The localization and assay of chemical elements by microprobe methods

Application of cryofixation and cryoultramicrotomy for biological electron microscopy

Conventional chemical fixation and embedding of specimens in resins are accompanied by many artifacts, including postmortem structural alterations. Antigenicity of constituents of specimens can be deteriorated and soluble elements relocated in the process of chemical fixation and resin-embedding. Cryofixation and cryoultramicrotomy will overcome many of these drawbacks of chemical fixation and resin embedding. The theoretical background, equipment, methods, and applications of cryofixation and cryoultramicrotomy for biological specimens are reviewed.

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.

Excitotoxic calcium overload in a subpopulation of mitochondria triggers delayed death in hippocampal neurons

In neurons, excitotoxic stimulation induces mitochondrial calcium overload and the release of pro-apoptotic proteins, which triggers delayed cell death. The precise mechanisms of apoptogen release, however, remain controversial. To characterize the linkage between mitochondrial calcium load and cell vulnerability, and to test the hypothesis that only a subpopulation of mitochondria damaged by calcium overload releases apoptogens, we have measured directly the concentrations of total Ca (free plus bound) in individual mitochondria and monitored in parallel structural changes and the subcellular localization of pro-apoptotic cytochrome c after NMDA overstimulation in cultured hippocampal neurons. Beyond transient elevation of cytosolic calcium and perturbation of Na+/K+ homeostasis, NMDA stimulation induced dramatic, but mainly reversible, changes in mitochondria, including strong calcium elevation, membrane potential depolarization, and variable swelling. Elevation of matrix Ca in the approximately one-third of mitochondria that were strongly swollen, as well as the absence of swelling when Ca2+ entry was abolished, indicate an essential role for Ca overload. Shortly after NMDA exposure, cytochrome c, normally localized to mitochondria, became diffusely distributed in the cytoplasm, coincident with the appearance of severely swollen mitochondria with ruptured outer membranes; under these conditions, cytochrome c was retained in intact mitochondria, implying that it was released mainly from damaged mitochondria. Consistent with the role of mitochondrial Ca overload, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone decreased Ca accumulation, prevented cytochrome c release, and was neuroprotective. These results support a mechanism in which delayed excitotoxic death involves apoptogen release from a subpopulation of calcium-overloaded mitochondria, whereas other, undamaged mitochondria maintain normal function.

Structure and function of human sweat glands studied with histochemistry and cytochemistry

The basic structure and the physiological function of human sweat glands were reviewed. Histochemical and cytochemical techniques greatly contributed the elucidation of the ionic mechanism of sweat secretion. X-ray microanalysis using freeze-dried cryosections clarified the level of Na, K, and Cl in each secretory cell of the human sweat gland. Enzyme cytochemistry, immunohistochemistry and autoradiography elucidated the localization of Na,K-ATPase. These data supported the idea that human eccrine sweat is produced by the model of N-K-2Cl cotransport. Cationic colloidal gold localizes anionic sites on histological sections. Human eccrine and apocrine sweat glands showed completely different localization and enzyme sensitivity of anionic sites studied with cationic gold. Human sweat glands have many immunohistochemical markers. Some of them are specific to apocrine sweat glands, although many of them stain both eccrine and apocrine sweat glands. Histochemical techniques, especially immunohistochemistry using a confocal laser scanning microscope and in situ hybridization, will further clarify the relationship of the structure and function in human sweat glands.

Correlated measurements of free and total intracellular calcium concentration in central nervous system neurons

Transient changes in the intracellular concentration of free calcium ([Ca2+])i) act as a trigger or modulator for a large number of important neuronal processes. Such transients can originate from voltage- or ligand-gated fluxes of Ca2+ into the cytoplasm from the extracellular space, or by ligand- or Ca2+(-)gated release from intracellular stores. Characterizing the sources and spatio-temporal patterns of [Ca2+]i transients is critical for understanding the role of different neuronal compartments in dendritic integration and synaptic plasticity. Optical imaging of fluorescent indicators sensitive to free Ca2+ is especially suited to studying such phenomena because this approach offers simultaneous monitoring of large regions of the dendritic tree in individual living central nervous system neurons. In contrast, energy-dispersive X-ray (EDX) microanalysis provides quantitative information on the amount and location of intracellular total, i.e., free plus bound, calcium (Ca) within specific subcellular dendritic compartments as a function of the activity state of the neuron. When optical measurements of [Ca2+]i transients and parallel EDX measurements of Ca content are used in tandem, and correlated simultaneously with electrophysiological measurements of neuronal activity, the combined information provides a relatively general picture of spatio-temporal neuronal total Ca fluctuations. To illustrate the kinds of information available with this approach, we review here results from our ongoing work aimed at evaluating the role of various Ca uptake, release, sequestration, and extrusion mechanisms in the generation and termination of [Ca2+]i transients in dendrites of pyramidal neurons in hippocampal slices during and after synaptic activity. Our observations support the long-standing speculation that the dendritic endoplasmic reticulum acts not only as an intracellular Ca2+ source that can be mobilized by a signal cascade originating at activated synapses, but also as a major intracellular Ca sink involved in active clearance mechanisms after voltage- and ligand-gated Ca2+ influx.

Depolarization-induced mitochondrial Ca accumulation in sympathetic neurons: spatial and temporal characteristics

Several lines of evidence suggest that neuronal mitochondria accumulate calcium when the cytosolic free Ca(2+) concentration ([Ca(2+)](i)) is elevated to levels approaching approximately 500 nM, but the spatial, temporal, and quantitative characteristics of net mitochondrial Ca uptake during stimulus-evoked [Ca(2+)](i) elevations are not well understood. Here, we report direct measurements of depolarization-induced changes in intramitochondrial total Ca concentration ([Ca](mito)) obtained by x-ray microanalysis of rapidly frozen neurons from frog sympathetic ganglia. Unstimulated control cells exhibited undetectably low [Ca](mito), but high K(+) depolarization (50 mM, 45 sec), which elevates [Ca(2+)](i) to approximately 600 nM, increased [Ca](mito) to 13.0 +/- 1.5 mmol/kg dry weight; this increase was abolished by carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP). The elevation of [Ca](mito) was a function of both depolarization strength and duration. After repolarization, [Ca](mito) recovered to prestimulation levels with a time course that paralleled the decline in [Ca(2+)](i). Depolarization-induced increases in [Ca](mito) were spatially heterogeneous. At the level of single mitochondria, [Ca](mito) elevations depended on proximity to the plasma membrane, consistent with predictions of a diffusion model that considers radial [Ca(2+)](i) gradients that exist early during depolarization. Within individual mitochondria, Ca was concentrated in small, discrete sites, possibly reflecting a high-capacity intramitochondrial Ca storage mechanism. These findings demonstrate that in situ Ca accumulation by mitochondria, now directly identified as the structural correlate of the "FCCP-sensitive store, " is robust, reversible, graded with stimulus strength and duration, and dependent on spatial location.

Functional characterization of thapsigargin and agonist-insensitive acidic Ca2+ stores in Drosophila melanogaster S2 cell lines

The role of acidic intracellular calcium stores in calcium homeostasis was investigated in the Drosophila Schneider cell line 2 (S2) by means of free cytosolic calcium ([Ca2+]i) and intracellular pH (pHi) imaging together with measurements of total calcium concentrations within intracellular compartments. Both a weak base (NH4Cl, 15 mM) and a Na+/H+ ionophore (monensin, 10 microM) evoked cytosolic alkalinization followed by Ca2+ release from acidic intracellular Ca2+ stores. Pretreatment of S2 cells with either thapsigargin (1 microM), an inhibitor of endoplasmic reticulum Ca(2+)-ATPases, or with the Ca2+ ionophore ionomycin (10 microM) was without effect on the amplitude of Ca2+ release evoked by alkalinization. Application of the cholinergic agonist carbamylcholine (100 microM) to transfected S2-DM1 cells expressing a Drosophila muscarinic acetylcholine receptor (DM1) emptied the InsP3-sensitive Ca2+ store but failed to affect the amplitude of alkalinization-evoked Ca2+ release. Glycyl-L-phenylalanine-beta-naphthylamide (200 microM), a weak hydrophobic base known to permeabilize lysosomes by osmotic swelling, triggered Ca2+ release from internal stores, while application of brefeldin A (10 microM), an antibiotic which disperses the Golgi complex, resulted in a smaller increase in [Ca2+]i. These results suggest that the alkali-evoked calcium release is largely attributable to lysosomes, a conclusion that was confirmed by direct measurements of total calcium content of S2 organelles. Lysosomes and endoplasmic reticulum were the only organelles found to have concentrations of total calcium significantly higher than the cytosol. However, NH4Cl (15 mM) reduced the level of total calcium only in lysosomes. Depletion of acidic Ca2+ stores did not elicit depletion-operated Ca2+ entry. They were refilled upon re-exposure of cells to normal saline ([Ca2+]o = 2 mM), but not by thapsigargin-induced [Ca2+]i elevation in Ca(2+)-free saline.

Stimulus-secretion coupling in neurohypophysial nerve endings: a role for intravesicular sodium?

It is generally accepted that Ca is essentially involved in regulated secretion, but the role of this cation, as well as others such as Na, is not well understood. An illustrative example occurs in neurohypophysial secretion, where an experimentally induced increase in the cytosolic concentration of Na+ can induce continuous neuropeptide release. In contrast, an increase in cytosolic Ca2+ will have only a transient stimulatory effect. The secretion-promoting targets for Ca2+ are not known; they may be cytosolic, as is usually assumed, but they may also be intravesicular, especially in view of evidence that Ca-rich secretory vesicles are preferentially secreted. In the present work, we have investigated the movements of these cations into and out of secretory vesicles during stimulus-secretion coupling. Isolated rat neurohypophysial nerve endings were stimulated by potassium (55 mM) depolarization, and at 6 min (peak secretion) and 20 min after the onset of stimulation, the elemental content of individual secretory vesicles was measured by quantitative x-ray microanalysis. A depolarization-induced transient increase in intravesicular Na+ concentration was found to coincide with the onset of secretion. Moreover, only a predicted small fraction of peripheral vesicles-presumably the docked ones-were Na+-loaded. The low sulfur concentration of Na+-rich vesicles most likely resulted from vesicle swelling. The results suggest that high intravesicular Na+ concentrations in docked vesicles, occurring by Na+/Ca2+ exchange or by transient fusion pore opening, is a proximal event in exocytosis.

The distribution and intracellular compartmentation of metals in the endogeic earthworm Aporrectodea caliginosa sampled from an unpolluted and a metal-contaminated site

The tissue distribution of Cd, Cu, Pb, Zn and Ca in the endogeic earthworm Aporrectodea caliginosa living in a non-polluted and a heavy metal polluted soil was investigated. The tissues of animals from the contaminated soil contained greater concentrations of Cd, Pb and Zn than the corresponding tissues of animals from the unpolluted soil. The greatest concentrations of Cd, Pb, Zn, and Ca were primarily accumulated within the posterior alimentary canal (PAC), a tissue fraction which contained the greatest proportion of the whole-worm burdens of the respective metals. Cu was distributed fairly evenly in the tissue fractions investigated. The pattern of accumulation for the 'heavy' metals is broadly similar to that for epigeic earthworms; in contrast, a different pattern of tissue accumulation was found for Ca. In animals from the uncontaminated site, the major elemental constituents of the chloragosomes were P, Ca, Zn and S. A significant positive correlation exists between P and Ca within the chloragosomal matrix. These intracellular vesicles are major foci for Pb and Zn accumulation within the PAC, with 'excess' metals associated with P ligands within the chloragosome matrix. The incorporation of Pb and Zn appears to involve the cationic displacement of Ca. Such compartmentation appears to prevent dissemination of large concentrations of these metals into other earthworm tissues, and may thus represent a detoxification strategy based on accumulative immobilization. No intracellular localization of Cd was identified in the study, although the Cd concentration in the metalliferous soils examined was not exceptionally high. The observations are discussed in the context of a contribution to enhanced understanding of metal ecotoxicology in earthworms by providing baseline data on a little investigated ecophysiological group of earthworms. Comparisons of metal distribution and mechanisms of metal sequestration are made with other ecophysiological groups of earthworms, and the significance of the findings to biomonitoring and toxicity-testing programmes is considered.

Activity-dependent calcium sequestration in dendrites of hippocampal neurons in brain slices

Synaptic activity-dependent changes in the spatio-temporal distribution of calcium ions regulate important neuronal functions such as dendritic integration and synaptic plasticity, but the processes that terminate the free Ca2+ transients associated with these changes remain unclear. We have characterized at the electron microscopic level the intracellular compartments involved in buffering free Ca2+ transients in dendritic cytoplasm of CA3 neurons by measuring the larger changes in the concentrations of total Ca that persist for several minutes after neuronal activity. Quantitative energy-dispersive x-ray microanalysis of cryosections from hippocampal slice cultures rapidly frozen 3 min after afferent synaptic activity identified a subset of dendritic endoplasmic reticulum (ER) as a high-capacity Ca2+ buffer. Calcium sequestration by cisterns of this subset of ER was graded, reversible, and dependent on a thapsigargin-sensitive Ca2+-ATPase. Sequestration was so robust that after repetitive high-frequency stimulation the Ca content of responsive ER cisterns increased as much as 20-fold. These results demonstrate that a subpopulation of ER is the major dendritic Ca sequestration compartment in the minutes after neuronal activity.

In situ compositional analysis of acidocalcisomes in Trypanosoma cruzi

We measured the elemental content of different compartments in Trypanosoma cruzi epimastigotes using quick freezing, ultracryomicrotomy, and electron probe microanalysis. Vacuoles identified by high electron density contained (in units of mmol/kg dry weight +/- S.E.) large amounts of phosphorus (1390 +/- 13), magnesium (646 +/- 19), calcium (171 +/- 5), sodium (161 +/- 18), and zinc (148 +/- 6). No other compartment had appreciable calcium or zinc content. Iron (128 +/- 16 mmol/kg) was detected only in vacuoles distinct from the electron-dense vacuoles and other organelles. Incubation of cells for 70 min in culture medium in the presence of ionomycin plus nigericin led to a very significant 3- or 2-fold increase in potassium in the electron-dense vacuoles and the iron-rich vacuoles, respectively, with no significant change in the other elements investigated. This indicated the acidic nature of the vacuoles and demonstrated that the electron-dense vacuoles correspond to what were described previously as acidocalcisomes, i.e. acidic compartments rich in Ca2+. The acidocalcisomes were investigated by separation of epimastigote fractions on Percoll gradients in combination with Triton WR-1339 treatment. This detergent caused a rapid vacuolation; these vacuoles were shown by electron microscopy to be largely transparent, with a diffuse matrix. Percoll gradient fractionation demonstrated decreases in the density of various organelle markers in detergent-treated cells compared with controls. Large decreases in the density of the acidocalcisome and the mitochondrion were seen, as well as smaller decreases in the density of the other markers. Conventional electron microscopy of epimastigotes loaded with gold-labeled transferrin indicated that the endosomal system was separate from vacuoles that probably corresponded to the calcium-containing organelles detected by electron probe microanalysis. The combined results provide evidence that acidocalcisomes are organelles different from lysosomes or other organelles previously described in these parasites.

Aluminium deposition in liver and kidney following acute intravenous administration of aluminium chloride or citrate in conscious rats

1. Plasma, urinary, liver and kidney cell aluminium (Al) levels were monitored in the rat, 1h after intravenous administration of 29630 nmol (800 micrograms) Al as either Al chloride or as Al citrate (Al chloride plus excess sodium citrate). Al levels were measured in plasma, urine and liver by atomic absorption spectroscopy (AAS). Liver and kidney Al content was measured at the cellular and subcellular level by electron probe X-ray microanalysis (EPXMA). 2. Urinary excretion of Al was significantly higher (P < 0.01), when Al was given as the citrate than as the chloride. After 1h, plasma Al levels were significantly lower in the Al citrate group than the Al chloride group (59 +/- 3.7 vs 877 +/- 214 nmol ml-1, respectively; P < 0.01). 3. Al concentrations were significantly higher in the livers of rats receiving Al chloride (818 +/- 252 nmol g-1 wet weight; P < 0.05), than in either control or Al citrate groups (122 +/- 41 and 107 +/- 26 nmol g-1 wet weight, respectively). Al concentrations derived from EPXMA measurements were in agreement with AAS values for the three groups, with significantly higher Al concentrations in the Al chloride group (1.7 +/- 0.4 nmol mg-1 dry weight; P < 0.05) than in the control or Al citrate groups, where Al was not detectable. EPXMA analysis showed that Al was distributed in all liver organelles analysed (cytoplasm, mitochondria, nucleus, ER) and was not preferentially taken up by any one organelle in Al chloride treated rats. 4. Significant amounts of Al were found in cytoplasm and mitochondria of proximal tubule cells of rats given Al citrate (0.64 +/- 0.15 and 0.80 +/- 0.11 nmol mg-1 dry weight, respectively), but not in nuclei or lysosomes of these cells. Al levels were not detectable in control kidneys, in proximal tubule cells after Al chloride administration or distal tubule cells after either Al treatment.

Electron microprobe study of the effect of long-term vacuum storage of freeze-dried cryosections on distribution of elements in cells

The distribution of elements in yeast cells was measured on freshly prepared, freeze-dried cryosections and compared with the distribution obtained on the same sections after storage for 20 months in a vacuum below 2.6 kPa. The average concentration of phosphorus remained unchanged but was equalized throughout the cells, i.e. it migrated from vacuoles into the cytoplasm and mitochondria. Zinc remained preferentially localized in the vacuoles, but the ratio between vacuolar and cytoplasmic zinc concentrations decreased about three-fold. Cytoplasmic and mitochondrial concentrations of potassium remained unchanged while partial release from the vacuoles and subsequently from the cells was observed. This resulted in a homogeneous distribution of potassium in the cells after 20 months and some of the vacuolar potassium appeared in spectra of formvar film measured several micrometres from the cells. A large increase in the sodium (from 160 to 360% more than in fresh sections), magnesium (from 110 to 200% more) and sulphur (from 70 to 350% more) contents was observed in all cellular compartments (except for vacuoles, where only a 20% increase in the magnesium content was observed), while chlorine was almost completely released from the cells. The limitations of the use of long-term vacuum-stored cryosections for electron microprobe analysis of cells are discussed.

Electron probe X-ray microanalysis of intracellular element concentrations in cryosections in the presence of changes in cell volume

The interpretation of element concentration data for X-ray microanalyses of biological tissues, which are subjected to some experimental treatment, can be complicated by changes in cell volume and total cell dry matter induced by the treatment. We have examined the manner in which such changes would affect the values measured in frozen-dried cryosections of soft tissues, and how they may be taken into account in the interpretation of the results. The element content (mass per unit dry weight) measured by the peak-to-continuum or Hall method is independent of changes in cell volume, but is sensitive to a change in the local dry mass. Conversely, intracellular concentrations in terms of mass per unit volume, as determined by the peripheral or internal standard technique, are dependent on volume changes but independent of dry mass. The estimated dry weight fraction is affected by changes in both volume and dry mass. The results obtained from both quantification methods can therefore provide information on the combination of changes in cellular element levels, volume and total dry mass that may occur following the experimental treatment. In a study of the late effect of the drug cisplatin on electrolyte concentrations in kidney proximal tubules, both quantification methods have been used to obtain wet weight and dry weight concentrations. By applying the above considerations, the analytical results have been interpreted as a combination of changes in element levels and a shrinkage of the tubule cells. Cell shrinkage was confirmed by morphometric analysis of tubular cross-sections.

Maturation-related changes in mass and elemental contents of secretory granules as measured by electron-microprobe

The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P less than 0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures.

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.

Electron probe microanalysis of calcium release and magnesium uptake by endoplasmic reticulum in bee photoreceptors

Honey bee photoreceptors contain large sacs of endoplasmic reticulum (ER) that can be located unequivocally in freeze-dried cryosections. The elemental composition of the ER was determined by electron probe x-ray microanalysis and was visualized in high-resolution x-ray maps. In the ER of dark-adapted photoreceptors, the Ca concentration was 47.5 +/- 1.1 mmol/kg (dry weight) (mean +/- SEM). During a 3-sec nonsaturating light stimulus, approximately 50% of the Ca content was released from the ER. Light stimulation also caused a highly significant increase in the Mg content of the ER; the ratio of Mg uptake to Ca released was approximately 0.7. Our results show unambiguously that the ER is the source of Ca2+ release during cell stimulation and suggest that Mg2+ can nearly balance the charge movement of Ca2+.

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.

Zinc sequestration by earthworm (Annelida: Oligochaeta) chloragocytes. An in vivo investigation using fully quantitative electron probe X-ray micro-analysis

The elemental compositions of chloragosome "granules" in the earthworm Lumbricus rubellus living in non-polluted and heavily Zn-polluted soils were determined by fully quantitative electron probe X-ray microanalysis. P, Ca, S and Zn were the major elemental components of the chloragosomes. The in vivo accumulation of Zn by the chloragosomes was accompanied by diminished chloragosomal Ca concentrations. Zn was apparently bound by at least two ligand pools (Pool 1 = uncharacterised; Pool 2 = P-containing ligands, binding approximately 45% and 55% of the Zn, respectively) in the "control" chloragosomes. In Zn-contaminated chloragosomes, most (approximately 70%) was bound by P-containing ligand(s) but some (less than 1%) was also bound by S-containing ligands. It is suggested that the sequestration of Zn in chloragosomes results in the detoxification of the metal by accumulative immobilisation.

X-ray microanalysis with continuous specimen cooling: is it necessary?

An X-ray microanalytical preparation technique using continuous specimen cooling and consisting of cryotransfer of frozen sections into the electron microscope, freeze-drying of the sections within the microscope and analysis at liquid nitrogen temperature is compared with a more conventional technique characterized by freeze-drying of sections in a vacuum evaporator with subsequent carbon coating, transfer of frozen-dried sections through the room air into the electron microscope and analysis at ambient temperature. For this comparison elemental concentrations in mitochondria, in areas of the rough endoplasmic reticulum and in the cytoplasm of rat hepatocytes, were measured. Si is found in abundance in specimens freeze-dried outside the microscope due to the use of a vacuum evaporator contaminated with pump oil. Radiation damage will be more severe at ambient temperature and is assumed to be the reason for observed differences in S concentration in mitochondria and the cytoplasm. However, peak-to-background ratios for Na, Mg, P, Cl and K are in general the same for both types of preparation.

Cytochemical applications of X-ray microanalysis

X-ray microanalysis (XRMA) has been applied to a wide variety of cytochemical problems, but the most valuable applications have been to the validation of cytochemical methods (by the qualitative or quantitative analysis of reaction products), and to the simultaneous localization of more than one substance, which cannot easily be achieved by using alternative methods. The latter applications involve stoichiometric studies (the quantitative relationships between reaction products and substrates), and distribution studies. Ultrastructural cytochemistry with XRMA is limited by the need to use high-brightness electron sources. Apart from the limited availability of such sources, they may cause unacceptable damage to the specimen. Preparation methods for cytochemistry using XRMA are reviewed; in principle these do not differ from those used for other cytochemical applications, but it is important not to introduce extraneous elements (from fixative, buffer, or embedding medium) into the specimen, where the additional X-ray peaks may interfere with the analysis. Quantification in XRMA of cytochemical preparations poses special problems, because the addition of the reaction product to the specimen alters the yield of continuum X rays, used for assessing the mass of the specimen, and also dilutes endogenous elements. However, measurement of ratios between characteristic elemental peaks is a useful method in X-ray microanalytical cytochemistry, and it is concluded that one of the most important attributes of XRMA for cytochemical purposes is the ease with which the substances of interest can be measured.

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.

Activity-dependent accumulation of calcium in Purkinje cell dendritic spines

The calcium content of synapses of parallel fibers on Purkinje cell dendritic spines was determined by electron probe x-ray microanalysis of freeze-dried cryosections from directly frozen slices of mouse cerebellar cortex. In fresh slices frozen within 20-30 sec of excision, calcium concentrations ranging from 0.8 to 18.6 mmol/kg of dry weight were measured in cisterns of smooth endoplasmic reticulum within Purkinje cell dendritic spines. The average calcium content of spine cisterns in rapidly excised slices (6.7 +/- 0.6 mmol/kg of dry weight +/- SEM) was higher than the average calcium content of spine cisterns in brain slices incubated without stimulation for 1-2 hr before direct freezing (2.5 +/- 0.4 mmol/kg of dry weight). Depolarization of incubated cerebellar slices by isotonic 55 mM KCl resulted in the accumulation within spine cisterns of very high amounts of calcium or isotonically substituted strontium, both derived from the extracellular fluid. These results suggest that one function of spine cisterns is to sequester free calcium that enters the spine through ligand-gated or voltage-gated channels during synaptic transmission.

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.

Proton microscopy and microanalysis--biological applications

Several nuclear and atomic techniques are available to a scanning proton microprobe for examining the elemental distributions and microstructure within a specimen. The instrument and its techniques are discussed together with some examples of non-destructive elemental microanalysis. The handling and display of quantitative maps and their relationship to regional spectra are shown with an emphasis in this paper on biological applications.

The ecology and pathogenicity of urease-producing bacteria in the urinary tract

Urease activity is a physiological function of many bacteria that enables these organisms to utilize urea as a source of nitrogen. The association of ureolytic bacteria with human or animal hosts varies widely from a commensal relationship as demonstrated with skin microflora, a symbiotic relationship in the gastrointestinal tract, to a pathogenic relationship in the urinary tract. Since similar or identical species of bacteria such as Staphylococcus aureus are found in all three environments, the effect of urease activity on the host must be solely a function of the environment of these organisms. In this review, the importance of urease to bacteria is discussed, identifying the gastrointestinal tract as a major reservoir of ureolytic bacteria and investigating the urinary tract environment and the infectious struvite stone production that often accompanies urease-producing bacteria there. Finally, an infection model is presented which explains the development and growth of these urinary calculi and their remarkable persistence in spite of modern urological treatments.

Quantitative electron energy loss spectroscopy in biology

The potential for applying electron energy loss spectroscopy (EELS) in biology is assessed. Some recent developments in instrumentation, spectrometer design, parallel detection and elemental mapping are discussed. Quantitation is demonstrated by means of the spectrum from DNA which gives an elemental ratio for N:P close to the expected value. A range of biologically important elements that can be usefully analyzed by EELS is tabulated and some possible applications for each are indicated. Detection limits and the effects of radiation damage are illustrated by spectra from the protein, insulin, and from the fluorinated amino-acid, histidine. Calcium detectability under optimum conditions may be as low as 1 mmol/kg dry weight. The application of EELS to analysis of cryosectioned adrenomedullary (chromaffin) cells is described in order to help determine the composition of the secretory granule. Water content can be determined from the amount of inelastic scattering as measured by the low-loss spectrum. The nitrogen/phosphorus ratio can be measured to provide information about the relative concentrations of ATP, chromogranin, and catecholamines. Quantitative EELS elemental maps are obtained in the STEM mode from chromaffin cells in order to measure the distribution of light elements.

Effects of topically-applied antiperspirant on sweat gland function

In subjects exposed to a hot environment, short-term topical pretreatment with aluminium zirconium tetrachlorhydrate delayed the onset of visible sweating although it failed to prevent the response. The delay was considered most probably to be due to the occlusive action, in the duct within the upper epidermis, of aluminium-containing conglomerates, which disappear after continuous sweating. However, microanalytical evidence indicated that ionic transport within the fundus secretory cells was also modified.

Quantitative energy dispersive X-ray microanalysis of eight elements in pancreatic endocrine and exocrine cells after cryo-fixation

Quantitative X-ray microanalysis of 8 elements was performed on ultrathin, freeze-dried sections of islets and pancreas pieces from non-inbred ob/ob-mice. Diffusion of elements was reduced to a minimum by rapidly freezing the tissue samples between nitrogen-cooled polished copper surfaces and avoiding the use of chemical fixatives and stains. The ultrastructural morphology was adequately maintained to allow measurements on secretory granules, mitochondria, cell nuclei, and cytoplasm free of these organelles. The distribution of the various elements between cellular compartments was similar in islet beta-cells and exocrine pancreas cells. However, the insulin secretory granules were outstanding in exhibiting the highest concentrations of zinc and calcium. In comparison with cytoplasm in the beta-cells, the insulin granules accumulated calcium 2-fold and zinc as much as 40-fold. As no correlation could be made for endoplasmic reticulum in the cytoplasmic measurements areas, the true accumulations above cytosol are likely to be even higher.

An ecological study of infected urinary stone genesis in an animal model

Direct molecular and morphological techniques of modern microbiology were used to monitor the sequential development of bacterial microcolonies and biofilms in a rat model of urinary infection and to demonstrate that the urease activity of the infecting organisms sets in course a series of reactions in which struvite and apatite crystals develop within the matrix of the enlarging bacterial aggregate. This forms multiple stone nidi on the uroepithelial surface upon which succeeding bacterial biofilms develop and with the incorporation of other urine components, such as urinary mucroproteins, establishes a matrix skeleton that becomes mineralised, thus allowing for the growth of the stone in concentric layers. To arrive at this hypothesis, we studied infection stone genesis using a newly developed model for infection-induced bladder stone formation in the rat. We examined in detail the sequential events in the evolving microbial ecology of progressive struvite calculogenesis, using conventional microbiological techniques, direct ultrastructural observation, newly developed ultrastructural cytochemical localisation techniques and immunological procedures for stabilisation of the biofilm glycocalix and stone matrix in the rat model. It was concluded that the organic glycocalix material secreted by the associated bacteria comprises a substantial and aetiologically important part of the infection stone calculogenesis and matrix production.

Properties of frozen sections relevant to quantitative microanalysis

Difficulties in the quantitative X-ray microanalysis of frozen sections may conceivably arise from ice-crystal damage and from electron-beam damage. X-ray peak-to-continuum ratios are commonly taken as a quantitative index of elemental concentrations. But recent reports suggest that in dehydrated frozen sections such ratios vary greatly with the scale of ice-crystal formation existing prior to sublimation. The experiments in these reports are re-interpreted here; it is argued that peak intensities may be affected by ice-crystal scale but that ratios of peak to continuum should not be affected after corrections for exogenous continuum. The accuracy of the peak-to-continuum method is affected by beam-induced loss of mass from microvolumes during analysis. Mass loss can be reduced or slowed by a cold-stage. For example, the radiation sensitivity for loss of chlorine from PVC is reduced by a factor of 1000 or more with reduction of temperature from 300 to 100 K. For sections of soft tissue the effectiveness of cooling is not nearly so striking but at 100 K, analyses of 1 micron frozen-hydrated sections by the continuum method, with spatial resolution of the order of 1 micron, can be completed before substantial mass loss occurs. However, analysis of frozen-hydrated sections by the continuum method at much higher resolution, say 100 nm resolution in 100 nm sections, is precluded by mass loss. Measurements of local mass can be achieved with much lower dose by observation and calibration of the electron transmission or backscattering. But even with these methods, several problems remain in achieving quantitative X-ray analysis at very high resolution.

Contrast and resolution of scanning transmission electron microscope imaging modes

Image blurring due to delocalization of inelastic events was studied for scanning transmission electron microscopy (STEM) of unstained thin sections. The delocalization probability was obtained from the angular distribution of inelastic scattering, which was calculated from experimental electron loss spectra of organic samples. This probability was implemented in a Monte Carlo program to simulate the effects of multiple scattering and delocalization for STEM images collected by either the annular detector or the spectrometer, and images generated by a combination of these two signals. Depending on the illumination, the detector geometry and the energy-loss range selected for imaging the annular detector image is blurred by a non-negligible fraction of inelastically scattered electrons. Simultaneous acquisition of an inelastic image using a spectrometer allows the blurring to be reduced by calculation of either the ratio or the difference of the two darkfield signals. While inherent nonlinearities reduce the interpretability of ratio-contrast images, difference-contrast improves the visibility of details submerged in a diffuse background without introducing artifacts.

Mitochondrial calcium and cellular electrolytes in brain cortex frozen in situ: electron probe analysis

Mitochondrial calcium in rat brain cortex frozen in situ and measured with electron probe X-ray microanalysis was 1.5 mmol Ca/kg dry weight. Cellular, extramitochondrial Ca was 6.4mmol/kg dry weight and cytoplasmic Na+ was 73mM. These results do not support the role of mitochondria regulating cytoplasmic Ca2+, but are compatible with the regulation of mitochondrial enzymes by matrix free Ca2+.

Application of X-ray microanalysis to cell suspensions of oil palm (Elaeis guineensis Jacq.)

X-ray microanalysis has been used to determine the elemental composition of oil-palm (Elaeis guineesis) cell suspensions without the use of cryoprotectants. Results based on individual cells were gathered over a typical growth cycle of 14 d. During the log phase (5-7 d) there is an increase in the number of cells containing high concentrations of both K (400 mmol kg(-1) dry weight) and P (400 mmol kg(-1) dry weight). Morphologically these cells had thin cell walls and were frequently joined to other cells (two to five cells per clump).

Calcium and sodium distribution and movements in smooth muscle

Electron probe microanalysis (EPMA) has been used to study the subcellular distribution of Ca, Na, K, Cl, and Mg in smooth muscle. The EPMA results indicate that the sarcoplasmic reticulum (SR) is the major intracellular source and sink of activator Ca: norepinephrine decreases the Ca content of the junctional SR in portal vein smooth muscle. Mitochondria do not play a significant role in regulating cytoplasmic free Ca2+, but mitochondrial Ca content can be altered to a degree compatible with suggestions that fluctuations in matrix Ca contribute to the control of mitochondrial metabolism. The rise in total cytoplasmic Ca during a maintained, maximal contraction is very much greater than the rise in free Ca2+, and is probably in excess of the known binding sites available on calmodulin and myosin. Cell Ca is not increased in normal cells that are Na-loaded. The non-Donnan distribution of Cl is not due to compartmentalization, but reflects high cytoplasmic Cl. Na-loading of smooth muscle in K-free solutions is temperature dependent, and may exhibit cellular heterogeneity undetected by conventional techniques. The total cell Mg is equivalent to approximately 12 mM, and less than 50% of it can be accounted for by binding to ATP and to actin. Mitochondrial monovalent cations in smooth muscle are relatively rapidly exchangeable.