Use of colloidal gold particles in double-labeling immunoelectron microscopy of ultrathin frozen tissue sections

Demonstration of an IgG-Fc receptor in ratcytomegalovirus infected cells

Infection of rat embryonic fibroblasts (REF) and R2 cells with rat cytomegalovirus (RCMV) resulted in the formation of cytoplasmic and membrane receptors for the Fc region of immunoglobulin G. The Fc receptors were demonstrated by the immunocytochemical techniques using indirect fluorescence and peroxidase technique at the light microscope level and using the protein A adsorbed to colloidal gold technique for demonstration at the electron microscope level. The production level of demonstration at the electron microscope level. The production level of the Fc receptor in the cell was depending on the input of virus. Experiments with ara C, cycloheximide and actinomycin D revealed that the formation of the Fc receptor was dependent on RNA, DNA and protein synthesis. Furthermore, results indicate that redistribution of the Fc receptor to form a cap, takes place within the first 60 minutes.

Identification of a nuclear protein component of interchromatin granules using a monoclonal antibody and immunogold electron microscopy

A monoclonal antibody, designated 780-3, has been generated which preferentially recognizes an antigenic component of interchromatin granules in human cells. By indirect immunofluorescence procedures, monoclonal antibody 780-3 produces a cell cycle-specific speckled nuclear staining pattern in adult human fibroblasts which is dramatically altered during metaphase. In contrast, transformed cells appear to express this antigen throughout the cell cycle in increased quantities. Immunogold electron microscopy revealed that the nuclear antigen is intimately associated with interchromatin granules in human cells. Analysis by immunoblot procedures showed that monoclonal antibody 780-3 recognizes two polypeptides of 105 and 41 kD. From these data, a possible nucleolar derivation of interchromatin granules is discussed. These studies demonstrate for the first time that monoclonal antibodies may be used in combination with immunogold electron microscopy to identify the ultrastructural location of nuclear antigens.

Immunocytochemical localization of the elongation factor Tu in E. coli cells

The localization of the elongation factor Tu (EF-Tu) in ultrathin cryosections of E. coli cells was determined with the electron microscope using a highly specific immunological labelling technique. EF-Tu is distributed almost homogeneously throughout the cytoplasm. Although it has often been suggested that EF-Tu could be part of a putative prokaryotic cytoskeleton, we did not find any evidence for supramolecular assemblies, such as fibres or filaments, containing a large amount of EF-Tu. EF-Tu was not observed in association with the outer cell membrane and periplasmic space. A topological relationship with the inner membrane is not apparent in our micrographs. In cells in which the EF-Tu level is raised significantly, the protein piles up in discrete cell regions.

Labelling of colloidal gold with protein. A quantitative study using beta-lactoglobulin

Markers prepared by labelling colloidal gold with macromolecules such as lectins, antibodies and protein A are gaining wide acceptance both in transmission and scanning electron microscopy. However detailed information on the process and extent of adsorption of macromolecules onto gold particles are still lacking. The adsorption isotherm of protein onto gold particles was studied quantitatively using goat beta-lactoglobulin (beta L) tritiated in vivo. When this protein was modified chemically by iodination with 125I, the adsorption isotherm was not significantly different (Langmuir type for monolayer). In the presence of saturating amount of beta L, a maximum of 13-14 molecules was adsorbed per particle of 12 nm in diameter for a theoretical maximum of 20 (compact monolayer). Ellipsometric measurements on nickel-coated slides indicated that betas L was adsorbed onto metallic surfaces as a compact monolayer. The molecules were irreversibly adsorbed on gold particles, kept to a large extent their capacity to bind anti-beta L antibodies and could not be displaced by polyethylene glycol, a stabilizer commonly used in the preparation of gold markers. Only markers labelled with more than 5 beta L molecules per particle could be completely bound by immobilized anti-beta L antibodies. Preliminary data indicated that the energetics of adsorption of beta L onto colloidal gold was in agreement with that expected from the mutual interaction of surface and adsorbate.

Immunoelectron microscopy of amylase in the human parotid gland. Ultrastructural localization by use of both the protein A-gold and the biotin-avidin-gold technique

Specimens of the human parotid gland were studied by immuno-electron microscopy for the presence of amylase. Both the protein A-gold technique and the biotin-avidin-gold technique were used on the same specimens. Different fixations were tried. Amylase was detected in the zymogen granules in high amounts. This enzyme could even be seen in glutaraldehyde fixed and routinely embedded material. The subcellular localization of this enzyme opens a new field of functional morphological studies and studies in special tumours including acinic cell carcinomas.

Ultrastructural immunogold labelling of human Langerhans cells enriched epidermal cell suspension

Colloidal gold particles are well suited as markers in electron microscopy. Indirect immunogold staining was used to identify cell membrane antigens defined by monoclonal antibodies OKT6 and BL6 on human Langerhans cells (LC) in suspensions. Isolated epidermal cells were obtained by skin trypsinization and enriched or depleted in OKT6 positive on BL6 positive LC using the panning method: incubation of OKT6 or BL6 preincubated cells on immunoglobulin coated dishes. Indirect immunogold staining was then performed after prefixation in 2% paraformaldehyde. In LC enriched suspensions, only LC exhibited a specific membrane labelling with OKT6 or BL6 recognized by the presence of small evently distributed gold granules. Neither Birbeck granules, nor other cytoplasmic organelles, were labelled. No other epidermal cells were found positive. In LC depleted suspensions, no labelling was observed. Immunogold labelling on LC enriched suspensions after panning is now in progress for the qualitative evaluation and the quantitative analysis of cell surface constituents and antigens expressed by human dendritic epidermal cells.

Vesicular stomatitis virus glycoprotein, albumin, and transferrin are transported to the cell surface via the same Golgi vesicles

Human hepatoma cells, infected by vesicular stomatitis virus, offer a good system to study simultaneously the intracellular localization of a well defined transmembrane glycoprotein (VSV-G), a secretory glycoprotein (transferrin), and a nonglycosylated secretory protein (albumin). We used monospecific antibodies in combination with 5- and 8-nm colloidal gold particles complexed with protein A to immunolabel these proteins simultaneously in thin frozen sections of hepatoma cells. VSV-G, transferrin, and albumin are present in the same rough endoplasmic reticulum cisternae, the same Golgi compartments, and the same secretory vesicles. In the presence of the ionophore monensin intracellular transport is blocked at the trans cisternae of the Golgi complex, and VSV-G, transferrin, and albumin accumulate in dilated cisternae, which are apparently derived from the trans-Golgi elements. Glycoproteins, synthesized and secreted in the presence of monensin, are less acidic than those in control cultures. This is probably caused by a less efficient contact between the soluble secretory proteins and the membrane-bound glycosyltransferases that are present in the most monensin-affected (trans) Golgi cisternae.

Immunocytochemical localization of fibrinogen, platelet factor 4, and beta thromboglobulin in thin frozen sections of human blood platelets

Affinity-purified monospecific antibodies against human fibrinogen and the platelet-specific proteins platelet factor 4 and beta thromboglobulin were used to localize these antigens in thin and ultra-thin frozen sections of mildly fixed, washed human blood platelets. By immunofluorescent double-labeling experiments the distribution of fibrinogen was compared to that of platelet factor 4 and beta thromboglobulin. All three antigens occurred in virtually all platelets and showed and identical, dotlike distribution. For immunoelectron microscopy we used protein A-colloidal gold on ultra-thin frozen sections to visualize the specific reaction indirectly. The staining for platelet factor 4, beta thromboglobulin, and fibrinogen localized exclusively over alpha-granules of washed platelets. Within the granules, platelet factor 4 was localized preferentially in the electron dense, alpha-granule nucleoid, whereas fibrinogen was more predominant in the electron-lucent granule periphery. Beta thromboglobulin localization did not show a preferential intragranular distribution.

Transport and topology of galactosyltransferase in endomembranes of HeLa cells

HeLa cell membranes were studied for the distribution and orientation of the Golgi marker enzyme uridine diphosphate-galactose:beta-D-N-acetylglucosamine beta, 1-4 transferase (GT). Short pulse labeling in the presence of [35S]methionine resulted in two precursor species (Mr = 44,000 and 47,000), present in a microsomal fraction with a density of 1.18 g/ml in sucrose, presumably derived from the rough endoplasmic reticulum. Processing of the N-linked oligosaccharide(s) occurred only after the precursor molecules migrated to lighter density fractions, presumably derived from the Golgi complex. The mature GT molecules (Mr = 54,000) contain O-linked oligosaccharides as shown by beta-elimination of metabolically incorporated [3H]galactose. The O-glycosylation occurred mainly in the light density fractions. The topology of GT was studied on membrane fractions after labeling with [35S]methionine as well as immunocytochemically on ultrathin cryosections at the electron microscope level. Our results indicate that both the antigenic determinants of GT as well as polypeptide chain are present intramembraneously and at the luminal side of the membranes of the Golgi complex and rough endoplasmic reticulum.

Biosynthesis of the major glycoprotein associated with zymogen-granule membranes in the pancreas

The biosynthesis of GP-2, the major glycoprotein associated with zymogen-granule membranes in the pancreas, was studied in acinar cell suspensions from rat pancreas. Pulse-chase experiments, using [35S]methionine, were performed and the processing of GP-2 was analyzed by immunoprecipitation and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. GP-2 is synthesized as a precursor glycoprotein with apparent molecular weight Mr = 73000. Within 60 min after synthesis it is almost completely converted to the mature form (Mr = 78000-80000). Only the precursor form of GP-2 is sensitive to digestion with the glycosidase endo-beta-N-acetylglucosaminidase H, indicating that the observed conversion reflects the processing of 'high-mannose' oligosaccharides into complex type oligosaccharides. Acinar cells cultured in the presence of increasing concentrations of the N-glycosylation inhibitor tunicamycin synthesize 5-6 distinct precursor GP-2 species with apparent molecular weights decreasing from 73000-61000. We conclude that GP-2 contains five or six N-linked carbohydrate chains. From cell fractionation studies it was established that the precursor GP-2 is present in a microsomal fraction with high density (greater than 1.169 g/ml) presumably derived from the rough endoplasmic reticulum; mature GP-2 is localized in low density microsomes (less than 1.130 g/ml) probably Golgi vesicles. The GP-2 in zymogen granule membranes is also in the mature form.

Localisation of 5'-nucleotidase in a rat liver cell line using a monoclonal antibody and indirect immunofluorescent labelling

A new method of labelling anti-IgG with rhodamine while protecting the IgG binding site is described. Indirect immunofluorescence of the ectoenzyme 5'-nucleotidase using a monoclonal antibody shows that it has a punctate distribution over the whole cell surface of a cultured rat liver cell line. The density of fluorescent spots representing the surface localisation of 5'-nucleotidase and of clathrin-coated pits was analysed by a computerised counting technique. The cell surface localisation of 5'-nucleotidase differs from clathrin-coated pits and microvilli. 5'-Nucleotidase within the cell is not uniquely associated with any clearly defined organelle.

Some recent developments in immunocytochemistry

Immunocytochemistry has become an indispensable tool both in basic biomedical research and in diagnostic histopathology. Several recent innovations have led to improvements in the sensitivity; specificity and precision of these techniques. Numerous modifications of the original methods have been developed, many with increased sensitivity. In particular, methods using protein A or the avidin-biotin complex as second steps appear to be promising. New methods for the quantitative determination of sensitivity have become available. The introduction of monoclonal antibodies as immunocytochemical reagents appears to be a major improvement. New methods of immunochemical analysis, such as immunoblotting and immunospotting of antigens extracted from tissue specimens, allow the molecular composition of immunoreactive antigenic sites in a tissue to be analysed. The accuracy of localization in immunoelectron microscopy has been improved significantly through the use of ultracryotomy of unfixed tissue in combination with colloidal gold particles as a label. In addition, gold particles can be counted and thus allow relatively simple quantification of the immune reaction. Using flow cytometry, especially in combination with monoclonal antibodies, quantitative immunofluorescence has become feasible.

Intracellular site of asialoglycoprotein receptor-ligand uncoupling: double-label immunoelectron microscopy during receptor-mediated endocytosis

In rats infused with asialoglycoprotein for 60 min, receptor-mediated endocytosis of the ligand occurred exclusively in hepatic parenchymal cells. We have used double-label immunoelectron microscopy on ultrathin cryosections of rat liver to identify the site at which the asialoglycoprotein receptor and its ligand dissociate following their common endocytosis. Asialoglycoprotein receptor, ligand and clathrin were identified and quantitated by the use of monospecific antibodies followed by gold-protein A complexes. Both receptor and ligand were found associated with the membrane of clathrin-coated vesicles close to the cell surface. We identified other vesicles that contained ligand accumulated within the lumen. The membranes of these latter vesicles contained little receptor, but receptor was concentrated in tubular extensions that were largely free of ligand. We call this organelle CURL (compartment of uncoupling of receptor and ligand). CURL vesicles appear to transform into secondary lysosomes, wherein the ligand is degraded. The tubular vesicles are, we propose, an intermediate in recycling the receptor to the cell surface.

Sorting and recycling of cell surface receptors and endocytosed ligands: the asialoglycoprotein and transferrin receptors

With few exceptions, receptor-mediated endocytosis of specific ligands is mediated through clustering of receptor-ligand complexes in coated pits on the cell surface, followed by internalization of the complex into endocytic vesicles. During this process, ligand-receptor dissociation occurs, most probably in a low pH prelysosomal compartment. In most cases the ligand is ultimately directed to the lysosomes, wherein it is degraded, while the receptor recycles to the cell surface. We have studied the kinetics of internalization and recycling of both the asialoglycoprotein receptor and the transferrin receptor in a human hepatoma cell line. By employing both biochemical and morphological/immunocytochemical approaches, we have gained some insight into the complex mechanisms which govern receptor recycling as well as ligand sorting and targeting. We can, in particular, explain why transferrin is exocytosed intact from the cells, while asialoglycoproteins are degraded in lysosomes. We have also localized the intracellular site at which endocytosed receptor and ligand dissociate.

Recycling of the asialoglycoprotein receptor: biochemical and immunocytochemical evidence

One of the best documented systems of receptor-mediated endocytosis is the clearance of asialoglycoproteins (ASGP) from the blood plasma by liver parenchymal cells. There are 200 000-500 000 ligand binding sites per cell, which makes this system favourable for molecular studies of receptor function. By using both biochemical and immunocytochemical approaches, we have obtained evidence for receptor recycling. We have also localized the intracellular site at which the endocytosed receptor and ligand dissociate. The human hepatoma cell Hep G2 contains abundant ASGP receptors (approximately 225 000 per cell). In growing cells approximately 85% of the functional receptors are on the cell surface and the remaining 15% are internal. The maximal rate of ligand uptake in this cell system at 37 degrees C is approximately 30 000 molecules per cell per minute. Each functional receptor can therefore bind and internalize more than 50 ligand molecules during a 6 h period (in the absence of new receptor synthesis), or one ligand each 8 min. To follow both ligand and receptor during their common endocytosis and to visualize the compartment in which the dissociation of ligand from receptor occurs, we have used our recently developed double-labelling immunocytochemical electron microscopic techniques with purified antibodies against ASGP ligand and ASGP receptor. In normal rat hepatocytes, both ligand and receptor are taken up from the sinusoidal cell surface in clathrin-coated vesicles. Both receptor and ligand are associated with the membrane of small clathrin-coated vesicles close to the cell surface. Larger vesicles, farther removed from the surface, contain ligand accumulated within the lumen. The membranes of these larger vesicles contain little receptor, but receptor was concentrated in detached vesiculotubular extensions, which were largely free of ligand. These vesicles represent the compartment of uncoupling of receptor and ligand (CURL) during their common endocytosis. Ligand contained within the vesicle lumen is then transferred to multivesicular bodies and lysosomes; the tubular extensions may carry receptor back to the cell surface.

Viral membrane proteins acquire galactose in trans Golgi cisternae during intracellular transport

Frozen, thin sections of baby hamster kidney (BHK) cells were incubated with either concanavalin A (Con A) or Ricinus communis agglutinin I (RCA) to localize specific oligosaccharide moieties in endoplasmic reticulum (ER) and Golgi membranes. These lectins were then visualized using an anti-lectin antibody followed by protein A conjugated to colloidal gold. All Golgi cisternae and all ER membranes were uniformly labeled by Con A. In contrast, RCA gave a uniform labeling of only half to three-quarters of those cisternae on the trans side of the Golgi stack; one or two cis Golgi cisternae and all ER membranes were essentially unlabeled. This pattern of lectin labeling was not affected by infection of the cells with Semliki Forest virus (SFV). Infected cells transport only viral spike glycoproteins from their site of synthesis in the ER to the cell surface via the stacks of Golgi cisternae where many of the simple oligosaccharids on the spike proteins are converted to complex ones (Green, J., G. Griffiths, D. Louvard, P. Quinn, and G. Warren. 1981. J. Mol. Biol. 152:663-698). It is these complex oligosaccharides that were shown, by immunoblotting experiments, to be specifically recognized by RCA. Loss of spike proteins from Golgi cisternae after cycloheximide treatment (Green et al.) was accompanied by a 50% decrease in the level of RCA binding. Hence, about half of the RCA bound to Golgi membranes in thin sections was bound to spike proteins bearing complex oligosaccharides and these were restricted to the trans part of the Golgi stack. Our results strongly suggest that complex oligosaccharides are constructed in trans Golgi cisternae and that the overall movement of spike proteins is from the cis to the trans side of the Golgi stack.

Localization at high resolution of antibody-induced mobilization of vaccinia virus hemagglutinin and the major histocompatibility antigens on the plasma membrane of infected cells

We examined the consequence of simultaneous or independent binding of monospecific antibody to the hemagglutinin (HA) of vaccinia virus and the A-, B- and -determinants of HLA on HeLa or Raji cells or KkDk determinants of H-2 on L929 cells. The bound antibodies were marked by goat-anti-mouse (GAM) or goat-anti-rabbit (GAR) fluorochrome conjugates suitable for light microscopy and GAM or GAR gold conjugates, used in electron microscopy. Specificity and amount of antibody adsorbed was ascertained by complement-mediated lysis of 51Cr-labeled cells and by fluorescence-activated cell sorter analysis. Regardless of the order of either antibody to major histocompatibility complex (MHC) or antibody to HA addition after warming to 37 degrees C, there was evidence by light microscopy for co-patching and co-capping of the viral and host antigens. Electron microscopic examination revealed that goat-anti-rabbit 20 nM gold conjugate and goat-anti-mouse 5 nM gold conjugate, marking respectively the HA and MHC molecules, became concentrated in patched or caps in which the two antigens frequently overlapped or were closely associated. The contiguous MHC and HA antigens were also engulfed, as evidenced from the of two sizes of gold particles inside endocytic vacuoles. The significance of these observations is discussed in relation to the cytotoxic T lymphocyte-mediated killing virus-infected targets.

Immunoelectron microscopic studies of the sites of cell-substratum and cell-cell contacts in cultured fibroblasts

Our object was to obtain information about the molecular structures present at cell-substratum and cell-cell contact sites formed by cultured fibroblasts. We have carried out double immunoelectron-microscopic labeling experiments on ultrathin frozen sections cut through such contact sites to determine the absolute and relative dispositions of the three proteins fibronectin, vinculin, and alpha-actinin with respect to these sites. (a) Three types of cell-substratum and cell-cell contact sites familiar from plastic sections could also be discriminated in the frozen sections by morphological criteria alone, i.e., the gap distances between the two surfaces, and the presence of submembranous densities. These types were: (i) focal adhesions (FA); (ii) close contacts (CC); and (iii) extracellular matrix contacts (ECM). This morphological typing of the contact sites allowed us to recognize and assign distinctive immunolabeling patterns for the three proteins to each type of site on the frozen sections. (b) FA sites were immunolabeled intracellularly for vinculin and alpha-actinin, with vinculin labeling situated closer to the membrane than alpha-actinin. Fibronectin was not labeled in the narrow gap between the cell surface and the substratum, or between two cells, at FA sites. Control experiments showed that this could not be ascribed to inaccessibility of the FA narrow gap to the immunolabeling reagents but indicated an absence or severe depletion of fibronectin from these sites. (c) CC sites were labeled intracellularly for alpha-actinin but not vinculin and were labeled extracellularly for fibronectin. (d) ECM sites were characterized by large separations (often greater than 100 nm) between the cell and substratum or between two cells, which were connected by long cables of extracellular matrix components, including fibronectin. In late (24-36 h) cultures, ECM contacts predominated over the other types. ECM sites appeared to be of two kinds, one labeled intracellularly for both alpha-actinin and vinculin, the other for alpha-actinin alone. (e) From these and other results, a coherent but tentative scheme is proposed for the molecular ultrastructure of these contacts sites, and specific functional roles are suggested for fibronectin, vinculin, and alpha-actinin in cell adhesion and in the linkage of intracellular microfilaments to membranes at the different types of contact sites.

Urothelium-specific antibody and lectin surface mapping of bladder urothelium

Coupled ligand-colloidal gold complexes were found to provide a convenient approach for the localization of scanning electron microscopy of cell surface membrane antigens and lectin-binding sites on bladder urothelium and for the immunocytochemical identification of urothelial cell populations at different stages of differentiation. The ligands used to prove the membrane were a urothelium-specific rabbit antibody raised to a urothelial membrane-associated antigen (UMA), and two lectins: Concanavalin A (Con A) and peanut agglutinin (PNA). A complex luminal surface distribution pattern was demonstrated by the UMA antigen related to the stage of urothelial cell maturation and differentiation. UMA could be detected on the surface of immature and early differentiating intermediate cells, but was absent from the late differentiation stage, becoming re-expressed as the cells matured and was found in greatest abundance on the terminally differentiated superficial cells. It was absent on cells in benign hyperplasia of the urothelium. Cellular and regional differences in lectin binding to the urothelial cell surface was suggested with Con A receptors localized uniformly over the superficial cells, and PNA receptors confined to linear arrays or occasional clusters over the apical surface but evenly dispersed over the lateral surface of these cells.

The preparation of protein A-gold complexes with 3 nm and 15nm gold particles and their use in labelling multiple antigens on ultra-thin sections

The preparation of a protein A-gold complex (pAg3) using 3 nm gold particles and its application for labelling of intracellular antigens on thin sections is reported. The 3 nm gold particle is the smallest metal particle currently available for cytochemistry and permits a higher resolution of the pAg technique. Furthermore, it can be used in double labelling experiments in conjunction with a pAg complex prepared from 15nm gold particles. For double labelling, the pAg3 complex must be used for staining of the first antigen since otherwise a non-specific co-labelling of the two pAg complexes results.

Immunocytochemical localization of the receptor for asialoglycoprotein in rat liver cells

We used high-resolution immunocytochemistry on ultrathin frozen sections labeled with colloidal gold to study the subcellular distribution of the asialoglycoprotein receptor in rat liver. The receptor was localized along the entire hepatocyte plasma membrane, including the bile capillary membrane, but was scarce intracellularly. Sinusoidal lining (Kupffer) cells and blood cells showed no immunoreactivity. In liver cells of rats injected with 1 to 100 micrograms of asialoorosomucoid (ASOR) 2-15 min before tissue fixation, endocytotic internalization of receptors at the blood front was conspicuous. At all times in this interval, receptor was present in approximately 100-nm vesicles and larger vacuoles adjacent to the sinusoidal plasma membrane. No other significant intracellular receptor was noted during the 15-min exposure to ASOR; in particular, lysosomes and Golgi complex were not labeled. Our observations, in combination with data from the literature which demonstrate that, under these conditions, the ligand is transferred further to the Golgi complex-lysosome region, suggest that the receptor and ligand are dissociated in the vicinity of the plasma membrane, after which the receptor rapidly returns to the cell surface.

The golgi apparatus: two organelles in tandem

The Golgi apparatus consists of distinct cis and trans compartments that may act sequentially to refine the protein export of the endoplasmic reticulum by removing escaped endoplasmic reticulum proteins. Refinement may be a multistage process akin to fractional distillation; the stack of cisternae comprising the cis Golgi may be the plates in this distillation tower. The trans Golgi, consisting of the last one or two cisternae, may be the receiver that collects from the cis Golgi only its most refined fraction for later distribution to specific locations throughout the cell.

Sizing of protein A-colloidal gold probes for immunoelectron microscopy

Gold particles in colloidal solutions often vary considerably in size. The finest sols (diameter less than 15 nm), especially, are very heterogeneous, as is indicated by coefficients of variance (CV) of 25-35%. We have complexed staphylococcal protein A with gold particles (PA/Au) and then fractionated the preparations by glycerol or sucrose gradient centrifugation into very homogeneous subfractions. In this way, PA/Au probes of almost any size between 4.5 and 15 nm could be prepared. The variation of the gold particles in these fractions resulted in CV's between 9 and 16%. The reactivity of the PA/Au complex was not affected by the gradient procedure, as was shown by single- and double-labeling immunocytochemistry of ultrathin cryosections of rat pancreatic tissue.