Tissue fixation and staining with osmium tetroxide: the role of phenolic compounds

First ultrastructural description of an apomictic opsiblastic egg in freshwater Gastrotricha

Freshwater gastrotrichs have a biphasic lifecycle that reputedly involves the production of three types of eggs: apomictic and fast hatching (tachyblastic ova), apomictic and delayed hatching (opsiblastic ova), and plaque-bearing eggs (potentially derived from mixis). While some details of oogenesis and eggshell structure are known for tachyblastic ova, there are few details on other egg types. Here, we provide the first ultrastructural description of the oviposited opsiblastic eggs of the freshwater gastrotrich, Lepidodermella squamata. Scanning electron microscopy revealed the eggshell surface to be ornamented with long flattened pillar-like structures centered on polygonal plates that are pitted along their periphery. Transmission electron microscopy showed the pits to lead to a vast labyrinth of tubular spaces and larger cavities throughout the thick apical layer of the shell. The basal layer of the shell is amorphous and connected to a network of fine fibers that traverse an extra-oocyte space and forms a protective sheet around the uncleaved oocyte. The uncleaved oocyte has a dense layer of peripheral ooplasm surrounding a core of organelles including mitochondria, membrane-bound secretion granules, endoplasmic reticulum, and a single nucleus in a granular, ribosome-rich cytoplasm. Secretion granules are the most abundant organelles and presumably contain lipid-rich yolk that will be used as energy for delayed cleavage, thus functioning in temporal dispersal. These data are compared to the fine structure of invertebrate resting eggs across the phylogenetic spectrum to determine the novelty of opsiblastic egg structure in L. squamata.

Phenolic Determination in Proembryogenic Cell Complexes of Buckwheat Morphogenic Cell Culture with Osmium Tetroxide, Toluidine Blue O Dye, and Iron Chloride

The study of the localization of secondary metabolites in both plants and the cell cultures on the intravital sections is hampered by the difficulty of obtaining thin, correctly oriented sections. Techniques for fixing tissues in resins allow these difficulties to be overcome. Properly selected tissue fixation techniques allow using different dyes to identify the compound of interest. In addition, some components of tissue fixation can act as fixatives and as a dye for identifying secondary metabolites. For example, osmium tetroxide, which fixes lipids in tissues, stains phenolic compounds black. This paper describes methods for the detection of phenolic compounds in morphogenic callus culture of buckwheat using osmium tetroxide, Toluidine Blue O dye, and ferric chloride as dyes in epoxy resin-embedded cell culture with double fixation of the material and when material fixed in Karnovsky's fixative.

Ultrastructural characterization of the putative defensive glands (warts) in the sessile, colonial rotifer Sinantherina socialis (Gnesiotrocha; Flosculariidae)

Female Sinantherina socialis are freshwater, sessile, colonial rotifers that possess two pairs of distinctive glands (warts) located below the corona. Previous studies demonstrated that colonies are unpalatable to many invertebrate and vertebrate predators; those authors suggested that the warts were a possible source of a chemical deterrent to predation. Here we explore wart ultrastructure and cytochemisty to determine whether the warts function as exocrine glands and if their contents display any allomone-like chemistry, respectively. Externally, the warts appear as elevated bulges without pores. Internally, the warts are specialized regions of the integumental syncytium and therefore acellular. The lipid stain Nile Red labels all four warts. Two lipid membrane probes (sphingomyelin and phosphatidylinositol) also bind the warts and may be staining internal secretion vesicle membranes. In fact, wart ultrastructure is defined by hundreds of membrane-bound secretion vesicles packed tightly together. The vesicles are mostly electron-lucent and crowded into a well-defined cytoplasmic space. The cytoplasm also contains abundant ribosomes, rough endoplasmic reticulum, mitochondria, and Golgi, but nuclei are generally positioned peripheral to the packed vesicles. Absence of muscles around the warts or any signs of direct innervation suggests expulsion of gland contents is forced by general body contraction. A single specimen with 'empty' warts implies that secretions are released en masse from all glands simultaneously. The identity of the chemical secretion remains to be determined, but the lack of osmium and uranyl acetate staining suggests a low abundance or absence of phenols, unsaturated lipids, or NH2 and -COOH groups. This absence, combined with the positive Nile Red staining, is interpreted as evidence that vesicles contain saturated fatty acids such as lactones that are unpalatable to predators.

Ultramicroscopy reveals a layer of multiply folded membranes around the tannin-accumulating vacuole in honeysuckle petal trichomes

Transmission electron microscopy was used to reveal a layer of multiply folded membranes that closely surrounded the tannin-accumulating vacuole in cells of honeysuckle petal trichomes. A huge amount of tannins were deposited in the peripheral region and the center of the vacuole. The prolific membranes extended to the tannins deposited along the vacuole periphery. It was difficult to distinguish the vacuole membrane, and it seemed as if it was the layer of multiply folded membranes that separated the vacuole lumen from the cytoplasm. In addition, there were also membrane assemblies in the cytoplasm away from the vacuole, which were continuous with the proliferated membranes bordering the vacuole. Therefore, the tannin-accumulating vacuole was in close association with a very large network of proliferated membranes. The occurrence of such a layer of multiply folded membranes around the tannin-accumulating vacuole might be a structural strategy for improvement of the efficiency of vacuolar accumulation of tannins.

Proton Conductivities of Lamellae-Forming Bioinspired Block Copolymer Thin Films Containing Silver Nanoparticles

Size-controlled metal nanoparticles (NPs) were spontaneously formed when the amphiphilic diblock copolymers consisting of poly(vinyl catechol) and polystyrene (PVCa-b-PSt) were used as reductants and templates for NPs. In the present study, the proton conductivity of well-aligned lamellae structured PVCa-b-PSt films with Ag NPs was evaluated. We found that the proton conductivity of PVCa-b-PSt film was increased 10-fold by the addition of Ag NPs into the proton conduction channels filled with catechol moieties. In addition, the effect of humidity and the origin of proton conductivity enhancement was investigated.

How did nature engineer the highest surface lipid accumulation among plants? Exceptional expression of acyl-lipid-associated genes for the assembly of extracellular triacylglycerol by Bayberry (Myrica pensylvanica) fruits

Bayberry (Myrica pensylvanica) fruits are covered with a remarkably thick layer of crystalline wax consisting of triacylglycerol (TAG) and diacylglycerol (DAG) esterified exclusively with saturated fatty acids. As the only plant known to accumulate soluble glycerolipids as a major component of surface waxes, Bayberry represents a novel system to investigate neutral lipid biosynthesis and lipid secretion by vegetative plant cells. The assembly of Bayberry wax is distinct from conventional TAG and other surface waxes, and instead proceeds through a pathway related to cutin synthesis (Simpson and Ohlrogge, 2016). In this study, microscopic examination revealed that the fruit tissue that produces and secretes wax (Bayberry knobs) is fully developed before wax accumulates and that wax is secreted to the surface without cell disruption. Comparison of transcript expression to genetically related tissues (Bayberry leaves, M. rubra fruits), cutin-rich tomato and cherry fruit epidermis, and to oil-rich mesocarp and seeds, revealed exceptionally high expression of 13 transcripts for acyl-lipid metabolism together with down-regulation of fatty acid oxidases and desaturases. The predicted protein sequences of the most highly expressed lipid-related enzyme-encoding transcripts in Bayberry knobs are 100% identical to the sequences from Bayberry leaves, which do not produce surface DAG or TAG. Together, these results indicate that TAG biosynthesis and secretion in Bayberry is achieved by both up and down-regulation of a small subset of genes related to the biosynthesis of cutin and saturated fatty acids, and also implies that modifications in gene expression, rather than evolution of new gene functions, was the major mechanism by which Bayberry evolved its specialized lipid metabolism. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Tissue and cellular localization of tannins in Tunisian dates (Phoenix dactylifera L.) by light and transmission electron microscopy

A histological approach including light microscopy and transmission electron microscopy (TEM) was used to provide accurate information on the localization of condensed tannins in the edible tissues and in the stone of date fruits (Phoenix dactylifera L.). Light microscopy was carried out on fresh tissues after staining by 4-dimethylaminocinnamaldehyde (DMACA) for a specific detection of condensed tannins. Thus, whether under light microscopy or transmission electron microscopy (TEM), results showed that tannins are not located in the epidermis but more deeply in the mesocarp in the vacuole of very large cells. Regarding the stones, tannins are found in a specific cell layer located at 50 μm from the sclereid cells of the testa.

Review article: The meristem in indeterminate root nodules of Faboideae

In this review, the anatomy of indeterminate legume root nodule is briefly summarized. Next, the indeterminate nodule meristem activity, organization and cell ultrastructure are described in species with a distinct nodule meristem zonation. Finally, the putative primary endogenous factors controlling nodule meristem maintenance are discussed in the context of the well-studied root apical meristem (RAM) of Arabidopsis thaliana.

A new technique for visualizing proanthocyanidins by light microscopy

We describe a new technique for visualizing proanthocyanidin-containing elements in plant tissues. Our innovation is the fixation of condensed tannins with an exogenous protein prior to alcohol dehydration. In this way, tannins do not undergo partial solubilization during the dehydration sequence and appear as sharply contoured globules of various diameters.

Multivesicular bodies participate in a cell wall-associated defence response in barley leaves attacked by the pathogenic powdery mildew fungus

Localized cell wall modification and accumulation of antimicrobial compounds beneath sites of fungal attack are common mechanisms for plant resistance to fungal penetration. In barley (Hordeum vulgare) leaves, light-microscopically visible vesicle-like bodies (VLBs) containing H(2)O(2) or phenolics frequently accumulate around cell wall appositions (syn. papillae), in which the penetration attempt of the biotrophic powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh) is halted. By ultrastructural analyses, we demonstrated that the Bgh-induced VLBs represent different structures. VLBs intensively stained by H(2)O(2)-reactive dyes were actually small papillae instead of cytoplasmic vesicles. Other VLBs were identified as osmiophilic bodies or multivesicular compartments, designated paramural bodies (PMBs) and multivesicular bodies (MVBs). MVBs seemingly followed two distinct pathways: either they were engulfed by the tonoplast for degradation in the vacuole or they fused with the plasma membrane to release their internal vesicles into the paramural space and hence could be the origin of PMBs. MVBs and PMBs appeared to be multicomponent kits possibly containing building blocks to be readily assembled into papilla and antimicrobial compounds to be discharged against fungal penetration. Finally, we propose that released paramural vesicles might be similar to exosomes in animal cells.

The Arabidopsis TDS4 gene encodes leucoanthocyanidin dioxygenase (LDOX) and is essential for proanthocyanidin synthesis and vacuole development

The anthocyanin and proanthocyanidin (PA) biosynthetic pathways share common intermediates until leucocyanidin, which may be used by leucoanthocyanidin dioxygenase (LDOX) to produce anthocyanin, or the enzyme leucoanthocyanidin reductase (LAR) to produce catechin, a precursor of PA. The Arabidopsis mutant tannin deficient seed 4 (tds4-1) has a reduced PA level and altered pattern PA accumulation. We identified the TDS4 gene as LDOX by complementation of the tds4-1 mutation either with a cosmid encoding LDOX or a 35S:LDOX construct. Independent Arabidopsis lines with a T-DNA insertion in the LDOX gene had a similar phenotype, and one was allelic to tds4-1. The seed phenotype of ban tds4 double mutants showed that LDOX precedes BANYULS (BAN) in the PA pathway, confirming recent biochemical characterisation of BAN as an anthocyanidin reductase. Double mutant analysis was also used to order the other TDS genes. Analysis of the PA intermediates in tds4-1 revealed three dimethylaminocinnamaldehyde (DMACA) reacting compounds that accumulated in extracts from developing seeds. Analysis of Arabidopsis PA and its precursors indicates that Arabidopsis, unlike many other plants, exclusively uses the epicatechin and not the catechin pathway to PA. Transmission electron microscopy (TEM) showed that the pattern observed when seeds of tds4 were stained with DMACA was a result of the accumulation of PA intermediates in the cytoplasm of endothelial cells. Fluorescent marker dyes were used to show that tds4 endothelial cells had multiple small vacuoles, instead of a large central vacuole as observed in the wild types (WT). These results show that in addition to its established role in the formation of anthocyanin, LDOX is also part of the PA biosynthesis pathway.

Identification and biochemical characterization of mutants in the proanthocyanidin pathway in Arabidopsis

Proanthocyanidin (PA), or condensed tannin, is a polymeric flavanol that accumulates in a number of tissues in a wide variety of plants. In Arabidopsis, we found that PA precursors (detected histochemically using OsO(4)) accumulate in the endothelial cell layer of the seed coat from the two-terminal cell stage of embryo development onwards. To understand how PA is made, we screened mature seed pools of T-DNA-tagged Arabidopsis lines to identify mutants defective in the synthesis of PA and found six tds (tannin-deficient seed) complementation groups defective in PA synthesis. Mutations in these loci disrupt the amount (tds1, tds2, tds3, tds5, and tds6) or location and amount of PA (tds4) in the endothelial cell layer. The PA intermediate epicatechin has been identified in wild type and mutants tds1, tds2, tds3, and tds5 (which do not produce PA) and tds6 (6% of wild-type PA), whereas tds4 (2% of wild-type PA) produces an unidentified dimethylaminocinnamaldehyde-reacting compound, indicating that the mutations may be acting on genes beyond leucoanthocyanidin reductase, the first enzymatic reduction step dedicated to PA synthesis. Two other mutants were identified, an allele of tt7, which has a spotted pattern of PA deposition and produces only 8% of the wild-type level of type PA as propelargonidin, and an allele of tt8 producing no PA. Spotted patterns of PA deposition observed in seed of mutants tds4 and tt7-3 result from altered PA composition and distribution in the cell. Our mutant screen, which was not exhaustive, suggests that the cooperation of many genes is required for successful PA accumulation.

An electron microscope study of resin production and secretion by the glands of seedlings of Betula pendula Roth

Transmission and scanning electron microscopy were used to study the ultrastructure and secretory processes of resin glands on shoot stems of Betula pendula seedlings during seasonal growth. The multicellular peltate glands possess a cortex of columnar cells surrounding a parenchymal medulla differing from the stem parenchyma below. Myelin-like deposits comprising concentric layers of membranes and osmiophilic substances accumulate mainly in the cortical cells, while only the medullar cells have chloroplasts. Both of these deposits appear to be synthesized, initially, in the endoplasmic reticulum. The myelin-like material is believed to consist of steroidal triterpenoids in cytoplasmic membranes, and the osmiophilic deposits to represent phenolics. Studies of glands of different ages suggest that the electron-transparent resin ultimately exported is formed by combining the two initial products. In the cortical cells the secretion first accumulates in vesicles that fuse with larger ones and the periplasmatic space. From the latter the secretion diffuses through the cell wall and the resin is finally deposited in the subcuticular space. Secretion vesicles, but no periplasmatic deposits were seen in the medullar cells. At the end of seasonal growth the cortical cells are markedly vacuolate and appear to have lost their organelles. Some of the cells accumulate an electron-opaque material not seen earlier. The medullar cells of the glands are suberized before winter dormancy, but usually later than in the surrounding stem bark.

Demonstration of phenolic compounds in plant tissues by an osmium-iodide postfixation procedure

A simple procedure to stain phenols in plant tissues is described. Postfixation with an aqueous solution prepared by mixing 2 cc of 2% osmium tetroxide and 8 cc of 3% potassium iodide yields brilliant visualization of phenol-containing vacuoles in different tissues of plants (e.g., coffee, oak, tobacco and spruce) bearing high concentration of phenolic compounds. Areas bearing phenols become dark gray to black. Chemical experiments demonstrate that osmium-potassium iodide (Os-KI) mixture reacts rapidly with several naturally occurring plant phenols, developing black solutions from which black solids precipitate. Phenols containing omicron-dihydroxy groups react with Os-KI solution more rapidly than other structurally different phenols. Therefore, omicron-dihydroxy units in an aromatic ring seem to function as primary sites of reactivity with the osmium-iodide complexes.

A technique for the electron microscopic autoradiography of Al adenosine receptors in brain tissue. Ligand-receptor fixation by osmium tetroxide

A procedure for the electron microscopic autoradiography of Al adenosine receptors is described. Fresh tissue slices from rat hippocampus were incubated with the radioactive adenosine analogs: Cyclohexyl[3H]adenosine, 5'-N-ethylcarboxamido[3H]adenosine or or [125I]-iodohydroxyphenylisopropyladenosine. Various fixation agents were tested with respect to the retention of these ligands by the tissue. While most of the ligands were lost in aldehyde fixation they were retained by osmium tetroxide probably via a crosslinking reaction. The final method of choice was an aldehyde prefixation (in the case of [125I]-iodohydroxyphenylisopropyladenosine with 4% buffered paraformaldehyde) during which more than 90% of the nonspecifically bound ligands were washed out while 40% of the specifically bound ligands remained. Subsequent fixation with osmium tetroxide (1%) allowed a standard protocoll for dehydration and embedding to be used with only minimal (less than 5%) further loss of the ligands. Electron microscopic autoradiography provided evidence for a specific distribution of the binding sites for [125I]-iodohydroxyphenylisopropyladenosine.

Solute production and net wall synthesis in the growing and non-growing cells of gravistimulated sunflower hypocotyls

Solute generation and cell wall synthesis were examined in sunflower hypocotyl peripheral layers, the growth rate of which had been altered by gravistimulation. Measurements of both the concentrations of the major solutes and the osmotic potential showed that although upper cells stopped growing, the solute levels in these cells continued to increase at rates comparable to those in lower cells. This indicated that altered growth rates, generated during gravicurvature, are not based on solute generation but must result from cell wall changes. Gravimetric and precursor incorporation studies showed that net wall synthesis continued in upper cells despite their lack of growth. An ultrastructural study of the epidermal cells on the uppermost (non-elongating) and lowermost (elongating) surfaces of horizontal cucumber hypocotyls showed that the relative amounts of the various membrane fractions were similar in upper and lower cells despite their very different growth rates.

Ice crystal damage in frozen thin sections: freezing effects and their restoration

Thin sections of unfixed kidney, fast frozen without cryoprotectants, were fixed in osmium tetroxide vapour directly after freeze drying or after 30 min in a moist atmosphere. Dry sections fixed in vapour showed ice crystal damage characteristic for the freezing procedure. This was demonstrated with freeze fracture replicas from the same preparation. Ice crystal holes were obscured in serial sections which were freeze dried and allowed to rehydrate in a moist atmosphere. The same ultrastructural appearance was observed in frozen sections brought to room temperature immediately after cutting. Frozen thin sections from unfixed tissue, if freeze dried, are very sensitive to atmospheric conditions and need some form of stabilization (e.g. osmium vapour fixation, sealing with an evaporated carbon film) before electron microscope images can be interpreted as representative for the frozen state. Restoration of ice crystal damage can occur by melting frozen sections or by rehydration of freeze dried frozen sections. Restoration phenomena will impair studies aimed at the localization of diffusible substances by autoradiography or X-ray microanalysis.

Oxidation of ruthenium red for use as an intercellular tracer

When ruthenium red (RR) is combined with OsO4, an electron-opaque complex forms which readily binds to the cell surface coat. However, the RR-OsO4 complex is often excluded from intercellular spaces in many cell types, and thus is not dependable as a tracer of regions continuous with the extracellular space. Postfixation of erythrocytes agglutinated by the lectin helix (Helix promatia) and intact carotid artery endothelium with a freshly prepared mixture of 1% OsO4 containing 0.1% ruthenium red (RR) resulted in a dense surface deposit of these cells, but intercellular regions were penetrated to a minimal degree by the stain. When a similar mixture of RR-OsO4 was allowed to stand 3 h before use, RR is oxidized by OsO4 to yield a ruthenium compound that has a spectrophotometric absorbance maximum at 365 nm. This RR molecule has a reduced number of cationic sites due to binding with osmium dioxide OsO2=. Postfixation of agglutinated RBCs and carotid artery endothelium with this oxidized ruthenium-OsO4 mixture resulted in a 50-80% decrease in surface deposition but markedly enhanced penetration into intercellular regions. The enhanced penetration is attributed to decreased binding affinity of the oxidized ruthenium for anionic surface membrane components, permitting effective stain penetration of cell-to-cell contact rather than extensive surface deposition. These studies indicate that the ruthenium compound formed by OsO4 oxidation of ruthenium red may be a useful tracer for ultrastructural visualization of intercellular spaces and junctions.