Reflection contrast microscopy. Visualization of (peroxidase-generated) diaminobenzidine polymer products and its underlying optical phenomena

Replication of Plasmodium in reticulocytes can occur without hemozoin formation, resulting in chloroquine resistance

Lin et al. generate Plasmodium berghei mutants lacking enzymes critical to hemoglobin digestion. A double gene deletion mutant lacking enzymes involved in the initial steps of hemoglobin proteolysis is able to replicate inside reticulocytes of infected mice with limited hemoglobin degradation and no hemozoin formation, and moreover, is resistant to the antimalarial drug chloroquine.

Most studies on malaria-parasite digestion of hemoglobin (Hb) have been performed using P. falciparum maintained in mature erythrocytes, in vitro. In this study, we examine Plasmodium Hb degradation in vivo in mice, using the parasite P. berghei, and show that it is possible to create mutant parasites lacking enzymes involved in the initial steps of Hb proteolysis. These mutants only complete development in reticulocytes and mature into both schizonts and gametocytes. Hb degradation is severely impaired and large amounts of undigested Hb remains in the reticulocyte cytoplasm and in vesicles in the parasite. The mutants produce little or no hemozoin (Hz), the detoxification by-product of Hb degradation. Further, they are resistant to chloroquine, an antimalarial drug that interferes with Hz formation, but their sensitivity to artesunate, also thought to be dependent on Hb degradation, is retained. Survival in reticulocytes with reduced or absent Hb digestion may imply a novel mechanism of drug resistance. These findings have implications for drug development against human-malaria parasites, such as P. vivax and P. ovale, which develop inside reticulocytes.

Enhanced 3D fluorescence live cell imaging on nanoplasmonic substrate

We have created a randomly distributed nanocone substrate on silicon coated with silver for surface-plasmon-enhanced fluorescence detection and 3D cell imaging. Optical characterization of the nanocone substrate showed it can support several plasmonic modes (in the 300-800 nm wavelength range) that can be coupled to a fluorophore on the surface of the substrate, which gives rise to the enhanced fluorescence. Spectral analysis suggests that a nanocone substrate can create more excitons and shorter lifetime in the model fluorophore Rhodamine 6G (R6G) due to plasmon resonance energy transfer from the nanocone substrate to the nearby fluorophore. We observed three-dimensional fluorescence enhancement on our substrate shown from the confocal fluorescence imaging of chinese hamster ovary (CHO) cells grown on the substrate. The fluorescence intensity from the fluorophores bound on the cell membrane was amplified more than 100-fold as compared to that on a glass substrate. We believe that strong scattering within the nanostructured area coupled with random scattering inside the cell resulted in the observed three-dimensional enhancement in fluorescence with higher photostability on the substrate surface.

Reflection contrast microscopy: The bridge between light and electron microscopy

Reflection contrast microscopy (RCM) is a light microscopic method to image cells at high definition and enhanced sensitivity compared to conventional bright-field microscopy. RCM images have very high contrast, which makes them easily applicable for digital image analysis. Because ultrathin sections are mostly used in this method, RCM also functions by bridging light with electron microscopy: the combination of ultrastructural with histochemical studies. RCM can also replace electron microscopy for rapid and simple screening of large quantities of samples for immunocytochemical staining. Special attention is paid to small biological objects, which have to be processed for RCM. If you encounter the limits of brightfield microscopy, in resolution, sensitivity or handling of the specimen, RCM will be a feasible option. Reflection contrast microscopy methods use only slightly adjusted electron microscopy methods for specimen preparation. Therefore, many familiar techniques for ultrathin specimen preparation can be applied. It is essential that only refractive index differences exist in those areas that are of interest and that the further specimen is as optically homogenic as possible, with a refractive index as close to that of glass as possible. Therefore, plastic embedding is recommended.

Local cell membrane deformations due to receptor-ligand bonding as seen by reflection microscopy

Understanding surface receptor clustering and redistribution processes at the cell-matrix contact zone requires detailed knowledge of the spatial integration of these molecules in the architecture of this complex interface. Here we present and discuss critically a procedure to extract such information combining reflection contrast microscopy (RCM) and reflection interference microscopy (RIM). As model system, we used living human umbilical vein endothelial cells (HUVEC) adhering to laminin-coated surfaces and investigated the distribution of the alpha2beta1 (CD29/CD49b) integrin at the contact zone of these cells. First, we applied freeze-fracture electron microscopy to gain information on microscopic details of the alpha2beta1 distribution at the contact zone. Next, we visualized and analyzed the overall lateral distribution of the integrins applying RCM using immunogold-labeling with 10 nm labels and a special silver enhancement technique. We found that RCM can be used to determine the lateral position of the marked receptor molecules to an accuracy of about 100-200 nm, instead of large morphological changes at the contact zone during silver enhancement. Finally, we combined RCM with RIM and analyzed the interference pattern of the contact zone around the label positions. Thus, we were able to detect changes of the average shape of the cell membrane due to receptor-ligand bonding of a size down to the resolution of the techniques.

Comparison of different detection methods in quantitative microdensitometry

Quantitative evaluation of immunohistochemical staining has become a focus of attention in research applications and in pathological diagnosis, such as Her-2/neu assessment in mammary carcinoma. Reproducibility of immunostaining techniques and microscopical evaluation are prerequisites for a standardized and reliable quantitation of immunostaining intensity. In the present study, different staining and microscopical techniques, including fluorescence microscopy, epipolarization microscopy of immunogold-silver, and absorbance microdensitometry were compared concerning suitability for quantitative evaluation. We describe a staining procedure using alkaline phosphatase-based immunohistochemistry with the substrate Vector Red and subsequent microdensitometry with a custom-designed absorbance filter. We have characterized linearity of the staining intensity in dependence of development time, antibody concentration, and section thickness by means of artificial standards consisting of agarose blocks into which immunogold- or alkaline phosphatase-conjugated antibodies were incorporated. Applicability of the different techniques was tested by anti-CD45 immunostaining of leukocytes within rat lung tissue detected by immunofluorescence, immunogold-silver epipolarization microscopy, as well as alkaline phosphatase-based Vector Red absorbance or fluorescence measurement. Excellent qualities of Vector Red for quantitative microdensitometric evaluation of staining intensity were particularly obvious for absorbance microscopy. Applicability in paraffin-embedded tissue as well as in cryosections, excellent segmentation, linearity over a wide range, light stability, and feasibility for permanent mounting and for long-term storage are the outstanding features of this technique for use in routine quantitative evaluation.

Reflection contrast microscopy within chrome-alum haematoxylin stained thick tissue-sections

This paper introduces two innovations in reflection contrast microscopy (RCM): (1) an extended application for qualitative light microscopic investigations; and (2) a novel method for quantification in cytochemistry. (1) We found out that RCM cannot only be used for surface characterizations and in thin sections but also within thick tissue-sections. The use of the RCM technique is demonstrated on slides of the supraoptic nucleus (SON) of the rat stained with chrome-alum haematoxylin: Among all the stained structures only neurosecretory granules are found to cause reflections. The visualization of the neurosecretion and its distribution is more distinct and of sharper contrast than in bright field microscopy. (2) The improved differentiation allows the quantification of neurosecretion in tissue-sections by combining RCM with grey-scale image analysis.

Multicolour preparations for in situ hybridization using precipitating enzyme cytochemistry in combination with reflection contrast microscopy

We have further developed a method for the detection of different enzyme cytochemical reaction products by means of reflection contrast microscopy (RCM). By embedding these enzyme precipitates in a protein matrix, we were able to prevent the reaction products from dissolving in immersion oil, which is required for RCM analysis. The applicability of the RCM procedure is, therefore, extended to a range of cytochemical enzyme precipitation methods, which normally result in oil soluble reaction products. To test their usefulness, these enzyme precipitates have been used in single- and well as double-label in situ hybridization (ISH) procedures to visualize a number of DNA target sequences by several different reflection colours, i.e. white, yellow and red. Three repetitive DNA probes for the (sub)centromeric regions of chromosomes 1, 7 and 17, as well as a repetitive DNA probe for the telomeric region of chromosome 1, and two cosmid DNA probes (40 kb each) for both arms of chromosome 11 could be detected with high efficiency in both interphase and metaphase preparations. Moreover the enzyme precipitates were shown to be stable upon exposure to excitation light or upon storage. It may be concluded that these findings render RCM a sensitive method for the visualization of multiple targets in biological specimens.

Reflection contrast microscopy of ultrathin sections in immunocytochemical localization studies: a versatile technique bridging electron microscopy with light microscopy

Reflection contrast microscopy (RCM) of ultrathin sections was recently introduced as a sensitive technique for visualization with enhanced definition in immunogold histochemistry. Experience of using RCM as a major tool in immunocytochemical research in different fields is summarized, e.g. oncology, nephrology and embryology. The sensitive visualization of immunocytochemical labels, gold particles or peroxidase-diaminobenzidine deposits in or on ultrathin sections, by RCM instead of electron microscopy is demonstrated. RCM of ultrathin sections is an adequate light microscopical alternative for immunoelectron microscopy, since an overview of both label and tissue is obtained with a high image definition and high contrast of label. In the studies presented, RCM is shown to provide a better gradation in staining intensity and staining pattern than other light microscopical methods. Moreover, a precise localization of multiple labels is obtained with this method. Besides the applications shown, ultrathin section visualization by RCM is very useful for correlative light- and electron microscopical studies of fine structures. Commercially available fluorescence microscopes can be adapted for proper RCM functioning; an adaptation scheme and list of microscopes tested is provided.

Comparison of reporter molecules for viral in situ hybridization

A number of streptavidin-linked reporter molecules at the endpoint of a five-step detection protocol for viral in situ hybridization using biotinylated probes were examined. DNA-DNA and RNA-RNA model systems were used. Streptavidin linked to either peroxidase or fluorescein was found to be optimal in terms of sensitivity and resolution within individual cells. All other reporter molecules labelled similar numbers of cells with low background reaction. However, streptavidin-5 nm gold followed by silver enhancement gave very high background staining making interpretation of positive signals very difficult.

Inorganic phosphors as new luminescent labels for immunocytochemistry and time-resolved microscopy

A new strongly luminescent marker consisting of inorganic crystals is described for time-resolved microscopy. These crystals, known as phosphors, show delayed luminescence, unlike prompt fluorescent labels such as FITC, TRITC and phycobiliproteins, and are therefore potentially suitable for time-resolved microscopy. The luminescence of these phosphors is strong and non-fading in comparison to FITC/TRITC, and not significantly influenced by pH or temperature. The phosphor yttriumoxisulfide activated with europium emits maximally at 620 nm with a typical half life-time of approximately 700 musec, upon excitation with near ultraviolet light (360 nm). Phosphors for immunocytochemical staining were made by ball milling and were stabilized in suspension with polycarboxylic acids. Proteins such as avidin, protein A or immunoglobulins were allowed to adsorb to the surface of the phosphors. The immunocytochemical properties of the conjugates were evaluated in a model system of latex beads with defined surface antigens and in a cellular system containing fixed human lymphocytes or erythrocytes. Specific cytochemical staining was observed in suspension as well as on glass slides. A specially constructed time-resolved microscope was used to suppress the fast decaying fluorescence, thereby permitting visualization of the specific, slowly decaying luminescence of the phosphor label without the necessity of integration. Finally, the use of multiple phosphors with different kinetic and spectral characteristics for multiparameter studies is indicated.

Improved detection and quantification of the (immuno) peroxidase product using reflection contrast microscopy

Reflection contrast microscopy (RCM) is a sensitive tool to detect minor amounts of precipitated diaminobenzidine (DABox) in immunoperoxidase stained specimens. One of the main issues in immunocytochemistry is the ongoing need for more sensitive and quantitative techniques. Therefore we applied RCM, using a new simple model system, to methods previously described for increased sensitivity in immunocytochemistry with bright field microscopy. Addition of imidazole was found the most sensitive method and addition of Nickel and Cobalt ions gave the most enhanced colour intensity. Variation of the enzyme reaction parameters yielded a continuous increase in reflection with time. This was then discussed in view of other model studies of peroxidase kinetics. A quantitative relationship between the amount of peroxidase and the reflection of DABox was observed, indicating that quantitative immunoperoxidase studies with RCM are feasible. In situ hybridization (ISH) was then used as a useful biological model for RCM to test the optimal conditions for DAB staining found in the model system (high concentrations of DAB and peroxidase and 2 h incubation time). There was no background staining in the model system, also after prolonged incubation time. The ISH experiments showed that the contrast (ratio) between specific signal and chromosome background did not increase in time, whereas only the use of high avPO concentrations yielded the highest contrast.