A Review on Colloidal Gold Post-Embedding Cytochemical Techniques

In 1971, colloidal gold was introduced in immunocytochemistry by Faulk and Taylor for the ultrastructural demonstration of antigenic sites at the surface of bacteria. Since then, application of this marker has been growing rapidly, extending its use to the various fields of cytochemistry. Indeed, it has been applied for pre- and post-embedding labeling of cell surface and intracellular binding sites respectively. It has been used in light microscopy as well as in both transmission and scanning electron microscopy, and thus appears today as the marker of choice in cytochemistry.

The present short review means to illustrate four different affinity techniques, which have been developed recently for the ultrastructural postembedding localization of various macromolecules using colloidal gold as the electron dense marker.

The origins and evolution of freeze-etch electron microscopy

The introduction of the Balzers freeze-fracture machine by Moor in 1961 had a much greater impact on the advancement of electron microscopy than he could have imagined. Devised originally to circumvent the dangers of classical thin-section techniques, as well as to provide unique en face views of cell membranes, freeze-fracturing proved to be crucial for developing modern concepts of how biological membranes are organized and proved that membranes are bilayers of lipids within which proteins float and self-assemble. Later, when freeze-fracturing was combined with methods for freezing cells that avoided the fixation and cryoprotection steps that Moor still had to use to prepare the samples for his original invention, it became a means for capturing membrane dynamics on the millisecond time-scale, thus allowing a deeper understanding of the functions of biological membranes in living cells as well as their static ultrastructure. Finally, the realization that unfixed, non-cryoprotected samples could be deeply vacuum-etched or even freeze-dried after freeze-fracturing opened up a whole new way to image all the other molecular components of cells besides their membranes and also provided a powerful means to image the interactions of all the cytoplasmic components with the various membranes of the cell. The purpose of this review is to outline the history of these technical developments, to describe how they are being used in electron microscopy today and to suggest how they can be improved in order to further their utility for biological electron microscopy in the future.

Correlative fluorescence and electron microscopy in tissues: immunocytochemistry

Correlative microscopy is a collection of procedures that rely upon two or more imaging modalities to examine the same specimen. The imaging modalities employed should each provide unique information and the combined correlative data should be more information rich than that obtained by any of the imaging methods alone. Currently the most common form of correlative microscopy combines fluorescence and electron microscopy. While much of the correlative microscopy in the literature is derived from studies of model cell culture systems we have focused, primarily, on correlative microscopy in tissue samples. The use of tissue, particularly human tissue, may add constraints not encountered in cell culture systems. Ultrathin cryosections, typically used for immunoelectron microscopy, have served as the substrate for correlative fluorescence and electron microscopic immunolocalization in our studies. In this work, we have employed the bifunctional reporter FluoroNanogold. This labeling reagent contains both a fluorochrome and a gold-cluster compound and can be imaged by sequential fluorescence and electron microscopy. This approach permits the examination of exactly the same sub-cellular structures in both fluorescence and electron microscopy with a high level of spatial resolution.

Immunogold Staining of Ultrathin Thawed Cryosections for Transmission Electron Microscopy (TEM)

INTRODUCTIONA pre-embedding method of immunochemical staining is used if antigens are damaged by resin embedding, or if the best preservation of membranes is required. Applying immunogold reagents to sections of lightly fixed tissue, free of embedding medium, can be a very sensitive method of immunochemical staining. Cells or tissues are fixed as strongly as possible and then treated with a cryoprotectant, which is usually a mixture of sucrose and polyvinylpyrrolidone (PVP). They are frozen onto pins in liquid nitrogen and sectioned at approximately -100°C. The frozen sections are thaw-mounted on to Formvar/nickel film grids and the cryoprotectant is removed by floating the grids on drops of phosphate-buffered saline (PBS). The immunogold staining is performed on the unembedded sections, which are subsequently contrast counterstained and infiltrated with a mixture of methylcellulose and uranyl acetate. In this protocol, samples are sectioned at low temperature, thaw-mounted onto film grids, immunochemically stained, contrast counterstained, and embedded/encapsulated in situ on the grid before viewing by transmission electron microscopy (TEM).

An advanced technique for immuno-labelling of microcystins in cryosectioned cells of Microcystis aeruginosa PCC 7806 (cyanobacteria): Implementations of an experiment with varying light scenarios and culture densities

The intracellular localisation of cyanobacterial toxins might well indicate production sites and possible shifts to destination points, thus giving information on possible functions of these toxins within algal cells or at the ecological level beyond. By preparing cells of Microcystis aeruginosa PCC 7806 by cryofixation/cryosectioning and using purified high quality antibodies for immunogold-localisation, excellent ultrastructural integrity and labelling of microcystins was shown. Compared to conventional techniques, including organic solvents, possible dislocation/extraction was significantly minimised, hence, the labelling density was enhanced and the labelling pattern changed. The microcystins were mainly localised within the inner nucleoplasmic area and accumulations of epitopes could be detected around/within intracellular inclusions, such as polyphosphate bodies and carboxysomes. Photosynthetic active radiation (PAR) had a significant effect on microcystin biosynthesis, and the medium light intensity of 25 microE m(-2) s(-1) induced the highest intracellular microcystin contents (up to 160 epitopes per cell and 26 epitopes per microm2). The restriction of the full light spectrum to blue (400-500 nm) or red (>610 nm) wavelengths did not result in any significant effect on microcystin production. However, the subcultures harvested at higher optical densities (>0.5) revealed significantly higher microcystin labelling compared to the less dense cell cultures (OD < 0.5). Altogether, the possibility was discussed whether microcystin might function as an inhibitor of RUBISCO under conditions of C-limitations. The effects of light intensity and cell suspension density on intracellular microcystin shown by immuno-detection matched the pattern of microcystin concentrations determined in parallel by HPLC and ELISA.

Ultrathin cryosections: an important tool for immunofluorescence and correlative microscopy

Here we show that ultrathin cryosections of placental tissue can be used as a substrate in immunofluorescence experiments. A high degree of spatial resolution can be achieved in these preparations because there is essentially no out-of-focus fluorescence. Therefore, immunofluorescence microscopy using ultrathin cryosections provides a very useful method for determining the precise subcellular localization of antigens in tissues. In addition, ultrathin cryosections of placenta also serve as a substrate for correlative immunofluorescence and immunoelectron microscopy using FluoroNanogold as the detection system. In correlative microscopy, the exact same structures in the same ultrathin section were observed by both fluorescence and electron microscopy. Using a particle counting procedure and electron microscopy, we compared the labeling obtained with colloidal gold and FluoroNanogold and found a higher number of particles with silver-enhanced FluoroNanogold than with colloidal gold.

Thrombin receptors on human platelets. Initial localization and subsequent redistribution during platelet activation

Platelet responses to thrombin are at least partly mediated by a G-protein-coupled receptor whose NH2 terminus is a substrate for thrombin. In the present studies we have examined the location of thrombin receptors in resting platelets and followed their redistribution during platelet activation. The results reveal several new aspects of thrombin receptor biology. 1) On resting platelets, approximately two-thirds of the receptors were located in the plasma membrane. The remainder were present in the membranes of the surface connecting system. 2) When platelets were activated by ADP or a thromboxane analog, thrombin receptors that were initially in the surface connecting system were exposed on the platelet surface, increasing the number of detectable receptors by 40% and presumably making them available for subsequent activation by thrombin. 3) Platelet activation by thrombin rapidly abolished the binding of the antibodies whose epitopes are sensitive to receptor cleavage and left the platelets in a state refractory to both thrombin and the agonist peptide, SFLLRN. This was accompanied by a 60% decrease in the binding of receptor antibodies directed COOH-terminal to the cleavage site irrespective of whether the receptors were activated proteolytically by thrombin or nonproteolytically by SFLLRN. 4) The loss of antibody binding sites caused by thrombin was due in part to receptor internalization and in part to the shedding of thrombin receptors into membrane microparticles, especially under conditions in which aggregation was allowed to occur. However, at least 40% of the cleaved receptors remained on the platelet surface. 5) Lacking the ability to synthesize new receptors and lacking an intracellular reserve of preformed receptors comparable to that found in endothelial cells, platelets were unable to repopulate their surface with intact receptors following exposure to thrombin. This difference underlies the ability of endothelial cells to recover responsiveness to thrombin rapidly while platelets do not, despite the presence on both of the same receptor for thrombin.

Identification of endoplasmic reticulum in the primitive eukaryote Giardia lamblia using cryoelectron microscopy and antibody to Bip

Giardia lamblia trophozoites contain a complex endomembrane system as demonstrated by fluorescence and cryoelectron microscopy. The endomembrane system was weakly detected in live cells using the fluorescent membrane dye 3,3′-dihexyloxacarbocyanine iodide. The definitive identification of endoplasmic reticulum required the development of a molecular label. We expressed Giardial Bip in Escherichia coli and raised a polyclonal antibody to the purified protein. In western blots, the antibody was specific for Giardial Bip and did not react with human, monkey and rodent homologs. By immunofluorescence microscopy in methanol fixed cells the antibody visualized tubular structures and other subcellular components that required characterization by electron microscopy. Using cryotechniques we directly demonstrate the presence of a complex endomembrane system at the ultrastructural level. In conjunction with Bip immunogold labeling of cryosections we identify: (1) endoplasmic reticulum cisternae and tubules; (2) stacked perinuclear membranes; and (3) Bip presence in the nuclear envelope. Both the endoplasmic reticulum and nuclear envelope were found either with or without a cleft region suggesting each may contain common specialized sub-regions. In stacked perinuclear membranes, which may represent either multilamellar endoplasmic reticulum or a Golgi apparatus, Bip labeling was restricted to peripheral layers, also suggesting specialized sub-regions. Labeled endomembrane systems could be observed associated with microtubule structures, including axonemes and the adhesive disk. The presence of an extensive endomembrane system in Giardia lamblia, which represents one of the earliest diverging eukaryotic species, supports the view that both the nucleus and endomembrane system co-evolved in a common ancestor of eukaryotic cells.

Platelets contain interleukin-1 alpha and beta which are detectable on the cell surface after activation

Activated platelets have been shown previously to exhibit membrane-bound IL-1 bioactivity, which leads to the question of localization of the cytokine in platelets. Using immunocytological and flow cytometric techniques, we found IL-1 alpha and IL-1 beta in the cytoplasma of both resting and thrombin-activated platelets. Immunogold-silver staining of the cell surface of activated platelets as well as preembedding antibody treatment of platelets revealed the presence of IL-1 (alpha and beta) in low density on the surface of intact cells in contrast to distinct enrichment in the cytoplasma of damaged platelets. Fibrin fibres present between cells indicated adsorbance of IL-1. There was also weak binding of anti-IL-1 alpha to the surface of thrombin-activated platelets as shown by flow cytometry. Following activation there appears to be some transfer of IL-1 onto the cell surface of activated cells, the bulk of the cytokine, however, is probably not released prior to platelet disintegration. In summary, we present evidence for the presence of both IL-1 alpha and IL-1 beta in resting and activated platelets without being able to demonstrate localization of the cytokines to specific subcellular structures.

Application of immunocryoultramicrotomy to free cells: ultrastructural localization of myeloperoxidase in neutrophils of human peripheral blood

Immunocryoultramicrotomy is generally considered the most suitable method for observing the immunostainings by monoclonal antibodies to intracellular antigens by electron microscopy. However, this procedure with the free cells such as peripheral blood cells or cultured cells is not widely used because of the difficulty in preparing ultrathin frozen sections. We report here a new technique for making ultrathin frozen sections of peripheral blood cells by positioning the cell in a semigelatinized medium for ultrasectioning. Monoclonal antibody against myeloperoxidase (MPO) was reacted with the ultrathin sections made with this new technique to observe the ultrastructural localization of intracellular MPO. MPO was observed in the primary granules of the neutrophils.

Use of ultrastructural immunohistochemistry in human pituitary pathology

Recent advances in ultrastructural immunohistochemistry have provided insight into not only the subcellular localization of single antigens but also the colocalization of two distinct antigens in the same cellular constituent. In the field of pituitary pathology, precise identification of cell types, mechanism of processing, and dynamic intracellular transportation of hormones, as well as production of multiple hormones in the same cells of nontumorous and neoplastic adenohypophyses, have been documented by use of these techniques. The present review deals with the use of major methods for ultrastructural immunohistochemistry including pre-, post-, and non-embedding methods, particularly focusing on their application to human pituitary pathology. Problems of tissue processing and a protocol for double labeling technique using the protein A-gold complex are also described.

Contractile proteins in podocytes: immunocytochemical localization of actin and alpha-actinin in normal and nephrotic rat kidneys

Actin and alpha-actinin immunoreactive sites have been localized at the electron microscope level by the protein A-gold immunocytochemical technique in podocytes of normal and nephrotic rat renal tissues. In normal renal glomeruli, fibrillar networks located in the core of foot processes or bundles of microfilaments interconnecting them were found to be labelled for these two cytoskeletal proteins. On the other hand, in nephrotic renal glomeruli, concomitant with the loss of podocytic foot processes a reorganization of the podocytic cytoskeleton and a concentration of some of its elements into thick uniform bands was observed. Actin and alpha-actinin were revealed in these bands. Control experiments confirmed the specificity of the labelling obtained. Our results suggest that normal podocytes contain an actin-based contractile system that might contribute to the maintenance of the particular cell shape of these cells and that the rearrangement of the podocytic cytoskeleton occurring in the nephrotic syndrome might account for the changes in the foot processes and contribute to the alteration in glomerular function.

The neutrophil selectin LECAM-1 presents carbohydrate ligands to the vascular selectins ELAM-1 and GMP-140

LECAM-1 (leukocyte-endothelial cell adhesion molecule 1), the lymphocyte lectin ("selectin") homing receptor for peripheral lymph nodes (PLNs), participates in the earliest interactions of polymorphonuclear neutrophilic leukocytes (PMNs) with inflamed venules. Here, we present evidence that LECAM-1 mediates this function through a novel mechanism--presentation of oligosaccharide ligands to the inducible vascular selectins endothelial-leukocyte adhesion molecule (ELAM-1) and granule membrane protein 140 (GMP-140). PMN, but not lymphocyte, LECAM-1 is modified with the vascular selectin ligand sialyl Lewis x (sLex) and specifically binds ELAM-1-transfected cells. Although only a small fraction of total cell surface sLex, LECAM-1-associated sLex appears to play a prominent role in PMN interactions with cell-associated ELAM-1 and GMP-140, as anti-LECAM-1 monoclonal antibodies or selective removal of cell surface LECAM-1 inhibits PMN binding to vascular selectin transfectants by up to 70%. The enhanced function of LECAM-1-associated sLex may reflect the striking concentration, shown here, of LECAM-1 on PMN surface microvilli, the site of initial cellular contact.

Evaluation of a gold-silver staining method for detection and identification of Candida species by light microscopy

A gold-silver staining procedure was evaluated for detection of Candida species of medical importance. Probes were prepared by coupling lectins or antibodies (polyclonal and monoclonal) directly or indirectly to colloidal gold particles. Structures reacting to these probes were specifically revealed by light microscopy in cells present in infected kidney tissue sections or in isolated yeast cells on glass slides. Definition, contrast and sensitivity were of a high order. Preliminary data showed that it was possible, using discriminating dilutions, to identify cells from different species of the genus Candida, grown in vitro, according to their ability to stain with polyclonal monospecific antisera. The advantages of gold-silver staining compared with other staining procedures currently used in routine mycological laboratories are its sensitivity, good definition, ease and rapidity, and long conservation of reaction. It is suggested that the procedure has applications for research and identification of yeasts in clinical samples.

The subcellular distribution of transferrin in rat choroid plexus studied with immunogold labelling of ultracryosections

Choroid plexus epithelium from third ventricle choroid plexus of 2-3-week-old rats was examined for transferrin-like immunoreactivity. In 5 microns paraffin sections most epithelial cells exhibited a pronounced immunoperoxidase staining for transferrin. The ultrastructure of the epithelium in question was examined by conventional electron microscopy. Immunolabelling of ultracryosections with IgG-gold, protein-A gold or protein-A gold-antiprotein-A protein-A gold showed an intense labelling of the basal extracellular space. The lateral intercellular space and the luminal surface showed a more variable labelling; no labelling of the tight junction zone was seen. Intracellularly a distinct labelling of the 'uptake and disposal pathway' (the endosomal-lysosomal system) was observed, but also the synthetic machinery (rough endoplasmic reticulum, stacked Golgi membranes) showed a characteristic labelling. Thus it seems likely that both uptake and synthesis of transferrin occur in choroid plexus epithelial cells.

Preparation and use of recombinant protein G-gold complexes as markers in double labelling immunocytochemistry

Recombinant protein G (RPG) was conjugated to colloidal gold particles and used for immunocytochemistry. In this report, the preparation of RPG-gold conjugates (RPGG) and the application of these conjugates in spot blot tests and in double immunolabelling are described. The immunolabelling was performed on ultracryosections of pig small intestine using antibodies directed against aminopeptidase N and sucrase-isomaltase. The labelling efficiency of RPGG was compared to that of protein A-gold conjugates (PAG) in different compartments of the enterocyte. Quantification showed that the labelling intensity was dependent on the size of the marker as well as on the kind of protein used for complex formation. The distributions for RPGG and PAG were respectively: for the 12 nm particles, 10.3 and 6.2 particles/micron of length of microvillar membrane, 3.5 and 1.0 particles/micron2 of Golgi profile and 5.9 and 2.0 particles/micron2 of multivesicular body profile; and for the 6 nm particles, 49.6 and 15.7 particles/micron of length of microvillar membrane, 24.4 and 5.0 particles/micron2 of Golgi profile and 25.4 and 3.4 particles/micron2 of multivesicular body profile. Controls showed very little non-specific gold labelling (less than 0.02 gold particles/micron2 of section).

Neuronal imaging with colloidal gold

Colloidal gold is easily prepared, and readily adsorbs to a number of immunoreagents and other proteins for a wide variety of uses for neuronal visualization. Gold probes serve a role as immunolabels for both light and electron microscopy. As an ultrastructural immunocytochemical marker for detection of proteins, peptides or amino acids, gold can be used for immunostaining thick or thin sections prior to embedding, or for immunostaining ultrathin sections after embedding tissue in conventional or unusual embedding matrices. By virtue of its particulate nature, gold as an immunolabel facilitates a semi-quantitative analysis of relative antigen densities on ultrathin sections. Various combinations of different size gold particles or dual immunolabelling with enzymatic immunolabels together with colloidal gold or silver-intensified gold serve well for ultrastructural immunocytochemical localization of two antigens in the same tissue section. Colloidal gold can be detected with light microscopy, transmission and scanning electron microscopy, and with confocal laser microscopy. Silver intensification allows detection of gold at both the light and electron microscope level, and increases the sensitivity of immunogold procedures. Colloidal gold is useful as a tracer for physiological studies of transport and internalization in neurons in vivo and in vitro; computer-assisted video imaging techniques allow detection and tracking of single gold particles in living cells.

Relations between resistance to chloroquine and acidification of endocytic vesicle of Plasmodium berghei

In order to visualize low-pH compartments of Plasmodium berghei strains we have used a basic congener of dinitrophenol, 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine (DAMP) which concentrates in acidic compartments, and can be detected by immunocytochemistry with anti-dinitrophenol antibodies. We have demonstrated that in a P. berghei chloroquine-sensitive strain (N strain), DAMP accumulates in the endocytic vacuoles where haemoglobin degradation is occurring. These compartments which have recently been shown to concentrate 4-aminoquinoline drugs (Moreau, Prensier, Maalla & Fortier, 1986) have an acidic pH. Conversely DAMP was found scattered all over the cytoplasm in a P. berghei chloroquine-resistant strain; the same phenomenon was previously observed (Moreau et al. 1986) in the localization of a 4-aminoquinoline on this same strain. Monensin-induced swelling of acidic compartments (Boss & Morre, 1984) was used as a complementary method for the determination of low-pH compartments on P. berghei strains. All the data reported here suggest that chloroquine resistance in P. berghei RC may be related to an impairment in the acidification of endocytic vesicles.

Cryostat sections for coexistence studies and preembedding electron microscopic immunocytochemistry of central and peripheral nervous system tissue

Perfusion-fixed tissue blocks were incubated in high molar sucrose solutions, shock frozen in melting isopentane, and sectioned on a conventional cryostat. Semithin sections (2-4 microns) alternatingly stained for parvalbumin and glutamate decarboxylase enabled us to demonstrate the coexistence of both antigens in the same cell. Thick sections (40 microns) of central and peripheral nervous system tissue were immunostained and processed for correlated light and electron microscopic studies. At the electron microscopic level, the preservation of ultrastructural features such as membranes and synaptic contacts was comparable to that normally seen in vibratome sectioned material. Hence, this technique can successfully be used for preembedding coexistence studies and electron microscopic preembedding immunocytochemistry when vibratome sectioning is problematic.

Purification and characterization of human lysosomal membrane glycoproteins

Two human cell lysosomal membrane glycoproteins of approximately 120 kDa, hLAMP-1 and hLAMP-2, were identified by use of monoclonal antibodies prepared against U937 myelomonocytic leukemia cells or blood mononuclear cells. The two glycoproteins were purified by antibody affinity chromatography and each was found to be a major constituent of human spleen cells, representing approximately 0.05% of the total detergent-extractable protein. Both molecules were highly glycosylated, being synthesized as polypeptides of 40 to 45 kDa and cotranslationally modified by the addition of Asn-linked oligosaccharides. NH2-terminal sequence analysis indicated that each was approximately 50% identical to the corresponding mLAMP-1 or mLAMP-2 of mouse cells. Electron microscopic studies of human blood monocytes, HL-60, and U937 cells demonstrated that the principal location of these glycoproteins was intracellular, in vacuoles and lysosomal structures but not in the peroxidase-positive granules of monocytes. Transport of the proteins between organelles was evidenced by their marked accumulation in the membranes of phagolysosomes. A fraction of each glycoprotein was also detected on the plasma membrane of U937 and HL-60 cells but not on a variety of other tissue culture cells. This cell-surface expression may be differentiation related, since the proteins were not detected in the plasma membrane of normal blood monocytes and their expression on U937 and HL-60 cells was reduced when the cells were treated with differentiating agents. Cell-surface expression of both glycoproteins was markedly increased in blood monocytes but not in U937 cells after exposure to the lysosomotropic reagent methylamine HCl, indicating differences in LAMP-associated membrane flow in these cell types.

Monensin turns on microtubule-associated translocation of secretory granules in cultured rat atrial myocytes

We have studied the effect of monensin on microtubule-associated translocation of atrial secretory granules in 5-7-day-old cultures of atrial myocytes from adult rats. Atrial granules and microtubules were localized by immunofluorescent microscopy of myocytes double-labeled with primary antibodies against atrial natriuretic polypeptide (ANP) and alpha-tubulin. In control myocytes, fluorescence due to atrial granules was predominantly localized to the perinuclear region containing the Golgi complex. After exposure for 30 minutes to monensin (0.5-5.0 microM), myocytes transiently contained conspicuous linear arrays of atrial granules associated with cytoplasmic microtubules. Thereafter, ANP fluorescence accumulated in subsarcolemmal foci at the cell periphery, while perinuclear ANP fluorescence faded. The monensin-induced redistribution of atrial granules was observable in both serum-containing and serum-free media and was unaffected by reducing external Ca2+ to low values, by inhibition of sarcoplasmic reticulum Ca2+ release with ryanodine, or by both. The redistribution was prevented by pretreatment with nocodazole, which fragmented microtubules and scattered Golgi complexes and the associated atrial granules throughout the cytoplasm. Radioimmunoassay showed that monensin seemingly decreased the rate of ANP secretion into the medium from 0.15 to 0.11 fmol/(hr.myocyte). These results suggest that monensin turns on microtubule-associated translocation of atrial granules from the perinuclear areas to the cell periphery by modifying the interaction between microtubules and atrial granules. Monensin also promotes movement of atrial granules along the microtubules but does not accelerate the release of ANP.

Ultrathin cryosections in the plane of cell monolayers: evaluation of their potential for antibody localization studies of the cytoskeleton

In spite of the fact that the preembedding method is satisfactory for the ultrastructural localization of cytoskeletal proteins, there is a need for a localization method that retains the cells' ground substance, delicate filament arrangements, and membrane-filament interactions and provides a good delineation of ultrastructural detail. Ultracryomicrotomy, a resinless sectioning method, can combine good morphology with optimal antibody labeling. Until now, however, it has not been possible to section cell monolayers parallel to their plane of growth. This is a prerequisite for the localization of proteins along segments of filaments, contained within the section thickness. We describe such a method and give a first appreciation of its potential for antibody localization studies of cytoskeletal proteins. The method consists of seeding cells on a parallel 0.75-mm-thick gelatin substrate that can later be cut and used as a mounting block. An adapted negative staining has yielded a very useful delineation of the well-preserved structures within the cells, even in combination with immunogold labeling. The latter has been in its indirect version less satisfactory in dense microfilament bundles because of penetration problems, and more satisfactory on microtubules. Clearly, the penetration properties of gold probes will have to be improved before this method will become widely applicable. The availability of a sectioning method like this will provide the basis for further progress. There will be many cases which will justify the use of this relatively more difficult approach.

[Cytologic analysis of the expression of an epitope carried by glycoproteins excreted by Candida albicans]

The present study concerns an epitope identified by a monoclonal IgM, named 5B2, generated against the parasitic phase of Candida albicans. The epitope was previously shown to be carried by excreted C. albicans glycoproteins and to be present in the sera of patients suffering from systemic candidiasis. The cytological analysis of the epitope expression was investigated in 3 different yeast strains: the C. albicans strain from which 5B2 was generated (VW.32); a C. albicans mutant, deficient in cell wall mannans (KD.102); and a Saccharomyces cerevisiae strain. Immunofluorescence assays using IgM-5B2 showed discontinuous labelling with the VW.32 strain and no labelling with the 2 other yeast strains; however, the superficial structures of the 3 strains reacted homogeneously with ConA. Ultrastructural immunodetection experiments performed with the VW.32 cells, using gold-conjugated monoclonal antibody, revealed the presence of the epitope in the vacuolo-vesicular system, the periphery of the cytoplasm, the periplasmic space and the cell wall. Under the same conditions, cells from the KD.102 strain only exhibited weak cytoplasmic labelling whereas the presence of the epitope in S. cerevisiae blastoconidia was restricted to the vesicles. Competition and double labelling experiments with IgM and ConA showed that the epitope, distributed on the great majority of VW.32 glycoproteins, is shared by a lesser proportion of the KD.102 glycoproteins and only by some vesicular glycoproteins of S. cerevisiae. Inhibition of the N-glycosylation process of the VW.32 strain by tunicamycin resulted in the absence of cytokinesis and germ tube formation. In such cells, epitope 5B2 was no longer expressed on the bud surface. These cytological results concerning the C. albicans epitope are discussed in relation to recent, more general biochemical data on the yeast glycosylation process.

Myelin-specific domain on the plasmalemma of oligodendroglia: differential expression in the rat and hypomyelinating mouse mutants jimpy and quaking

Monoclonal antibody, 1A9, prepared against bovine white matter, recognizes a proteinaceous, myelin-specific domain in the CNS that is restricted to the surface of oligodendroglia in primary dissociated cell cultures. The antigen is not detected in the PNS or non-neural tissues. Antibody binding is abolished by heating, exposure to SDS and delipidation, indicating that a conformationally sensitive epitope is recognized. The antigen is present in tracts of developing white matter in rat cerebellum beginning at 5 days postnatally. In developing cultures of fetal rat brain the period of rapid onset for the phenotypic expression of 1A9 antigen is similar to that of galactocerebroside, corresponding to 2-4 postnatal days of age. The 1A9 antigen is not observed in white matter or cultured oligodendroglia of the hypomyelinating jimpy mutant mouse, but its expression is qualitatively normal in the quaking mutant. The possibility is raised that 1A9 may be the primary target of the jimpy mutation.

Application of cryoultramicrotomy to immunocytochemistry

This paper reviews the most recent status of immuno-cryoultramicrotomy. The technical aspects of each step of the method are also analysed in detail with the intention of providing a useful source of information for investigators using this method.

Gamma-aminobutyrate, gastrin releasing peptide, serotonin, somatostatin, and vasopressin: ultrastructural immunocytochemical localization in presynaptic axons in the suprachiasmatic nucleus

An ultrastructural immunocytochemical study was undertaken to identify neuroactive substances contained in presynaptic boutons in the hypothalamic suprachiasmatic nucleus. Axonal boutons containing immunoreactive gamma-aminobutyrate, glutamate decarboxylase, neurophysin/vasopressin, gastrin releasing peptide/bombesin, somatostatin and serotonin were localized within the hypothalamic suprachiasmatic nucleus with pre-embedding peroxidase immunostaining. Synaptic contacts were found between boutons containing each of these substances and postsynaptic structures. While some variation in synaptic morphology existed, most of the immunoreactive contacts were of the symmetrical type. Previous work has indicated that neuroactive peptides may be found in highest concentrations in dense-core vesicles, to examine the subcellular localization of the amino acid inhibitory transmitter gamma-aminobutyrate, ultrastructural immunocytochemistry with pre-embedding peroxidase was compared with post-embedding immunocytochemistry with colloidal gold. Ultracryothin sections were also used for ultrastructural localization of gamma-aminobutyrate and glutamate decarboxylase immunoreactivity. Both gamma-aminobutyrate and glutamate decarboxylase immunoreactivity were found throughout the cytoplasm of immunoreactive boutons when pre-embedding peroxidase was used; with post-embedding colloidal gold immunostaining, label was found over areas containing small clear vesicles, and over mitochondria of immunoreactive axons. At the dilutions used in this study, strongly immunoreactive gamma-aminobutyrate dendrites, boutons forming asymmetrical synapses, and cell bodies were not found. Differences between pre-embedding and post-embedding immunostaining may be due to antigen and label diffusion caused by mild fixation and membrane damage necessary for antisera penetration during pre-embedding immunostaining. These results suggest that gamma-aminobutyrate, gastrin releasing peptide, somatostatin and vasopressin are contained in axons making contact with neurons of the suprachiasmatic nucleus, and may function as neurotransmitters here. Since all of these substances can also be localized in perikarya within the suprachiasmatic nucleus, there is a strong possibility that at least some of the axons containing immunoreactivity for each of these substances may be involved in local circuit interactions between neurons within the suprachiasmatic nucleus.

Controlled growth of colloidal gold particles and implications for labelling efficiency

A new method is reported for the preparation of colloidal gold particles with diameters ranging between 5 and 12 nm. The initial gold particle population, with an average diameter of 5.6 +/- 0.9 nm, is prepared by reduction of chloroauric acid with white phosphorous. An increase in particle diameter by growth is obtained by reduction of chloroauric acid with white phosphorous in the presence of colloidal gold particles. The labelling efficiency of these gold particles, conjugated with protein A, in indirect immunolabelling experiments is investigated by labelling of beta-galactosidase on ultrathin cryosections of Escherichia coli cells. We demonstrate that the labelling efficiency is at least dependent on particle diameter, probe concentration and preparation method. In addition it is shown, that with this new method, gold particle populations can be prepared with minor overlap in diameter spreading. Therefore these gold probes are suitable for qualitative double labelling experiments. The quantitative aspect of immunolabelling is discussed.

Comparative immunoelectron microscopy with monoclonal antibodies on yellow fever virus-infected cells: pre-embedding labelling versus immunocryoultramicrotomy

To study the intra- and extracellular distribution of yellow fever virus 17D (YFV)-specific antigens, pre-embedding immunoelectron microscopy (IEM) and IEM on ultrathin frozen sections were carried out comparatively using monoclonal antibodies (MAB) and YFV-infected cells. In addition, three electron-dense marker systems (IgG-ferritin and IgG-gold and protein A-gold) were compared for their efficiency in detecting bound MAB. Pre-embedding immuno-labelling was performed in microtest plates followed by in situ embedding and immunocryoultramicrotomy was performed using pellets of sucrose-infused cells. In both procedures, cells were prefixed with different concentrations of glutaraldehyde (GA). In pre-embedding IEM virus-specific antigens could be detected on the envelopes of extracellular virions with YFV-neutralizing MAB. Using immunocryoultramicrotomy, neutralizing MAB bound to intracellular mature virions as well as to viral antigens incorporated into cytoplasmic membranes. A concentration of 1% GA destroyed antigenicity entirely, while with 0.25% and 0.1% GA immunoreactivity was retained for more than 3 mth. Some highly reactive MAB labelled antigen significantly in pre-embedding IEM, when used at concentrations of 1 ng/ml. Immunocryoultramicrotomy was 10-100 times less sensitive. On cryosections colloidal gold was the marker of choice, due to the fact that it showed less nonspecific sticking to intracellular components and that it was easily detectable on highly contrasted cryosections. Owing to their higher sensitivity, IgG-ferritin conjugates were preferred in pre-embedding IEM.

Labelling of colloidal gold with protein A. A quantitative study

Colloidal gold complexes with protein A are extensively used in immunocytochemistry as secondary reagents for the localization of antigens. However detailed information on the process and extent of adsorption of protein A onto gold particles, the optimal condition of preparation and the stability of such complexes are lacking. The adsorption isotherm of 125I-protein A onto gold particles (11.2 nm in diameter) was studied quantitatively with gold sols buffered at pH 4-7. At low coverage of the particles, the isotherm was independent of pH. However in the presence of a large excess of protein A, the highest coverage was obtained with a gold sol buffered at pH 5.1, the isoelectric point of the protein. The association constant was decreased at high coverage of the particles. Maximum binding of the complex to immobilized IgG occurred with particles labelled with at least 9 molecules of protein A. The complex was stable under storage with up to 12 molecules adsorbed per particle. At high coverage (26 molecules per particle), a progressive loss of protein A was observed. The optimum condition for preparing the complex are reported.

Intercellular junctions and the cardiac intercalated disk

Cardiac muscle cells are equipped with three distinct types of intercellular junction--gap junctions, "spot" desmosomes, and "sheet" desmosomes (or fasciae adherentes)--located in a specialized portion of the plasma membrane, the intercalated disk. Gap junctions are responsible for electrical coupling and the transfer of small molecules between cells, whereas the desmosomelike junctions (also known as adherens junctions) provide strong intercellular adhesion. The adhesion sites formed by the "spot" desmosome anchor the intermediate-filament cytoskeleton of the cell; those formed by the fascia adherens anchor the contractile apparatus. An understanding of the ultrastructure of these junctions helps explain how they carry out their functions, and new observations in this field have been made through the application of ultrarapid freezing techniques in conjunction with freeze-fracture electron microscopy. With recent findings from biochemical and immunocytochemical studies, this understanding is now being extended to the molecular level.