A review of cell surface markers and labelling techniques for scanning electron microscopy

Backscattered-electron imaging of the colloidal gold surface marker

A variety of markers have been used for surface labeling with the scanning electron microscope (1). They are all recognized in the secondary electron imaging (SEI) mode on the basis of their characteristic sizes and shapes. This makes it difficult, however, to recognize unambiguously markers of small size (smaller than 40 nm). Unfortunately, markers of such size are needed if one wishes to minimize steric hindrance phenomena (2) and therefore obtain high efficiency labeling. We have found that the atomic number contrast of the colloidal gold marker, expressed in the backscattered electron imaging (BEI) mode, alleviates this difficulty to a significant extent (3).

Human peripheral blood leukocytes, separated by sedimentation in presence of Dextran, were attached to poly-1-lysine pretreated carbon coated grids. The cells were then fixed for 5 min with 0.25% solution of buffered glutaraldehyde, pH 7.2, extensively rinsed with 0.1% glycine in PBS, and incubated for 30 min, at room temperature with an IgM monoclonal antibody (D2) specifically recognizing, a surface antigen of mature granulocytes.

Mutagenicity of diesel exhaust particles mediated by cell-particle interaction in mammalian cells

Diesel exhaust particle (DEP) has been identified as a class 2A human carcinogen and closely related to the increased incidence of respiratory allergy, cardiopulmonary morbidity and mortality, and risk of lung cancer. However, the molecular mechanisms of DEP mutagenicity/carcinogenicity are still largely unknown. In the present study, we focused on the mutagenicity of DEPs in human-hamster hybrid (A(L)) cells and evaluated the role of cell-particle interaction in mediating mutagenic process. We found that DEPs formed micron-sized aggregates in the medium and located mainly in large cytoplasmic vacuoles of cells by 24h treatment. The cellular granularity was increased by DEP treatment in a dose-dependent manner. DEPs resulted in a dose-dependent increase of mutation yield at CD59 locus in A(L) cells, while inflicting minimal cytotoxicity. There was a more than two-fold increase of mutation yield at CD59 locus in A(L) cells exposed to DEPs at a dose of 50mug/ml. Such induction was significantly reduced by concurrent treatment with phagocytosis inhibitors, cytochalasin B and ammonium chloride (p<0.05). These results provided direct evidence that DEPs was mutagenic in mammalian cells and that cell-particle interaction played an essential role in the process.

Galanin and cholecystokinin in cultured magnocellular neurons isolated from adult rat supraoptic nuclei: a correlative light and scanning electron microscopical study

Cultured magnocellular neurons, isolated from adult rat supraoptic nuclei, were characterized by immunocytochemistry, using the avidin-biotin-peroxidase complex and antisera to vasopressin, oxytocin, galanin and cholecystokinin. Light microscope examination of the immunostained cultures revealed the presence of vasopressin- and oxytocin-like immunoreactivity, as well as neurons containing either galanin- or cholecystokinin-like immunoreactivity. In contrast, no significant galanin- or cholecystokinin-like immunoreactivity could be observed in freshly dispersed cells. Correlative scanning electron microscopical observations in the secondary electron imaging mode revealed that the stained neurons appeared significantly brighter than the unstained structures. Complementary observations with toad brain sections (preoptic area), immunostained for galanin, led to the same result. Considering previous results, it is suggested that the presence of galanin- and cholecystokinin-like immunoreactivity in the cultured neurons and its virtual absence in freshly dispersed cells is indicating a participation of these peptides in the regenerative processes taking place during culture. It is further concluded that the avidin-biotin-peroxidase method is suitable for correlative light and scanning electron microscopical studies of smooth surfaces and cultured cells.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

Visualization and rapid quantification of autoradiographic labeling in scanning electron microscopy applied to localization of receptor sites on the surface of whole cells

The use of backscattered electron imaging (BEI) as a routine procedure for examining autoradiographic reactions in scanning electron microscopy (SEM) is described. This technique allows the determination of the number of receptor sites occupied by 125I-epidermal growth factor (EGF) on whole cells. The effect of 1.25 dihydroxyvitamin D3 (1,25 (OH)2D3) on the number of epidermal growth factor receptors (EGF-R) in the BT 20 human mammary carcinoma cell line (which is known to possess a very high number of EGF-R) has been evaluated with this method. To compare the silver grain density over the cells (controls and 1,25 (OH)2D3-treated cells) we used an image analysis system Quantimet 900. The results were compared with those of a previous study using transmission electron microscopy (TEM). This study confirmed the results obtained with TEM and showed the even distribution of receptors sites on a single cell and a large difference in the number of receptor sites from one cell to another. The use of BEI to visualize the autoradiographic reaction in SEM allowed the examination of a large surface with good contrast and resolution and eliminated artefacts not corresponding to the silver grains. It gave new information not delivered by quantitative TEM autoradiography and was easier and faster to use. The efficient use of SEM autoradiography combined with BEI could facilitate whole area distribution mapping of radioactive labeling.

Scanning electron microscopy in the backscattered electron imaging (BEI) mode: applications to clinical hematology

Elements of high atomic number backscatter electrons that carry information of cytochemical and of immunocytochemical significance in high resolution scanning electron microscopy (SEM). Reaction products of enzyme cytochemistry containing lead or osmium have been used to localize the sites of phosphatase and of endogenous peroxidase, respectively. Particles of colloidal gold, ranging in diameter from 40 to 13 nm, have been successfully used to label cell surface antigens specifically identified by murine monoclonal antibodies. The diagnostic potential of the scanning electron microscope in clinical hematology appears to be considerably enhanced by the use of the backscattered electron imaging (BEI) mode. Quantitation of the number of gold-labeled epitopes on cell surfaces cannot be achieved in the conventional secondary electron mode of the SEM, but is an attractive possibility in the backscattered electron imaging mode.

Scanning electron microscopy of cell surface antigens labeled with colloidal gold

A method is described and discussed that permits the specific labeling of the surface of prefixed cells with the colloidal gold marker viewed with the scanning electron microscope. Its value depends exclusively on the use of backscattered electron imaging. Its advantages include the possibility of preserving the surface features of the labeled cells, the ease with which specificity can be established, the possibility of making total counts of the labeled surface antigenic sites, and the possibility of achieving distinct labeling for two different antigens expressed on the surface of the same cell.

A method for performing scanning electron microscopy of colonies growing in a human tumor cloning system

Scanning electron microscopy (SEM) is a valuable tool for examining cell surface morphology and cell-cell interactions. We used SEM to study 38 patients' tumors, representing 16 histological malignancies, growing in soft agar. Using our method, we obtained high quality micrographs without residual agar or preparation artifacts. We present our method for obtaining high quality SEM of tumor colonies growing in soft agar, which provides micrographs free of debris and necrotic host tissue.

Surface localization of an endogenous lectin in rabbit bone marrow

A lectin, which may be involved in cell to cell adhesion during erythropoiesis in rabbit bone marrow, has been isolated and characterized. Several electron microscopical techniques have been used to investigate the cell surface distribution of this lectin in bone marrow utilizing colloidal gold conjugates of anti-lectin IgG or protein A. The lectin is present at the surface of erythroid cells at all stages of development but no lectin was detected on the surface of myeloid cells. The limitations and complementary nature of the techniques used are discussed.

A methodological basis for SEM autoradiography: biosynthesis and radioligand binding

A method is described for scanning electron microscope (SEM) autoradiography whereby preservation of high resolution cell surface details is retained together with degelatination of the emulsion without gross loss or redistribution of silver grains. This method should provide a convenient medium-sized marker for SEM (using secondary, backscattered electron and X-ray imaging) topographic studies of biosynthesized molecules, and of cell surface receptors and antigens, using indirect or direct labelling procedures with radio-labelled ligands.

The use of colloidal gold complexes in an ultrastructural study of lectin-binding sites and matrix deposition of normal human primary breast epithelium on collagen gels

Colloidal gold probes were used to study the distribution of peanut agglutinin binding sites and the deposition of extracellular fibronectin and type IV collagen in cultured human breast cells grown on type I collagen gels. Qualitative analysis was performed at the ultrastructural level and appraised in relation to the possible role of peanut agglutinin, fibronectin and type IV collagen as functional markers for distinguishing cell types using this methodology. Peanut agglutinin bound to the surface of cuboidal epithelial cells but not on basal, putative myoepithelial cells in the cell islands, suggesting that it may be a useful functional marker. The binding on the epithelial cells was markedly increased by pre-treatment of the cells with neuraminidase. No correlation was seen between the amount of binding and either the surface topography or cellular ultrastructure. Fibronectin and type IV collagen were demonstrated on the fibrillar network left on the collagen gels after removal of the cell sheet. Any cells still adhering to the gel surface showed no evidence of gold probe binding on their upper surfaces. Examination of the under surfaces of the cell sheet showed gold probe binding equivalent to that found on the gels under the cells. However, it was not proven conclusively which cells produce the fibronectin and type IV collagen.

A novel approach for scanning electron microscopy of colloidal gold-labeled cell surfaces

A method is described for the use of scanning electron microscopy on the surface of gold-labeled cells. It includes the use of 45- or 20-nm colloidal gold marker conjugated with Staphylococcal protein A. The marker is best recognized on the basis of its atomic number contrast by using the backscattered electron imaging mode of the scanning electron microscope. When the backscattered electron signal is mixed with the secondary electron signal, an optimum correlation between the distribution of the labeled sites and the cell surface structures is demonstrated. The method is illustrated by its application to the identification of human circulating granulocytes. Its good resolution, high contrast, and good labeling efficiency offers a promising approach to the specific localization of cell surface antigenic sites labeled with particles of colloidal gold.

Utilization of monoclonal antibodies and immuno-scanning electron microscopy for the positive identification of human leukemic cells

Monoclonal antibodies generated against normal and leukemic human leukocytes were tested for their differential reactivity with leukemia and lymphoma cell lines as well as with circulating lymphoid and myeloid leukemic cells by means of immuno-scanning electron microscopy (immuno-SEM). Anti-T (OKT3), anti-mu-chain, anti-CALLA (J5), anti-BA-1, anti-BA-2, and anti-nonlymphoid (Mol) monoclonal antibodies were covalently conjugated to polystyrene latex microspheres (immunolatex), using a two-step glutaraldehyde reaction, and subsequently incubated with the various cell types. Cultured B-type Burkitt lymphoma cells (Daudi) and chronic lymphocytic leukemia (CLL) cells displayed extensive labeling with monoclonal anti-mu, anti-B1, and anti-BA-1 immunolatex conjugates, while cultured malignant T cells (HD-Mar) showed positive labeling with OKT3 immunolatex alone. Cultured myelomonocytic cells (GDM-1) and cells obtained from patients with acute myeloblastic (AML) and monoblastic leukemia (AMoL) labeled only with anti-Mol immunolatex, while cultured promyelocytic cells (HL-60) displayed far less labeling with this conjugate. Common-type acute lymphoblastic leukemia (C/ALL) cells were labeled predominantly with the J5 (anti-CALLA) and anti-BA-2 immunolatex conjugates. Evidence is presented indicating that immuno-SEM employing monoclonal antibodies is a reproducible technique which may be used in the study of leukocyte maturation and may provide additional information in the classification of poorly differentiated leukemias.

Immunocytochemical demonstration of Con A-receptors on the cell surface of guinea pig peritoneal macrophages

By means of protein A-gold in combination with the replica technique the distribution of Con A-receptors on the surface of guinea pig peritoneal macrophages was investigated. In nonspecifically activated macrophages the arrangement of Con A-binding sites was demonstrated as homogeneous. It could be evidenced that by the combined technique the advantages of the high resolution of transmission electron microscopy could be joined with the possibility of the stereological demonstration of cell surface structures.

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.

Immunospecific ferromagnetic iron-dextran reagents for the labeling and magnetic separation of cells

Ferromagnetic iron dextran particles were prepared by reacting a mixture of ferrous chloride and ferric chloride with dextran polymers under alkaline conditions. Particles purified by gel filtration chromatography were in the size range of 30-40 nm, had an electron dense core of about 15 nm, were stable against aggregation in physiological buffer, showed little non-specific binding to cells and had a magnetic moment. Protein A from Staphylococcus aureus was covalently coupled to periodate-oxidized ferromagnetic iron-dextran particles. These conjugates were used to indirectly label antigen sites on human red blood cells and thymocytes for visualization by scanning and transmission electron microscopy. Cells labeled with these immunospecific ferromagnetic particles are were quantitatively retained by a simple permanent magnet and could be separated from unlabeled cells. Applications of these novel reagents in the separation of cells, cell membranes and receptors in drug targeting studies are discussed.