mM in dendritic knobs and 55 mM in olfactory mucus, we obtained an ECl value of +6 +/- 12 mV. This indicates that Ca2+-gated Cl- channels in olfactory cilia conduct inward currents in vivo carried by Cl- efflux into the mucus. Our results show that rat OSNs are among the few known types of neurons that maintain an elevated level of cytosolic Cl-. In these cells, activation of Cl- channels leads to depolarization of the membrane voltage and can induce electrical excitation. The depolarizing Cl- current in mammalian OSNs appears to contribute a major fraction to the receptor current and may sustain olfactory function in sweet-water animals.

Effects of cadmium on electrolyte ions in cultured rat hepatocytes studied by X-ray microanalysis of cryosections

The distribution of elements in isolated and cultured rat hepatocytes was measured by energy dispersive electron probe X-ray microanalysis of freeze-dried ultrathin cryosections. The intracellular compartmentation of electrolyte ions, in particular the content of sodium, chloride, and potassium, was found to depend on culture conditions and on the amount of cadmium chloride added to the culture medium. In cells exposed to 1-10 microM cadmium without carbon dioxide supply, the potassium/sodium ratio decreased from control values of about 10 to values below 1 within 30 min. Changes of potassium and sodium content were followed by an increase in the intracellular chloride content. In cells exposed to 1-10 microM cadmium with carbon dioxide supply, changes of the electrolyte composition were delayed to 1-2 days. An increase of intracellular chloride preceded the inversion of the intracellular potassium/sodium ratio. High cadmium doses induced a cytoplasmic calcium increase and finally disintegration and decay of cell structure. Almost normal potassium and sodium contents were found in cells exposed to 10 microM cadmium in the presence of 100 microM zinc with carbon dioxide for 1 day. Changes in the intracellular electrolyte composition by adverse or toxic conditions were detected before any structural damage became visible. Thus, energy dispersive electron probe X-ray microanalysis of cryosections proved to be a sensitive probe of cell viability and cytotoxicity.

Carbonic anhydrase supports electrolyte transport in Drosophila Malpighian tubules. Evidence by X-ray microanalysis of cryosections

Electron probe X-ray microanalytical studies on the role of carbonic anhydrase in electrolyte transport in the cells of Drosophila Malpighian tubules indicate that carbonic anhydrase delivers protons and bicarbonate ions to ion transport systems in the cell membrane. After injection and after feeding acetazolamide or hydrochlorothiazide, known inhibitors of carbonic anhydrase, the contents of potassium, magnesium and chloride in the apical cytoplasm and in the cytoplasm close to the basal plasma membrane decreased. We explain our measurements by the hypothesis of a basal Mg-H-antiport system in parallel with Cl-HCO(3)-antiport, inhibitable by DIDS. Zinc is supposed to enters cells and intracellular Zn storage vacuoles by a negatively charged Zn-anion-complex in exchange for HCO(3)(-) ions. This antiport is inhibitable by SITS. The content of the Zn storage vacuoles is acid, as shown by red fluorescence after incubation of Malpighian tubules with acridine orange. Red fluorescence is absent after preincubation in a medium containing an inhibitor of carbonic anhydrase. Carbonic anhydrase was demonstrated cytochemically in the Golgi-ER complex, Golgi vesicles and intercellular space. We suppose that carbonic anhydrase is synthesized and stored in the Golgi-ER-complex from where it is released into the tubule lumen.

Changes in element concentrations induced by agonist in pig pancreatic acinar cells

Changes in electrolytes of pig pancreatic acinar cells following application of gastrin-cholecystokinin (CCK) were investigated using the technique of X-ray microanalysis of hydrated and dehydrated sections of freshly frozen pancreas. After stimulation by CCK (10(-9) M), Na and Cl increased significantly in the cytoplasm [Na, from 10 mmol/kg wet wt. (48 mmol/kg dry wt.) to 19 mmol/kg (95 mmol/kg); Cl, from 22 mmol/kg (105 mmol/kg) to 49 mmol/kg (245 mmol/kg)] as well as in the luminal interspace [Na, from 53 mmol/kg (189 mmol/kg) to 65 mmol/kg (283 mmol/kg); Cl, from 65 mmol/kg (232 mmol/kg) to 102 mmol/kg (443 mmol/kg)]. In the secretory granules Cl increased significantly from 30 mmol/kg (86 mmol/kg) to 67 mmol/kg (203 mmol/kg). K decreased significantly from 120 mmol/kg (571 mmol/kg) to 81 mmol/kg (405 mmol/kg) in the cytoplasm, while both increased from 38 mmol/kg (109 mmol/kg) to 58 mmol/kg (176 mmol/kg) in the granules and from 46 mmol/kg (164 mmol/kg) to 48 mmol/kg (209 mmol/kg) in the luminal interspace. Ca increased significantly in the cytoplasm as well as in the luminal interspace, and decreased significantly in the secretory granules. CCK evoked Ca release from secretory granules in the secretory pole of acinar cells. The values were measured from dehydrated sections, and agreed well with those from hydrated sections. The effect of furosemide, an inhibitor of the Na+-K+-2Cl- co-transporter, on the ion transport of acinar cell was studied. When furosemide (10(-5) M) was added to the external solution, the cytoplasmic Cl and Ca concentrations decreased significantly, while there was a little decrease in Na and K concentrations under the secretory condition. These results indicate that Na+-K+-2Cl- co-transport, and Na+, Cl- and K+ exits into the lumen are involved in the mechanism of ion secretion in pig pancreatic acinar cells.

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.

X-ray microanalysis of ion contents in vacuoles and cytoplasm of the growing tips of a hydroid polyp as related to osmotic changes and growth pulsations

Growth pulsations (GP) in hydroid polyps are associated with changes in vacuolar patterns which can be imitated by altering external osmolarity. With the use of X-ray spectroscopy we measured the elemental contents in the vacuoles and cytoplasm of the growing tips of a hydroid polyp, Podocoryne carnea, under various tonicity conditions. Under hypertonic condition which arrested the samples at the retraction phase of normal GP, the elemental content within the vacuolar compartment appeared to be similar to that of the external medium, confirming our previous conclusion about the dehermetization of the vacuolar compartment under these conditions. Under hypotonical condition which arrested samples at the extension GP phase (vacuoles isolated) element ratio data displayed an obvious bimodality. At least one of the data groups could be characterized by a significant increase in the concentrations of sodium and potassium, as related to Cl, Ca and Mg, and in comparison to the same ratios in hypotonical samples and those in the external medium. We suggest that under hypotonical conditions the isolated vacuolar compartment is formed by influx of sodium and potassium ions. These cations are accompanied by anions other than chloride. Potassium appears to be transferred into the vacuoles from the cytoplasm while the sodium derives from the external environment.

Accumulation of calcium in degenerating photoreceptors of several Drosophila mutants

The hypothesis that a large, possibly toxic, increase in cellular calcium accompanies photoreceptor cell degeneration in several different Drosophila mutants was tested. The calcium content of wild type and mutant photoreceptors of Drosophila was measured using rapid freezing of the eyes and energy-dispersive x-ray analysis (e.d.x.) of cryosections and semithin sections of cryosubstituted material. Light- and dark-raised mutants of the following strains were studied: retinal degeneration B (rdgB); retinal degeneration C (rdgC); neither inactivation nor afterpotential C (ninaC), and no receptor potential A (norpA). These are light-dependent retinal degeneration mutants in which the affected gene products had been previously shown as myosin-kinase (ninaC), calcium-dependent phosphoprotein phosphatase (rdgC), phosphoinositide transfer protein (rdgB), and phospholipase C (norpA). In light-raised mutants, ommatidia of variable degrees of degeneration were observed. Mass-dense globular bodies of 200-500 nm diameter in relatively large quantities were found in the degenerating photoreceptor of all the mutants tested. These subcellular globules were found to have a very high calcium content, which was not found in wild type or in nondegenerating photoreceptors of the mutants. Nondegenerating photoreceptors were found not only in dark-raised mutants, but in smaller quantities also in light-raised mutants. Usually these globular structures contained high levels of phosphorus, indicating that at least part of the calcium in the mutant photoreceptors is precipitated as calcium phosphate. The results indicate that a large increase in cellular calcium accompanies light-induced photoreceptor degeneration in degenerating Drosophila mutants even when induced by very different mutations, suggesting that the calcium accumulation is a secondary rather than a primary effect in the degeneration process.

Cryofixation, cryosubstitution, cryoembedding for ultrastructural, immunocytochemical and microanalytical studies

Cryofixation, cryosubstitution and cryoembedding are a set of low-temperature methods for immunocytochemical and microanalytical ultrastructural studies. This review covers the theoretical and practical aspects of these cryomethods, simple, low-cost, safe devices that provide reproducible results and a summary of recent results. Sections prepared by these three cryomethods can be used to determine elemental composition, molecular composition, functions and 3-D ultrastructure. The information obtained can be treated by multivariate statistical methods. Thus, each cellular compartment can be identified by its morphology, molecular and elemental composition and function and changes in these data during physiological and pathological processes can be monitored.

Experimental morphology of insect olfaction: tracer studies, X-ray microanalysis, autoradiography, and immunocytochemistry with silkmoth antennae

The general morphology and methodological peculiarities of insect sensilla are briefly reviewed. The stimulus conducting pore-tubule systems of pheromone-sensitive sensilla of the silkmoths Bombyx mori and Antheraea polyphemus are described. Lipophilic tracers readily enter the hair lumen, while hydrophilic tracers do so only after prolonged extraction with lipid solvents and/or pronase. X-ray microanalysis demonstrates a high potassium content of the sensillum lymph; calcium was only found in the haemolymph above detection limit. Auxiliary cells rapidly take up radioactive leucine administered via the haemolymph. Antibodies against pheromone-binding protein of Antheraea polyphemus label the sensillum lymph of sensilla trichodea, but not of sensilla basiconica in A. polyphemus as well as in B. mori. The cytoplasm of auxiliary cells of the sensilla trichodea is also labelled. The results are discussed in context with present hypotheses on the role of sensillum lymph in stimulus transport and inactivation.

X-ray microanalysis of cell nuclei

The principal component analysis, a multivariate statistical analysis of data, has been used to process X-ray microanalytical data from cell nuclei. Sixty-seven measurements from different areas of chromatin, nucleoli of rat follicular cells, and nucleoli of rat oocyte cells in their antral stage have been studied. The variables are the X-ray characteristic signals for P, S, Al, Fe, Cu, and Zn. This method demonstrates four distinct groups, the chromatin area, which is associated with a higher concentration of P; the compact mass of oocyte nucleolus which possesses the highest content in S, Al, and Zn, and two groups of nucleolar areas. The fibrillar component is richer in S, Al, and Zn than the granular component. The high degree of correlation between these three elements proves the chemical affinity of metals for the proteins (S being the signature for proteins). Cryoembedding in Lowicryl resin at even lower temperatures (213 degrees K in K11M) after quick cryofixation and cryosubstitution in the absence of chemical fixatives gives good ultrastructural preservation and the possibility of simultaneously performing X-ray microanalysis and immunocytochemistry.

Freeze-substitution and the preservation of diffusible ions

Freeze-substitution of biological material in pure acetone followed by low-temperature embedding in the Lowicryls K11M and HM23 yields stable preparations well suited for sectioning and subsequent morphological and microanalytical studies. Transmission electron microscopy of dry-cut sections shows that diffusible cellular thallium ions (T1+) of T1(+)-loaded muscle are localized at similar protein sites in freeze-substituted as in frozen-hydrated preparations. A comparison of X-ray microanalytical data obtained from freeze-dried cryosections and sections of freeze-substituted normal (potassium-containing) muscle shows that K+ ion retention in the freeze-substituted sample is highly dependent on the freeze-substitution procedure used so far, in the best case, about 67% of the cellular K+ is retained after freeze-substitution in pure acetone and low-temperature embedding. It is concluded that the macromolecules during the preparative steps and that ion retention may be increased by further optimizing freeze-substitution and low-temperature embedding.

Cryofixation methods for ion localization in cells by electron probe microanalysis: a review

Electron probe microanalysis data on the intracellular content and distribution of electrolyte ions depends critically on the functional state of the cells at the moment of cryofixation. Whereas tissue specimens often require special in-situ freezing techniques, isolated and cultured cells can be frozen within their environmental medium under physiologically controlled conditions. Thus, they represent a feasible system to study functional ion-related intracellular parameters such as the K/Na ratio. Specifically modified freezing devices allow the study of ion shifts related to dynamic processes in cells, for example, locomotion and exocytosis. The time resolution achieved by time-controlled cryofixation is approximately 1 ms.

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.

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.

Standards for the application of X-ray microanalysis to biological specimens

A review on the subject of compounds used as standards for biological X-ray microanalysis is presented. The general approach used for standardization has been to use standards which resemble the specimen closely in composition. Thus, standards based on proteins have been used for analysis of quench-frozen cryosectioned specimens, whereas standards based on embedding resins have been used for resin-embedded material. The properties of, and problems associated with, each type of standard are recognized and have been well documented. The choice and analysis of biological material. Attention is drawn to the fact that the problems associated with any quantification procedure need to be kept in mind when analysis of standards is undertaken.

In situ elemental analysis and visualization in cryofixed nervous tissues. X-ray microanalytical investigations of embryological and mature brain, inner ear, photoreceptors, muscle and muscle spindles. Comparison of preparation methods for analysis and visualization at cellular and subcellular levels

For meaningful X-ray microanalysis (XRMA) in biology and medicine, the development of preparative and quantitative methods has been necessary. The methods need to preserve close to in vivo distribution of diffusible ions with at the same time reasonable morphological preservation of the tissue. Analyses at low and middle microanalytical resolution are useful at the initial stages of an investigation or when data from large populations of samples have to be acquired. Cryomicrotomy, which makes it possible for the single cells within semi-thin and thick cryosections examined by X-ray microanalysis to be further characterized histochemically (enzyme and substrate content), has been adopted for several pathophysiological studies. The method is particularly suitable for the analysis of complex morphological tissues with many cell types as in the brain or sensory organs of the internal ear. For microanalysis at the subcellular level, we developed a preparative procedure based on the frozen fixed preparation which is freeze-dried in vacuo at -80 degrees C and then at the same temperature, without breaking the vacuum, impregnated with a low-temperature Lowicryl-type resin. The resin is polymerized by u.v. light. This method prevents redistribution of the ions in the tissue and retains the antigenicity of the tissue. A considerable number of cells can be analysed simultaneously and the elemental composition in different cell compartments can be compared due to the similar analytical conditions within the section. An alternative to thin plastic sections of freeze-dried material is thin cryosections cut at -150 degrees C and analysed at low temperatures. Although some methodological problems still exist in preparation of cryosections, this type of section is potentially the most useful in analysis of diffusible ions, especially calcium which in most biological systems is present in very low concentrations. New preparative techniques for XRMA brought severe problems in visualization of the specimens prepared by cryomethods. Charging, low contrast, mass loss and contamination, which are often negligible in conventional electron microscopy, have still to be solved in XRMA of cryoprepared specimens. However, the methods of semi-thin and thick cryosectioning and low-temperature embedding were successfully used for analysis of cells and organelles and for the study of fluids in restricted biological spaces such as the inner ear, muscle spindles and ventricles of the brain in rats. Accordingly, examinations which were impossible by micropuncture and ion selective techniques could be carried out by XRMA.(ABSTRACT TRUNCATED AT 400 WORDS)

Elemental concentrations in air-exposed and vacuum-stored cryosections of rat liver cells

Elemental concentrations in different compartments of cryosections of isolated rat liver cells cryotransferred and freeze-dried were compared with those obtained after storage under vacuum for 12 or 60 h and after exposure to room air for 2 min. Poorer image contrast and segregation artefacts are frequently found in air-exposed sections, together with a slight but significant decrease of the K concentration in the cytoplasm and an increase of the S concentration in the liver cell nuclei and the extracellular medium. Extreme distortions of both ultrastructure and elemental distributions are observed if the sections are even slightly colder than the surrounding atmosphere. While storage of frozen-dried cryosections under vacuum for less than 12 h does not lead to alterations in the sections, gross changes are found both in morphology and elemental distribution in sections stored under vacuum for about 60 h. Long-time vacuum storage of frozen-dried cryosections is, therefore, not recommended.