Morphological findings and energy dispersive X-ray microanalysis of oral amalgam tattoos

Oral amalgam tattoos (AT) are distinct pigmentations of the oral mucosa resulting from accidental incorporation of dental amalgam in the oral soft tissues. Dental amalgams and in particular mercury, one of the constituents of dental amalgams, have for long been considered toxic. Oral ATs are easily accessible to study soft tissue reaction to amalgam and its degradation products. In this study, 17 oral ATs were examined by transmission electron microscopy and energy dispersive X-ray microanalysis. Ultrastructurally, in the ATs, three kinds of electron-dense particles were observed. The largest particles ranged in size from 0.5 up to several 100 microm. Smaller electron-dense inclusions (0.5-0.1 microm) were seen extracellularly associated with meshworks of elastic fibers and collagen bundles. The third and smallest type of particles (5-30 nm in diameter) was found with basement membranes of small vessels and pericytes and particularly decorating collagen bundles. Element analysis regularly revealed the presence of silver, sulphur, copper and lead in the AT decay products. Mercury was found in only one instance. Tissue reactions due to ATs seem to be minimal. No acute inflammatory changes were seen. Larger inclusions occasionally were surrounded by macrophages and multinucleated cells. TEM and element analysis may in specific cases be helpful in the differential diagnosis of pigmented lesions of the oral mucosa.

Abnormal ion content, hydration and granule expansion of the secretory granules from cystic fibrosis airway glandular cells

The absence or decreased expression of cystic fibrosis transmembrane conductance regulator (CFTR) induces increased Na(+) absorption and hyperabsorption of the airway surface liquid (ASL) resulting in a dehydrated and hyperviscous ASL. Although the implication of abnormal airway submucosal gland function has been suggested, the ion and water content in the Cystic Fibrosis (CF) glandular secretory granules, before exocytosis, is unknown. We analyzed, in non-CF and CF human airway glandular cell lines (MM-39 and KM4, respectively), the ion content in the secretory granules by electron probe X-ray microanalysis and the water content by quantitative dark field imaging on freeze-dried cryosections. We demonstrated that the ion content (Na(+), Mg(2+), P, S and Cl(-)) is significantly higher and the water content significantly lower in secretory granules from the CF cell line compared to the non-CF cell line. Using videomicroscopy, we observed that the secretory granule expansion was deficient in CF glandular cells. Transfection of CF cells with CFTR cDNA or inhibition of non-CF cells with CFTR(inh)-172, respectively restored or decreased the water content and granule expansion, in parallel with changes in ion content. We hypothesize that the decreased water and increased ion content in glandular secretory granules may contribute to the dehydration and increased viscosity of the ASL in CF.

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.

Preparation of cells cultured on silicon wafers for mass spectrometry analysis

The distribution of specific atoms and molecules within living cells is of high interest in bio-medical research. Laser secondary neutral mass spectrometry (laser-SNMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) detect atoms with high sensitivity and spatial resolution. The application of these methods to cultured cells requires special preparation techniques preserving morphological and chemical integrity of the living cells. The cells should, therefore, be grown on a conducting material preventing charging of the sample during ion bombardment. Silicon is currently used as the preferred support material for non-biological samples in mass spectrometry. This study investigates (1) the influence of silicon surfaces on cell growth and (2) the suitability of a sandwiched, rapid freezing method to analyse transmembrane ion gradients. Human melanoma cells were grown on silicon with polished or etched surfaces. Growth kinetics were studied using the Sulforhodamine-B assay. Number, shape, and morphology of the cells were assessed by epifluorescence microscopy of calcein AM- and DAPI-stained cells. Cells were subjected to rapid freezing, freeze-fracturing, and freeze-drying prior to analysis by TOF-SIMS and laser-SNMS. While cell numbers and morphology on the rough silicon wafers were impaired, morphology and growth kinetics of cells on polished silicon were identical to control cells on cell culture tested polystyrene. TOF-SIMS and laser-SNMS resulted in high-resolution elemental images and mass spectra. Measurement of the intracellular Na+ and K+ concentrations revealed a ratio as observed in living cells. In conclusion, culturing cells on polished silicon wafers followed by sandwiched, rapid freezing is an adequate preparation method to study intracellular ion distribution with mass spectrometry.

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.

Magnesium transport through the basal plasma membrane of larval malpighian tubules of Drosophila hydei studied by electron probe X-ray microanalysis

Magnesium besides calcium is the most important excretion product. In the anterior Malpighian tubules of Drosophila, excretion of magnesium takes place via the hindgut by proteoglycan containing concretions. This study reports on magnesium transport through the basal plasma membrane of the principal cells of the proximal segment of the anterior Malpighian tubules. Measurements by electron probe X-ray microanalysis indicate the existence of two antiporters which transfer magnesium in still unknown stoichiometry from the hemolymph space into the cell: Mg/H and Mg/Na.

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.

X-ray microanalysis of organic thin sections in TEM using an UTW Si(Li) detector: comparison of quantification methods

We compared Hall's peak to continuum ratio method with a peak ratio method in order to quantify light elements (C, N, and O) in organic specimens as a model for biological thin sections. X-ray spectra were recorded by an energy dispersive X-ray spectrometer equipped with an ultra thin window detector. Spectra were processed by means of a top-hat filter adapted to peak full-width half maximum. The peak intensities were measured by multiple least square fitting to reference spectra. For most elements of biological interest, theoretical and experimental k-factors were determined. Absorption correction was found to be important for quantitation of carbon, nitrogen, and oxygen. Boron was efficiently detected; however, quantitative analysis was not possible. We conclude from our experiments that the peak ratio method is more suitable for quantitation of elemental concentrations in biological thin sections than the peak to continuum method.

In vitro effects of zirconia and alumina particles on human blood monocyte-derived macrophages: X-ray microanalysis and flow cytometric studies

The cytocompatibility of two particulate bioceramics, zirconia and alumina, was studied using human blood monocytes driven to differentiate into mature macrophages with granulocyte macrophage-colony-stimulating factor. Changes in individual cell elemental composition, particularly sodium and potassium content, were assessed by X-ray microanalysis of ultrathin freeze-dried sections. Phagocytosis and respiratory burst of macrophages exposed to biomaterial for 7 days were analyzed under flow cytometry using uptake of fluorescent latex beads and 2'7'-dichlorofluorescien diacetate oxidation, respectively. Zirconia and alumina particles were found to decrease the intracellular potassium/sodium ratio (an index of cell vitality) significantly (p<.01) in 7-day-cultured macrophages compared to control cells cultured out of material. Phagocytosis of both ceramic particles by macrophages was followed by a concomitant decrease in cell phagocytic ability (27%) and a marked altered oxidative metabolism (>2 times reduced by zirconia and >5 times reduced by alumina). The present study clearly demonstrates that reduction of the phagocytic capacity of macrophages associated with altered oxidative metabolism caused by biomaterial particles is characterized by changes in intracellular elemental content. Thus, investigation of cellular homeostasis by electron probe microanalysis together with analysis of functional changes may improve estimation of biomaterial cytocompatibility.

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.

Tachyzoite calcium changes during cell invasion by Toxoplasma gondii

The invasion of host cells by the obligate intracellular protozoan parasite Toxoplasma gondii is calcium dependent. We have identified two calcium storage areas in tachyzoites, the endoplasmic reticulum and vesicles that contain high concentrations of calcium as amorphous calcium phosphate precipitates. Our data indicate that these vesicles slowly lose their calcium during the intracellular development of the tachyzoite as their nucleus phosphorus content increases. We found fluctuations in the sulfur content of the tachyzoite during invasion following the exocytosis of protein from the secretory organelles, with a loss of sodium and chlorine, and the uptake of potassium from the host cell cytoplasm. We demonstrated that penetration of the tachyzoite into the host cell was accompanied by increases in the concentrations of phosphorus and sulfur in the host cell nucleus, probably due to increased transcription. The cytosol sodium concentrations decreased, while the potassium content increased. Thus, the subcellular element distribution of tachyzoites and host cells changes during invasion and intracellular growth of the parasites. In addition, our results indicate that tachyzoite calcium might be involved in the egress of the parasite from the host cell.

Extracellular compartments of the blowfly eye: ionic content and topology

To analyze the elemental composition and topology of the extracellular compartments of the compound eye, the eyes of blowflies Calliphora vicina were rapidly frozen and ultrathin cryosections were freeze dried. Three zones of an ommatidium, peripheral cytosol of visual cells, rhabdomeres, and ommatidial cavities were analyzed by X-ray microprobe analysis. The ommatidial cavity was found to contain sodium and potassium in proportion similar to that in the blowfly hemolymph. Potassium-to-sodium ratio in a cytosol was typical for a cytosol. The rhabdomeres displayed an electrolyte content intermediate between the above compartments. Three topologically connected extracellular compartments were characterized by the experiments with tracers, monastral blue and lanthanum: (1) common intercellular space of ommatidia including peripheral clefts between the visual cells, both tracers entered this compartment; (2) the ommatidial cavity, which is not accessible for monastral blue, however, as revealed by our X-ray microanalysis, it was reachable for lanthanum; (3) rhabdomeric loops, which were accessible for lanthanum entering either via the cavity or from the common intercellular clefts. The above characteristics of the ionic content and topology of ommatidial compartments might suggest higher sodium and lower potassium content in the microvilli as compared with the cytosol. The rhabdomeric and "cavital" plasma membranes are assumed to be permeable for these ions so that a voltage of only 25-30 mV, negative inside, is probably formed across them, much lower than the known resting potential -60 mV across the peripheral plasma membrane of a visual cell.

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.

The influence of nickel and cobalt on putative members of the oxygen-sensing pathway of erythropoietin-producing HepG2 cells

Cobalt and nickel stimulate, as does hypoxia, the production of erythropoietin (EPO) in HepG2 cells. Under hypoxic conditions, a decrease in the level of intracellular reactive oxygen species (ROS) is thought to stimulate EPO expression. Cobalt and nickel may interact with the putative oxygen sensor by changing the redox state of the central iron atom of heme proteins, similar to the effects of hypoxia. It was investigated, therefore, whether cobalt and nickel interact with hemeproteins or ROS scavenging systems in the control of intracellular ROS level. Cobalt chloride (100 microM, 24 h) oxidized non respiratory as well respiratory hemeproteins and increased the oxygen consumption. In contrast, nickel chloride (300 microM, 24 h) primarily reduced respiratory hemeproteins and decreased the oxygen consumption. In HepG2 cells treated with CoCl2, iron and cobalt were localized in cytosolic granules close to the cell nucleus and in mitochondria at concentrations up to 12 mM or 41 mM, respectively. Intracellular nickel was not measurable. Three-dimensional reconstruction of confocal laser microscopy images revealed hot spots of hydroxyl radical generation by a Fenton reaction at the sites of cytosolic iron accumulation. The .OH levels decreased in cobalt-treated (to 81%) as well as in nickel-treated (to 67%) HepG2 cells, accompanied by an increase of EPO expression to 167% and 150%, respectively. Our results underline the importance of .OH formed by a Fenton reaction for triggerimg EPO production. Identification of the primary hemeprotein being the oxygen sensor was not possible due to the antagonistic effects of cobalt and nickel on the redox state of detectable hemeproteins.

Potassium is involved in apatite biomineralization

The biogenetic formation of mineral crystals, one aspect of biomineralization, is a multistep process of apatite formation throughout the growth of dentin tissue. An important step is the transformation of the non-mineralized predentin matrix to mineralizing dentin matrix and its biological control. In this study, the high capacity of elemental mapping is combined with single x-ray point measurements to elucidate whether special elements are involved in initiation or regulation of mineral nucleation. Directly at the mineralization front, micro-areas with a strong co-enrichment of phosphorus (e.g., as phosphate) and potassium are found. During the beginning of the calcium enrichment and the subsequent apatite mineral formation in the characteristic micro-areas, the content of potassium decreases significantly. These findings indicate that potassium is involved in the process of dentin mineralization.

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.

Magnesium in newly formed dentin mineral of rat incisor

Small amounts of magnesium are always detectable in addition to calcium and phosphorus in mineralized tissues such as dentin or bone. Magnesium has been considered to influence the mineralization process, especially crystal growth. The present study reports on the location and enrichment of magnesium in the newly mineralized dentin by using the high lateral resolution of energy dispersive X-ray microanalysis combined with scanning transmission electron microscopy. To this end, we have used the continuously growing rat incisor as a model for a collagenous mineralizing system. Dental tissue was dissected free and cryofixed in liquid nitrogen-cooled propane. The distribution of elements was measured in freeze-dried ultrathin cryosections. The magnesium distribution of the newly formed dentin area near the predentin area was found to be inhomogeneous. In certain small dentin areas, characteristical magnesium enrichments were observed. Further, high magnesium-to-phosphate molar ratios were found in these areas, and these were correlated with low calcium-to-phosphate molar ratios. Our results support the theory that magnesium is involved in the process of biological apatite crystal formation.

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.

The importance of the Golgi complex for epithelial ion transport in Drosophila Malpighian tubules, studied by electron microscopy, cytochemistry and X-ray microanalysis

The distribution of potassium in the cells of Drosophila Malpighian tubules is not homogeneous. In the microvilli of the apical part of the cell the cytoplasmic potassium content was found to be 2 to 3 times higher than in the neighboring intermediate cytoplasm. Data obtained by electron microscopy, histochemistry and electron probe X-ray microanalysis indicate that glucosaminoglycans (GAGs), synthesized by the Golgi-ER complex, are responsible for potassium accumulation in the apical microvilli. Vesicles bud from the Golgi complex and then move to the apical cell region, where they discharge their contents into the cytoplasm or into the lumen. Budded vesicles also discharge their contents into the hemolymph space between the folds of the basal plasma membrane. GAGs, transformed to proteoglycans (PGs), were identified on the folds of the basal cell surface including basal lamina by reaction with alcian blue. Brefeldin A (BFA) was found to disintegrate Golgi-ER structures to vesicles, whereas budded vesicles vanished. Within the microvilli the K+-content decreased to 32%, the water content to 77%. These data provide evidence that the ER-Golgi complex is involved in the delivery of GAGs (and PGs) into the luminal space and the hemolymph. After disintegration of the Golgi complex, GAGs are missing as temporary ion stores from the vicinity of the membrane transporters.

Elemental distributions in predentine associated with dentine mineralization in rat incisor

Electron probe microanalysis was applied to study quantitatively and semi quantitatively the elemental concentrations and distributions that occur in predentine during the dentine mineralization of rat incisor. Apex regions of the continuously growing incisors were rapidly dissected and cryofixed in liquid nitrogen-cooled propane. Ultrathin cryosections were prepared from the dentine tissue. On the average in the extracellular predentine element concentrations of calcium and phosphorus were about 0.5% (w/w) and 0.5-1% (w/w), respectively; so the calcium content in the extracellular predentine is higher while the phosphorus content is much lower than in the odontoblast area. Due to the high content of glycosaminoglycans in the extracellular matrix the concentration of sulfur in the predentine was more than 1% (w/w); the potassium content was found in the range of 0.6-0.8% (w/w) which is quite high for an extracellular area and the concentrations of sodium and chlorine were higher than 2% (w/w). Elemental mapping analysis was carried out to demonstrate the distribution of some important elements at the predentine/dentine border during mineralization.

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.

Calcium-sequestering organelles of Dictyostelium discoideum: changes in element content during early development as measured by electron probe X-ray microanalysis

Starving Dictyostelium discoideum amoebae aggregate within a few hours by chemotaxis towards the attractant cAMP to form a multicellular organism. The differentiating cells possess rapid and efficient calcium buffering and sequestration systems which enable them to restrict changes in the cytosolic free calcium concentration temporally and spatially during their chemotactic reaction and allow the continuous accumulation of Ca2+ during development. In order to identify and to characterize calcium storage compartments, we analyzed the element content of amoebae at three consecutive stages of differentiation. Determination of the element distribution was done using energy-dispersive X-ray microanalysis of freeze-dried cryosections of rapid-frozen cells. Amoebae were frozen in the vegetative and aggregation-competent state and after formation of aggregates. Aggregation-competent as well as aggregated cells contained mass dense granules with large amounts of calcium together with phosphorous and either potassium or magnesium: in aggregation-competent cells calcium was colocalized with potassium, whereas in aggregated cells the mass dense granules contained calcium and magnesium. Although mass dense granules were also present in undifferentiated, vegetative cells, they contained only low amounts of phosphorous and potassium together with little Ca and Mg. We conclude that during their differentiation D. discoideum cells use an intracellular storage compartment to sequester Ca and other cations constantly throughout development.

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.

Morphology and element distribution of magnesium-secreting epithelium: the proximal tubule segment PII of dogfish, Scyliorhinus caniculus (L.)

Proximal tubule segment PII cells of marine elasmobranch fish were studied by transmission electron microscopy of thin sections, and X-ray microanalysis was performed with freeze-dried cryosections. Epithelial cells of PII are characterized by high and dense brush border at the apical side, elaborate folding of the lateral cell membrane and large basal extracellular labyrinth confined by a system of meandering cell extensions. Basal cytoplasmic zone, apical cytoplasmic zone, nuclei, mitochondria and apical small vacuoles were accessible for X-ray microanalysis. Concentrations of Na, Mg, P, S, Cl and K were different in the cytoplasmic zones along the basal-apical axis of the cell and in the organelles. PII cells lacked an apical tubulovesicular apparatus, instead they displayed an apical zone of smooth clear vesicles and small apical vacuoles. After freeze-drying, the small apical vacuoles and the smooth clear vesicles contained flocculent mass-dense material. Small apical vacuoles showed high concentrations of Mg (229 mmol/kg water), Na (132 mmol/kg water) and Cl (148 mmol/kg water). Sequestration of Mg in vesicles and small apical vacuoles and subsequent exocytosis between the microvilli of the brush border are supposed to be important steps in the transepithelial transport (tubular secretion) of magnesium by PII cells of marine fish.

Electron probe X-ray microanalysis of epithelial cells: aspects of cryofixation

Content and distribution of diffusible ions in epithelial cells were studied by scanning transmission electron microscopy and energy dispersive electron probe X-ray microanalysis of freeze-dried cryosections from trout kidney, rat liver and Malpighian tubules of Drosophila larvae. Cryofixation of small excised kidney and liver samples by rapid immersion into liquid propane resulted in intracellular K/Na-ratios < 1. In contrast, K/Na-ratios > 7 were obtained after in situ cryofixation by means of a cryopunching device which allows tissue pieces to be frozen during excision from the intact organ. Isolated hepatocytes cryofixed in a small droplet of culture medium had a K/Na-ratio of 3.7. After culturing the hepatocytes, the K/Na-ratio increased to 24. Effects of extracellular media of different composition on the intracellular element content were studied. Malpighian tubules of Drosophila larvae were cryofixed by rapid immersion into liquid propane, and the distribution of K across the cells forming the tubules from the basal to the apical cell membrane was measured. An increasing K gradient was found from the intermediate to the apical cytoplasm. The intracellular K distribution was dependent on ions and transport inhibitors present in the fluid surrounding the Malpighian tubules within the larvae. Content and distribution of ions in epithelial cells sensitively depend on the physiological state immediately before cryofixation. Thus, electron probe X-ray microanalysis of cells and cell functions requires careful selection and control of the cell system to be studied.

[The mapping of the local content of water and dry matter by using ultrathin frozen sections]

The possibility and applications of quantitative dry mass and water content mapping using an image analysis of darkfield intensity digitalization were shown on ultrathin freeze-dried cryosections. The distribution map obtained may be further used both for direct measurements of local dry mass and water content under physiological state and pathological disorders and for calculation of the concentration values obtained by quantitative X-ray mapping on freeze-dried cryosections in terms of millimoles per litre of cellular water.

Heterogeneous distribution of elemental contents in the larval Malpighian tubules of Drosophila hydei: X-ray microanalysis of freeze-dried cryosections

The elements Na, Cl and K are distributed heterogeneously in the larval cells of the four Malpighian tubules of Drosophila hydei. Elemental gradients are steeper when they are related to the dry weight in comparison to the weight of water. The high contents of Na, Cl and K in the cytoplasm close to the basal plasma membrane differ significantly from the low values found in the neighboring basal "labyrinth" and in the intermediate cytoplasm. Elemental contents (K, Cl) increase from the intermediate cytoplasm to the apical microvilli and further to the lumen. The K/Na-ratio changes across the cells from 0.9 in the hemolymph to 2 in the basal cytoplasm close to the plasma membrane, to 27 in the microvilli, and 89 in the fluid phase of the lumen. In the lumen, K is accumulated in proteoglycan containing type-II concretions. The water content increases from the distal to the proximal segments of anterior and posterior tubules.

Analysis of early hard tissue formation in dentine by energy dispersive X-ray microanalysis and energy-filtering transmission electron microscopy

Thin cryosections and sections of embedded tissue were prepared from dentine of cryofixed rat incisors. Energy dispersive X-ray microanalysis (EDX) and electron energy-loss spectroscopy (EELS) have been applied to study the calcium and phosphorus distribution in predentine of these incisors. A small enrichment of calcium and phosphorus was found in the predentine zone near the dentine border. Element distributions were correlated with analyses of the early crystal formation in dentine. These investigations were carried out by parallel applications of electron spectroscopic diffraction (ESD) and electron spectroscopic imaging (ESI) using zero-loss filtering. It was found that the earliest crystal formations already showed the lattice of the hexagonal mineral apatite. They form parallelly arranged chains of dots which coalesce rapidly to form "needle-like" crystallites along the collagen microfibrils.

Calcium localization in the shell-forming tissue of the freshwater snail, Biomphalaria glabrata: a comparative study of various methods for localizing calcium

The routes calcium might take across the mantle to the shell have been investigated with various electron-microscopical techniques in the freshwater snail Biomphalaria glabrata (Planorbidae, Basommatophora). In chemically-fixed tissue, calcium was precipitated with a tannic acid-antimonate technique in predominantly the intercellular spaces of the outer mantle epithelium and the interstitium below it. Some vacuoles of the outer mantle epithelium and one type of mucus cell in the inner mantle epithelium also contained precipitate. The presence of calcium in the precipitates was proved by electron energy loss spectroscopy combined with electron spectroscopic imaging. Incubation with lead acetate and uranyl acetate revealed binding-sites for calcium in the intercellular spaces of the epithelia interstitium and the mucus cells of the inner mantle epithelium. Precipitates were also seen after all incubations in the calcium spherites of the connective tissue. The concentrations of calcium and other elements were analysed in freeze-dried ultrathin sections of cryofixed mantle tissue by means of energy-dispersive X-ray microanalysis. Only in mitochondria of the musculature could high amounts of calcium and phosphorous be detected.

Comparison of cryopreparation techniques for electron probe microanalysis of cells as exemplified by human erythrocytes

Erythrocytes in human blood were used to evaluate the reliability of cryopreparation techniques for electron probe X-ray microanalysis of biological cells and tissues. The elemental content determined by X-ray microanalysis of ultrathin freeze-dried cryosections was found to be consistent with data known from the literature. Considerable redistribution of the intracellular elemental composition was found after freeze-substitution as well as after freeze-drying followed by resin embedding. Two conclusions are drawn from this study: 1. Erythrocytes in human blood are a suitable reference specimen for evaluation of specimen preparation techniques for microanalysis. 2. At present, freeze-dried cryosections are the most reliable specimen type for quantitative electron probe microanalysis of cells.

Element distribution in organelles of pancreatic acinar cells of rat, mouse, and pig investigated by energy-dispersive X-ray microanalysis

The exocrine pancreas was long thought to be composed of identical subunits, the acinar cells that store the inactive forms of the digestive enzymes in zymogen granules (ZGs). These were generally seen as a homogeneous population of vesicles. This homogeneity was recently questioned: Digestive demands are answered by the release of specific enzymes and immunocytochemical labeling showed distinctive nonidentical populations of ZGs. We have aimed at finding concomitant differences in element contents. We analyzed by energy-dispersive x-ray microanalysis (EDX) the subcellular distribution of elements in acinar cells of resting and stimulated rat, resting mouse, and resting pig pancreas and compared the results with values from the literature. We found large variances in the concentrations of Na, Mg, P, S, Cl, K, and Ca in cytoplasm rich in endoplasmic reticulum (C/E), whereas the concentrations of P, Cl, K, and Ca in mitochondria and ZGs had surprisingly small variations. Na and Mg were detected in measurable amounts only in C/E and mitochondria and Ca was detectable only in ZGs. We could not find any other elements. We have not found clearly distinguishable populations of ZGs. We critically discuss our findings in comparison with the literature. Many discrepancies can be explained by the different preparation procedures. We show that it is questionable to present absolute values of concentration in biological specimens on the basis of EDX. The technique should, in our opinion, be used only for the study of relative concentrations.

Photoaffinity labeling of plasma membrane proteins involved in the transport of loop diuretics into hepatocytes

To identify proteins involved in the hepatocellular uptake of loop diuretics, [3H]bumetanide was photoactivated by light flash in the presence of either intact isolated rat hepatocytes, rat liver basolateral plasma membranes or integral membrane proteins extracted from the basolateral plasma membranes. Proteins of 52-54, 48, 33, 27, 25 and 23 kDa in sodium dodecyl sulfate (SDS) gel electrophoresis were radiolabeled on intact hepatocytes. On liver basolateral plasma membranes a 50-52 kDa protein was the most intensely labeled protein. After separation into integral and associated membrane proteins by extraction with Triton X-114, radioactive labeling was only found in integral membrane proteins with a molecular weight of 50-52 kDa. Photoactivated bumetanide irreversibly inhibited the hepatocellular uptake of cholate, taurocholate but not of serine. Binding proteins for photoactivated bumetanide were absent on AS 30-D ascites hepatoma cells. Labeling of all proteins was sodium dependent in intact hepatocytes but was sodium independent in plasma membranes. Labeling was prevented by non-labeled bumetanide and by the loop diuretics piretanide and furosemide. Labeling protection was further achieved with organic anions such as bromosulfophthalein, rifampicin, probenecid and by the bile acids taurocholate, deoxycholate and dehydrocholate. The radiolabeled proteins did not belong to the bumetanide-sensitive NaCl/KCl co-transport system which apparently does not occur in intact isolated rat hepatocytes.

Effects of amiloride treatment on U-118 MG and U-251 MG human glioma and HT-29 human colon carcinoma cells

Human glioma (U-118 MG, U-251 MG) and human colon carcinoma (HT-29) spheroids and monolayers were continuously exposed to amiloride under physiological Na+ and HCO3- conditions. Amiloride in concentrations of 0.1-0.2 mM inhibited growth, while 0.5 mM or higher induced disintegration of the glioma spheroids within 4-6 days. Growth retardation of the HT-29 spheroids was achieved at concentrations of 0.4-0.5 mM and total growth inhibition and disintegration were achieved at 1.0 mM. Monolayer cultures of glioma cells were also more sensitive to amiloride than those of colon carcinoma cells. The higher amiloride concentrations induced pyknotic nuclei mainly in the central areas of the spheroids where the extracellular pH (pHe) was low. The amiloride-sensitive glioma spheroids had lower pHe than the colon carcinoma spheroids. The intracellular pH (pHi), measured in monolayers, was higher (7.11-7.18) in glioma cells than in colon carcinoma cells (6.94). High concentrations of amiloride, 1.0 mM for 1 h in combination with low Na+ concentrations, caused a strong pHi decrease in glioma cells but only a slight decrease in the colon carcinoma cells. The pHi measurements in glioma monolayers were carried out after 2-6 days of continuous exposure to 0.1 mM amiloride at physiological levels of Na+ and HCO3- to simulate the conditions during growth inhibition. After several days this caused, when growth already was inhibited, an acidification of pHi. Parallel measurements with X-ray microanalysis showed an increase of intracellular sodium and a decrease of intracellular potassium in the gliomas, while no such changes were seen in the colon carcinoma cells under identical conditions. It is concluded that the two glioma cell lines were more sensitive to amiloride, both as monolayers and spheroids, than the corresponding cultures of the colon carcinoma cell line. The inhibition of proliferation by amiloride seemed not to have a clear connection to pHi regulation.

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.

High-resolution scanning electron microscopy of frozen-hydrated and freeze-substituted kidney tissue

Inner surfaces and fracture faces of rabbit kidney tissue were investigated with high-resolution scanning electron microscopy using two different cryopreparation techniques: (i) for the observation of fracture faces, cryofixed tissue was fractured and coated in a cryopreparation chamber dedicated to SEM, vacuum transferred onto a cold stage and observed in the frozen-hydrated state; (ii) for the observation of inner surfaces of the nephron, water was removed after freezing and fracturing by freeze substitution and critical-point drying of the tissue. By both methods, macromolecular structures such as intramembranous particles on fracture faces and particles on inner surfaces were imaged. The latter method was used to investigate in more detail surface structures of cells in the cortical collecting duct. These studies revealed a heterogeneity of intercalated cells not described thus far.

Mass dense vacuoles in Drosophila Malpighian tubules contain zinc, not sodium. A reinvestigation by X-ray microanalysis of cryosections

The intracellular storage of zinc in Malpighian tubules of Drosophila hydei was studied by X-ray microanalysis of freeze-dried cryosections. Mass dense vacuoles in the proximal region of the anterior larval Malpighian tubule cells were found to accumulate zinc, not sodium. The zinc content was enhanced considerably after addition of zinc to the food of the larvae. Zinc-containing vacuoles were also found after pupation. After starvation of larvae in sea water, Na was detected in these vacuoles in addition to Zn. A small increase of Na and a remarkable increase of Zn was found in the vacuoles after injection of Ringer solution with ouabain into the larvae. Similar vacuoles in cells of untreated posterior tubules exhibit only low zinc levels.

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.

Intracellular storage of sodium and magnesium in Drosophila Malpighian tubules. X-ray microanalysis of native cryosections

Using electron probe X-ray microanalysis after cryofixation, cryosectioning and freeze-drying we investigated the content of electron-dark vacuoles in the intermediate cell region of the proximal segment of Malpighian tubules in Drosophila larvae. According to this method these vacuoles store sodium and magnesium in a high correlation (r = 0.98) of 5:1 ratio. Phosphorus, potassium and sulfur are also stored. In the intermediate groundplasm surrounding the vacuoles the element content is different from that in the vacuoles. The significance of vacuolar sodium and magnesium storage for the ionic metabolism is unknown. In addition to Na, Mg, P, K and S the vacuoles also contain 3-OH-kynurenine and other fluorochromes. With the pyroantimonate technique intravacuolar precipitates were demonstrated. X-ray microanalysis of the precipitates revealed sodium and calcium, although following cryofixation calcium was not detectable in the vacuoles by X-ray analysis.

Primary liver cell cultures grown on gas permeable membrane as source for the collection of primary bile

Isolated rat hepatocytes maintained in primary culture on gas permeable membrane for 20 h form monolayers and establish at their cell borders a network of canaliculi (approximate diameter 3.5 micron). In the presence of the known choleretic bile acid dehydrocholate, dilation of canaliculi occurs. When nonfluorescent carboxyfluorescein diacetate ester is added to the culture medium, fluorescent carboxyfluorescein appears in the intracanalicular space. In the dilated state, fluid containing the fluorescent compound could be collected from the canaliculi by puncture with a micropipette. The intracanalicular space shows a negative electrical potential difference of 31 mV in reference to the bath solution and is 13.5 mV more positive with reference to recordings from the cytosol of cultured rat hepatocytes. Cultured rat hepatocytes grown on gas permeable membrane are energetically stable over 3 d. On Day 4, ATP levels increase markedly, whereas Na+-K+-ATPase activity declines. Ionic composition of hepatocytes, as measured by electronprobe element analysis on cryosection samples, does not change markedly during monolayer formation. With formation of bile canaliculi, the activity of alkaline phosphatase rapidly increases within 24 h and is stable for the next 3 d. Within that time the activity of gamma-glutamyltranspeptidase, however, increases steadily, reaching a 1.6-fold higher activity than freshly isolated hepatocytes. Bile acids appear in the culture supernatant after 1 d. When unconjugated [14C]cholic acid is added to the cultures the supernatant contains also [14C]tauro- and [14C]glycocholic acid, indicating the preservation of conjugation capacity in these cultures. Total bile acid concentrations in the supernatant increase from 5 to 26 microM on Day 4. The cultures do not secrete alpha-fetoprotein. Monolayer cultures of hepatocytes in the presence of choleretic bile acids seem to be a suitable model system to collect and to analyze the composition of primary bile. In conjunction with the electrical parameters, it is possible to describe directly properties of bile secretion at the canalicular pole of the intact hepatocyte.

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.