Extraction of proteins and membrane lipids during low temperature embedding of biological material for electron microscopy

Rehydration of Freeze Substituted Brain Tissue for Pre-embedding Immunoelectron Microscopy

Electron microscopy (EM) volume reconstruction is a powerful tool for investigating the fundamental structure of brain circuits, but the full potential of this technique is limited by the difficulty of integrating molecular information. High quality ultrastructural preservation is necessary for EM reconstruction, and intact, highly contrasted cell membranes are essential for following small neuronal processes through serial sections. Unfortunately, the antibody labeling methods used to identify most endogenous molecules result in compromised morphology, especially of membranes. Cryofixation can produce superior morphological preservation and has the additional advantage of allowing indefinite storage of valuable samples. We have developed a method based on cryofixation that allows sensitive immunolabeling of endogenous molecules, preserves excellent ultrastructure, and is compatible with high-contrast staining for serial EM reconstruction.

One for All, All for One: A Close Look at In-Resin Fluorescence Protocols for CLEM

Sample preparation is the novel bottleneck for high throughput correlative light and electron microscopy (CLEM). Protocols suitable for both imaging methods must therefore balance the requirements of each technique. For fluorescence light microscopy, a structure of interest can be targeted using: 1) staining, which is often structure or tissue specific rather than protein specific, 2) dye-coupled proteins or antibodies, or 3) genetically encoded fluorescent proteins. Each of these three methods has its own advantages. For ultrastructural investigation by electron microscopy (EM) resin embedding remains a significant sample preparation approach, as it stabilizes the sample such that it withstands the vacuum conditions of the EM, and enables long-term storage. Traditionally, samples are treated with heavy metal salts prior to resin embedding, in order to increase imaging contrast for EM. This is particularly important for volume EM (vEM) techniques. Yet, commonly used contrasting agents (e.g., osmium tetroxide, uranyl acetate) tend to impair fluorescence. The discovery that fluorescence can be preserved in resin-embedded specimens after mild heavy metal staining was a game changer for CLEM. These so-called in-resin fluorescence protocols present a significant leap forward for CLEM approaches towards high precision localization of a fluorescent signal in (volume) EM data. Integrated microscopy approaches, combining LM and EM detection into a single instrument certainly require such an “all in one” sample preparation. Preserving, or adding, dedicated fluorescence prior to resin embedding requires a compromise, which often comes at the expense of EM imaging contrast and membrane visibility. Especially vEM can be strongly hampered by a lack of heavy metal contrasting. This review critically reflects upon the fundamental aspects of resin embedding with regard to 1) specimen fixation and the physics and chemistry underlying the preservation of protein structure with respect to fluorescence and antigenicity, 2) optimization of EM contrast for transmission or scanning EM, and 3) the choice of embedding resin. On this basis, various existing workflows employing in-resin fluorescence are described, highlighting their common features, discussing advantages and disadvantages of the respective approach, and finally concluding with promising future developments for in-resin CLEM.

Combining high-pressure freezing with pre-embedding immunogold electron microscopy and tomography

Immunogold labeling of permeabilized whole-mount cells or thin-sectioned material is widely used for the subcellular localization of biomolecules at the high spatial resolution of electron microscopy (EM). Those approaches are well compatible with either 3-dimensional (3D) reconstruction of organelle morphology and antigen distribution or with rapid cryofixation-but not easily with both at once. We describe here a specimen preparation and labeling protocol for animal cell cultures, which represents a novel blend of specifically adapted versions of established techniques. It combines the virtues of reliably preserved organelle ultrastructure, as trapped by rapid freezing within milliseconds followed by freeze-substitution and specimen rehydration, with the advantages of robust labeling of intracellular constituents in 3D through means of pre-embedding NANOGOLD-silver immunocytochemistry. So obtained thin and semi-thick epoxy resin sections are suitable for transmission EM imaging, as well as tomographic reconstruction and modeling of labeling patterns in the 3D cellular context.

The sleeping beauty kissed awake: new methods in electron microscopy to study cellular membranes

Electron microscopy (EM) for biological samples, developed in the 1940–1950s, changed our conception about the architecture of eukaryotic cells. It was followed by a period where EM applied to cell biology had seemingly fallen asleep, even though new methods with important implications for modern EM were developed. Among these was the discovery that samples can be preserved by chemical fixation and most importantly by rapid freezing without the formation of crystalline ice, giving birth to the world of cryo-EM. The past 15–20 years are hallmarked by a tremendous interest in EM, driven by important technological advances. Cryo-EM, in particular, is now capable of revealing structures of proteins at a near-atomic resolution owing to improved sample preparation methods, microscopes and cameras. In this review, we focus on the challenges associated with the imaging of membranes by EM and give examples from the field of host–pathogen interactions, in particular of virus-infected cells. Despite the advantages of imaging membranes under native conditions in cryo-EM, conventional EM will remain an important complementary method, in particular if large volumes need to be imaged.

VIS2FIX: rapid chemical fixation of vitreous sections for immuno-electron microscopy

Immuno-electron microscopy uniquely allows high-resolution localization of proteins in their cellular context. Usually, affinity labeling with an electron-dense marker, e.g., small gold particles, is performed on sections of chemically fixed cells or tissues. In this chapter, we describe two novel protocols, the VIS2FIX methods, for chemical fixation of sections of cryo-immobilized biological samples. This method involves production of thin sections of high-pressure frozen cells that are statically adhered to a TEM grid. Subsequent steps involve chemical fixation of the samples by either the VIS2FIX(H) ("H" for "hydrated") or the VIS2FIX(FS) ("FS" for "freeze substitution") techniques. Following chemical fixation, the samples are ready for immunolabeling. The described methods are fast and efficient, yield excellent preservation of intracellular structures, and offer the possibility to maintain lipids in the sample.

Herpes simplex virus 1 Us3 deletion mutant is infective despite impaired capsid translocation to the cytoplasm

Herpes simplex virus 1 (HSV-1) capsids are assembled in the nucleus bud at the inner nuclear membrane into the perinuclear space, acquiring envelope and tegument. In theory, these virions are de-enveloped by fusion of the envelope with the outer nuclear membrane and re-enveloped by Golgi membranes to become infective. Us3 enables the nucleus to cytoplasm capsid translocation. Nevertheless, Us3 is not essential for the production of infective progeny viruses. Determination of phenotype distribution by quantitative electron microscopy, and calculation per mean nuclear or cell volume revealed the following: (i) The number of R7041(∆US3) capsids budding at the inner nuclear membrane was significantly higher than that of wild type HSV-1; (ii) The mean number of R7041(∆US3) virions per mean cell volume was 2726, that of HSV-1 virions 1460 by 24 h post inoculation; (iii) 98% of R7041(∆US3) virions were in the perinuclear space; (iv) The number of R7041(∆US3) capsids in the cytoplasm, including those budding at Golgi membranes, was significantly reduced. Cell associated R7041(∆US3) yields were 2.37×10(8) and HSV-1 yields 1.57×10(8) PFU/mL by 24 h post inoculation. We thus conclude that R7041(∆US3) virions, which acquire envelope and tegument by budding at the inner nuclear membrane into the perinuclear space, are infective.

NAADP and the two-pore channel protein 1 participate in the acrosome reaction in mammalian spermatozoa

A TPCN1 gene–deficient mouse strain is used to show that two convergent working NAADP-dependent pathways with nonoverlapping activation and self-inactivation profiles for distinct NAADP concentrations drive acrosomal exocytosis, by which TPC1 is central for the pathway activated by low-micromolar NAADP concentrations.

The functional relationship between the formation of hundreds of fusion pores during the acrosome reaction in spermatozoa and the mobilization of calcium from the acrosome has been determined only partially. Hence, the second messenger NAADP, promoting efflux of calcium from lysosome-like compartments and one of its potential molecular targets, the two-pore channel 1 (TPC1), were analyzed for its involvement in triggering the acrosome reaction using a TPCN1 gene–deficient mouse strain. The present study documents that TPC1 and NAADP-binding sites showed a colocalization at the acrosomal region and that treatment of spermatozoa with NAADP resulted in a loss of the acrosomal vesicle that showed typical properties described for TPCs: Registered responses were not detectable for its chemical analogue NADP and were blocked by the NAADP antagonist trans-Ned-19. In addition, two narrow bell-shaped dose-response curves were identified with maxima in either the nanomolar or low micromolar NAADP concentration range, where TPC1 was found to be responsible for activating the low affinity pathway. Our finding that two convergent NAADP-dependent pathways are operative in driving acrosomal exocytosis supports the concept that both NAADP-gated cascades match local NAADP concentrations with the efflux of acrosomal calcium, thereby ensuring complete fusion of the large acrosomal vesicle.

Nanometer-resolution fluorescence electron microscopy (nano-EM) in cultured cells

Nano-resolution fluorescence electron microscopy (nano-fEM) pinpoints the location of individual proteins in electron micrographs. Plastic sections are first imaged using a super-resolution fluorescence microscope and then imaged on an electron microscope. The two images are superimposed to correlate the position of labeled proteins relative to subcellular structures. Here, we describe the method in detail and present five technical advancements: the use of uranyl acetate during the freeze-substitution to enhance the contrast of tissues and reduce the loss of fluorescence, the use of ground-state depletion instead of photoactivation for temporal control of fluorescence, the use of organic fluorophores instead of fluorescent proteins to obtain brighter fluorescence signals, the use of tissue culture cells to broaden the utility of the method, and the use of a transmission electron microscope to achieve sharper images of ultrastructure.

Nano-fEM: protein localization using photo-activated localization microscopy and electron microscopy

Mapping the distribution of proteins is essential for understanding the function of proteins in a cell. Fluorescence microscopy is extensively used for protein localization, but subcellular context is often absent in fluorescence images. Immuno-electron microscopy, on the other hand, can localize proteins, but the technique is limited by a lack of compatible antibodies, poor preservation of morphology and because most antigens are not exposed to the specimen surface. Correlative approaches can acquire the fluorescence image from a whole cell first, either from immuno-fluorescence or genetically tagged proteins. The sample is then fixed and embedded for electron microscopy, and the images are correlated (1-3). However, the low-resolution fluorescence image and the lack of fiducial markers preclude the precise localization of proteins. Alternatively, fluorescence imaging can be done after preserving the specimen in plastic. In this approach, the block is sectioned, and fluorescence images and electron micrographs of the same section are correlated (4-7). However, the diffraction limit of light in the correlated image obscures the locations of individual molecules, and the fluorescence often extends beyond the boundary of the cell. Nano-resolution fluorescence electron microscopy (nano-fEM) is designed to localize proteins at nano-scale by imaging the same sections using photo-activated localization microscopy (PALM) and electron microscopy. PALM overcomes the diffraction limit by imaging individual fluorescent proteins and subsequently mapping the centroid of each fluorescent spot (8-10). We outline the nano-fEM technique in five steps. First, the sample is fixed and embedded using conditions that preserve the fluorescence of tagged proteins. Second, the resin blocks are sectioned into ultrathin segments (70-80 nm) that are mounted on a cover glass. Third, fluorescence is imaged in these sections using the Zeiss PALM microscope. Fourth, electron dense structures are imaged in these same sections using a scanning electron microscope. Fifth, the fluorescence and electron micrographs are aligned using gold particles as fiducial markers. In summary, the subcellular localization of fluorescently tagged proteins can be determined at nanometer resolution in approximately one week.

Visualizing proteins in electron micrographs at nanometer resolution

To understand protein function, we need a detailed description of the molecular topography of the cell. The subcellular localization of proteins can be revealed using genetically encoded fluorescent proteins or immunofluorescence. However, the precise localization of proteins cannot be resolved due to the diffraction limit of light. Recently, the diffraction barrier has been overcome by employing several microscopy techniques. Using super-resolution fluorescence microscopy, one can pinpoint the location of proteins at a resolution of 20 nm or even less. However, the cellular context is often absent in these images. Recently, we developed a method for visualizing the subcellular structures in super-resolution images. Here we describe the method with two technical improvements. First, we optimize the method to preserve more fluorescence without compromising the morphology. Second, we implement ground-state depletion and single-molecule return (GSDIM) imaging, which does not rely on photoactivatable fluorescent proteins. These improvements extend the utility of nano-resolution fluorescence electron microscopy (nano-fEM).

The effects of glycol methacrylate as a dehydrating agent on the dimensional changes of liver tissue

The dimensional changes of liver sections during the course of processing with glycol methacrylate (GMA) or with ethanol are described. Tissue processing with ethanol served as a control. During prolonged processing steps (24 h each), linear shrinkage of tissue specimens dehydrated with GMA at room temperature was 13.2%. Subsequent infiltration with GMA resulted in trivial swelling, and polymerization in slight shrinkage (2.3%). In comparison, processing with cold GMA resulted in shrinkage during dehydration (about 10.8%), a slight swelling in pure GMA, followed by shrinkage during polymerization (2.2%). Short routine processing schedules resulted in similar shrinkage/swelling patterns, although precise values differed slightly. In all experiments, ethanolic dehydration resulted in smaller dimensional tissue changes than did GMA dehydration. The dimensional changes of tissue sections during stretching on water, mounting and drying compensated for the major part of the shrinkage manifested during processing.

VIS2FIX: a high-speed fixation method for immuno-electron microscopy

Immuno-transmission electron microscopy (TEM) is the technique of choice for high-resolution localization of proteins in fixed specimen. Here we introduce 2 novel methods for the fixation of sections from cryo-immobilized samples that result in excellent ultrastructural preservation. These high-speed fixation techniques, both called VIS2FIX, allow for a reduction in sample preparation time from at least 1 week to only 8 h. The methods were validated in immuno-TEM experiments on THP-1 monocytes, human umbilical vein endothelial cells (HUVECs) and Madin-Darby canine kidney (MDCK-II) cells. The fixation and retention of neutral lipids is demonstrated, offering unique prospects for the application of immuno-TEM in the lipidomics field. Furthermore, the VIS2FIX methods were successfully employed in correlative fluorescence and electron microscopy.

Protein localization in electron micrographs using fluorescence nanoscopy

A complete portrait of a cell requires a detailed description of its molecular topography: proteins must be linked to particular organelles. Immunocytochemical electron microscopy can reveal locations of proteins with nanometer resolution but is limited by the quality of fixation, the paucity of antibodies and the inaccessibility of antigens. Here we describe correlative fluorescence electron microscopy for the nanoscopic localization of proteins in electron micrographs. We tagged proteins with the fluorescent proteins Citrine or tdEos and expressed them in Caenorhabditis elegans, fixed the worms and embedded them in plastic. We imaged the tagged proteins from ultrathin sections using stimulated emission depletion (STED) microscopy or photoactivated localization microscopy (PALM). Fluorescence correlated with organelles imaged in electron micrographs from the same sections. We used these methods to localize histones, a mitochondrial protein and a presynaptic dense projection protein in electron micrographs.

Electron microscopy of viruses and virus-cell interactions

Electron microscopy is a powerful tool to visualize viruses in diagnostic as well as in research settings for investigating viral structure and virus-cell interactions. Here, a simple but efficient method is described for demonstrating viruses by negative staining, and its limit is discussed. A prerequisite to obtain reliable information on virus-cell interactions is excellent preservation of cellular and viral ultrastructure. The crux is that during fixation and embedding, by applying conventional protocols about 50% of the lipids are lost, which results in loss of integrity of cell membranes. To achieve good preservation of cellular architectures, good contrast, and both high spatial and temporal resolution, methods for freezing, freeze-substitution, and freeze-etching are described and their applicability discussed mostly taking complicated built herpes viruses as examples.

Freeze-substitution: the addition of water to polar solvents enhances the retention of structure and acts at temperatures around -60 degrees C

High-pressure freezing followed by freeze substitution and plastic embedding is becoming a more widely used method for TEM sample preparation. Here, we have investigated the influence of solvents, fixative concentrations and water content in the substitution medium on the sample quality of high-pressure frozen, freeze-substituted and plastic embedded mammalian cell culture monolayers. We found that the visibility of structural details was optimal with acetone and that extraction increased with both increasing and decreasing solvent polarity. Interestingly, the addition of water to polar solvents increased the sample quality, while being destructive when added to apolar solvents. The positive effect of water addition is saturable in acetone and ethanol at 5%(v/v), but even addition of up to 20% water has no negative effect on the sample structure. Therefore, a medium based on acetone containing fixatives and 5% water is most optimal for the substitution of mammalian cell cultures. In addition, our results suggest that the presence of water is critical for the retention of structure at temperatures around -60 degrees C.

Immunogold Labeling of Cryosections from High-Pressure Frozen Cells

Immunogold labeling of cryosections according to Tokuyasu (Tokuyasu KT. A technique for ultracyotomy of cell suspensions and tissues. J Cell Biol 1973;57:551-565), is an important and widely used method for immunoelectron microscopy. These sections are cut from material that is chemically fixed at room temperature (room temperature fixation, RTF). Lately in many morphological studies fast freezing followed by cryosubstitution fixation (CSF) is used instead of RTF. We have explored some new methods for applying immunogold labeling on cryosections from high-pressure frozen cells (HepG2 cells, primary chondrocytes) and tissues (cartilage and exocrine pancreas). As immunolabeling has to be carried out on thawed and stable sections, we explored two ways to achieve this: (1) The section fixation method, as briefly reported before (Liou W et al. Histochem Cell Biol 1996;106:41-58 and Möbius W et al. J Histochem Cytochem 2002;50:43-55.) in which cryosections from freshly frozen cells were stabilized in mixtures of sucrose and methyl cellulose and varying concentrations of glutaraldehyde, formaldehyde and uranyl acetate (UA). Only occasionally does this method reveal section areas with excellent cell preservation and negatively stained membranes like Tokuyasu sections of RTF material. (Liou et al.) (2) The rehydration method, a novel approach, in which CSF with glutaraldehyde and/or osmium tetroxide (OsO4) was followed by rehydration and cryosectioning as in the Tokuyasu method. Especially, the addition of UA and low concentrations of water to the CSF medium favored superb membrane contrast. Immunogold labeling was as efficient as with the Tokuyasu method.

New comprehension of the apicoplast of Sarcocystis by transmission electron tomography

BACKGROUND INFORMATION

Apicomplexan parasites (like Plasmodium, Toxoplasma, Eimeria and Sarcocystis) contain a distinctive organelle, the apicoplast, acquired by a secondary endosymbiotic process analogous to chloroplasts and mitochondria. The apicoplast is essential for long-term survival of the parasite. This prokaryotic origin implies that molecular and metabolic processes in the apicoplast differ from those of the eukaryotic host cells and therefore offer options for specific chemotherapeutic treatment. We studied the apicoplast in high-pressure frozen and freeze-substituted cysts of Sarcocystis sp. from roe deer (Capreolus capreolus) to get better insight in apicoplast morphology.

RESULTS AND CONCLUSIONS

We observed that the apicoplast contains four continuous membranes. The two inner membranes have a circular shape with a constant distance from each other and large-sized protein complexes are located between them. The two outer membranes have irregular shapes. The periplastid membrane also contains large-sized protein complexes, while the outer membrane displays protuberances into the parasite cytoplasm. In addition, it is closely associated with the endoplasmic reticulum by 'contact sites'.

The ultrastructure of Ignicoccus: evidence for a novel outer membrane and for intracellular vesicle budding in an archaeon

A novel genus of hyperthermophilic, strictly chemolithotrophic archaea, Ignicoccus, has been described recently, with (so far) three isolates in pure culture. Cells were prepared for ultrastructural investigation by cultivation in cellulose capillaries and processing by high-pressure freezing, freeze-substitution and embedding in Epon. Cells prepared in accordance with this protocol consistently showed a novel cell envelope structure previously unknown among the Archaea: a cytoplasmic membrane; a periplasmic space with a variable width of 20 to 400 nm, containing membrane-bound vesicles; and an outer sheath, approximately 10 nm wide, resembling the outer membrane of gram-negative bacteria. This sheath contained three types of particles: numerous tightly, irregularly packed single particles, about 8 nm in diameter; pores with a diameter of 24 nm, surrounded by tiny particles, arranged in a ring with a diameter of 130 nm; and clusters of up to eight particles, each particle 12 nm in diameter. Freeze-etched cells exhibited a smooth surface, without a regular pattern, with frequent fracture planes through the outer sheath, indicating the presence of an outer membrane and the absence of an S-layer. The study illustrates the novel complex architecture of the cell envelope of Ignicoccus as well as the importance of elaborate preparation procedures for ultrastructural investigations.

Impairment of nuclear pores in bovine herpesvirus 1-infected MDBK cells

Herpesvirus capsids originating in the nucleus overcome the nucleocytoplasmic barrier by budding at the inner nuclear membrane. The fate of the resulting virions is still under debate. The fact that capsids approach Golgi membranes from the cytoplasmic side led to the theory of fusion between the viral envelope and the outer nuclear membrane, resulting in the release of capsids into the cytoplasm. We recently discovered a continuum from the perinuclear space to the Golgi complex implying (i) intracisternal viral transportation from the perinuclear space directly into Golgi cisternae and (ii) the existence of an alternative pathway of capsids from the nucleus to the cytoplasm. Here, we analyzed the nuclear surface by high-resolution microscopy. Confocal microscopy of MDBK cells infected with recombinant bovine herpesvirus 1 expressing green fluorescent protein fused to VP26 (a minor capsid protein) revealed distortions of the nuclear surface in the course of viral multiplication. High-resolution scanning and transmission electron microscopy proved the distortions to be related to enlargement of nuclear pores through which nuclear content including capsids protrudes into the cytoplasm, suggesting that capsids use impaired nuclear pores as gateways to gain access to the cytoplasmic matrix. Close examination of Golgi membranes, rough endoplasmic reticulum, and outer nuclear membrane yielded capsid-membrane interaction of high identity to the budding process at the inner nuclear membrane. These observations signify the ability of capsids to induce budding at any cell membrane, provided the fusion machinery is present and/or budding is not suppressed by viral proteins.

Immunomolecular mapping of adherens junction and desmosomal components in normal human epidermis

Adherens junctions (AJs) are cell-cell and cell-matrix junctions that are known to comprise the transmembrane and cytoplasmic components linked to the f-actin cytoskeleton. Although the presence of AJs han been confirmed in normal human epidermis, previous studies immunolocalizing AJ-related antigens have been controversial. The purpose of this study was to produce a more precise molecular mapping of AJs and their constituents in relation to desmosomes in normal human epidermal keratinocytes. Using an electron microscope (EM) method to optimally fix plasma membranes. AJ structures were typically seen as a narrowing of the intercellular space between two keratinocytes that was distinct from desmosomes and gap junctions. Such structures were consistently found more frequently in the upper epidermis than in the basal layer. Immunogold electron microscopy showed an absence of the AJ components (E-cadherin and beta-catenin) from desmosomal areas but they were present at interdesmosomal areas at sites of close membrane association. Conversely, the desmosomal components plakoglobin and plakophilin 1 were restricted only to the outer attachment plaque of the desmosome. These results further confirm that AJs have a distinct molecular composition and distribution from desmosomes and that they regularly occur between desmosomes along the keratinocyte plasma membrane to provide alternative cell-cell adhesion mechanisms.

Histochemical and immunohistochemical protocols for routine biopsies embedded in Lowicryl resin

Tissue processing and analysis require good preservation of both the shape and content of cells. Lowicryl resin is one of the few embedding media that allow good preservation of both tissue architecture and cellular contents. Therefore, different histochemical and immunohistochemical reactions can be applied to semithin sister sections from one biopsy. Further examination of a zone of interest can be carried out under the electron microscope. The hydrophilic property of Lowicryl resins makes possible different histochemical reactions; however, the technique used for paraffin sections must be adapted for each reaction. Antigenic preservation of cells by low temperature embedding allows immunolabeling on either semithin sections or in the zone of interest on ultrathin sections. We have shown the application and adaptation of different histochemical and immunohistochemical reactions on semithin and ultrathin sections from hepatic biopsies that were large, but thin. The variety of techniques that can be used on sister Lowicryl sections of a single biopsy makes this medium useful for extensive pathological studies of precious needle biopsies.

Accumulation of a GPI-anchored protein at the cell surface requires sorting at multiple intracellular levels

Proteins modified by glycosylphosphatidylinositol membrane anchors have become popular for investigating the role of membrane lipid microdomains in cellular sorting processes. To this end, trypanosomatids offer the advantage that they express these molecules in high abundance. The parasitic protozoan Trypanosoma brucei is covered by a dense and nearly homogeneous coat composed of a glycosylphosphatidylinositol-anchored protein, the variant surface glycoprotein, which is essential for survival of the parasite in the mammalian blood. Therefore, T. brucei must possess mechanisms to selectively and efficiently deliver variant surface glycoprotein to the cell surface. In this study, we have quantified the steady-state distribution of variant surface glycoprotein by differential biotinylation, by fluorescence microscopy and by immunoelectron microscopy on high-pressure frozen and freeze-substituted samples. These three techniques provide very similar estimates of the fraction of variant surface glycoprotein located on the cell surface, on average 89.4%. The intracellular variant surface glycoprotein (10.6%) is predominantly located in the endosomal compartment (75%), while 25% are associated with the endoplasmic reticulum, Golgi apparatus and lysosomes. The density of variant surface glycoprotein in the plasma membrane including the membrane of the flagellar pocket, the only site for endo- and exocytosis in this organism, is 48-52 times higher than the density in endoplasmic reticulum membranes. The relative densities of the Golgi complex and of the endosomes are 2.7 and 10.8, respectively, compared to the endoplasmic reticulum. This data set provides the basis for an analysis of the dynamics of sorting. Depending on the intracellular itinerary of newly formed variant surface glycoprotein, the high surface density is achieved in two (endoplasmic reticulum --> Golgi complex --> cell surface) or three enrichment steps (endoplasmic reticulum --> Golgi complex --> endosomes --> cell surface), suggesting sorting between several membrane compartments.

The significance of the Golgi complex in envelopment of bovine herpesvirus 1 (BHV-1) as revealed by cryobased electron microscopy

Nucleocapsids of herpesviruses originate in the nucleus of host cells and bud through the inner nuclear membrane acquiring tegument and envelope. The release of the enveloped virus particle from the perinuclear space is unknown. Cryobased electron microscopic imaging revealed enveloped virus particles within cisterns associated with the perinuclear space, a pre-Golgi compartment connecting Golgi cisterns to the perinuclear space, and enveloped virus particles in Golgi cisterns where they are packaged into transport vacuoles by membrane fission. To our knowledge, our images show for the first time the connectivity from the perinuclear space to Golgi cisterns. The data strongly indicate an intracisternal transport of enveloped virus particles from the budding site to the packaging site. Budding starts by condensation at the inner membrane. Condensation involving the viral envelope and peripheral tegument was persistent in virus particles within perinuclear space and associated cisterns. Virus particles within Golgi cisterns and transport vacuoles originating by Golgi membrane fission, however, lacked condensation. Instead, spikes were clearly evident. The phenomenon of condensation is considered likely to be responsible for preventing fusion of the viral envelope with cisternal membranes and/or for driving virions from the perinuclear space to Golgi cisterns. Glycoprotein K is discussed to likely play a role in the intracisternal transportation of virions. In addition to the pathway including intracisternal transport and packaging, there were clear indications for the well-known pathway involving wrapping of cytoplasmic nucleocapsids by Golgi membranes. The origin of the cytoplasmic nucleocapsids, however, remains obscure. Lack of evidence for release of nucleocapsids at the outer nuclear membrane suggests that the process is very rapid, or that nucleocapsids pass the nucleocytoplasmic barrier via an alternative route.

Freeze-substitution: origins and applications

Freeze-substitution is a physicochemical process in which biological specimens are immobilized and stabilized for microscopy. Water frozen within cells is replaced by organic solvents at subzero temperatures. Freeze-substitution is widely used for ultrastructural and immunocytochemical analyses of cells by transmission and scanning electron microscopy. Less well recognized is its superiority over conventional chemical fixation in preserving labile and rare tissue antigens for immunocytochemistry by light microscopy. In the postgenome era, the focus of molecular genetics will shift from analyzing DNA sequence structure to elucidating the function of gene networks, the intercellular effects of polygenetic diseases, and the conformational rearrangements of proteins in situ. Novel strategies will be needed to integrate knowledge of chemical structures of normal and abnormal macromolecules with the physiology and developmental biology of cells and tissues from whole organisms. This review summarizes the progress and future prospects of freeze-substitution for such explorations.

Improved post-embedding immunocytochemistry of myelinated nervous tissue for electron microscopy

The particularly high lipid content of normal mature adult myelin sheaths, together with the light fixation protocols usually necessary to retain antigenicity, combine to make white matter nervous tissue an especially problematical subject for post-embedding immuno-electron microscopy using modern acrylic resins. Fixation and infiltration modifications to standard processing schedules for Lowicryl were found to greatly improve the embedding and therefore the resulting morphology. This in turn improved the signal to noise ratio by reducing the high non-specific backgrounds usually found in poorly infiltrated areas. Using Lowicryl HM20, we have been able to obtain satisfactory immunostaining for myelin basic protein with good retention of structural integrity in the myelin of both normal and lesioned adult cortico spinal tract.

Primary structure of the antigen-binding domains of a human oligodendrocyte-reactive IgM monoclonal antibody derived from a patient with multiple sclerosis

Several murine IgM monoclonal antibodies (mAbs) promoting remyelination in mice were shown to be germline gene-encoded natural autoantibodies that react with oligodendrocytes and intracellular antigens. Here, we show that human oligodendrocyte-reactive IgM mAb DS1F8 derived from a patient with multiple sclerosis targets microtubule-like structures similar to the murine mAbs. Sequencing of the cDNAs of the variable regions revealed that the antigen-binding domains are also encoded by germline genes. These similarities of mAb DS1F8 to the murine mAbs promoting remyelination suggest that this human mAb is a natural autoantibody. This may imply that the engineering of human autoantibodies for therapy of demyelinating diseases is feasible.

Resins for combined light and electron microscopy: a half century of development

The last fifty years have seen enormous improvements in the way biological specimens are prepared for microscopy. The Fifties produced the essential groundwork upon which many of our current methodologies are based. Acrylic resin embedding was introduced in 1949, with subsequent publications seeking improvements to resin formulations, embedding protocols, and modes of polymerisation. Procedures for progressive lowering of temperature processing, cryosubstitution, freeze-drying and polymerisation by ultra-violet light at low temperatures, all had their genesis in this decade of great innovation. The Sixties marked the period when the acrylics were eclipsed by the more stable and reliable epoxy resins, and much of our present-day understanding of ultrastructure was elucidated. The Seventies carried on this work with advances in technical developments concerned mainly with freezing methodologies. The beginning of the Eighties saw a resurrection of the acrylic resins, with new formulations of these resins giving reliable and stable embeddings. The low temperature and freezing methodologies pioneered in the Fifties, backed up by recent improvements to low temperature technologies, were used to further our understanding of ultrastructure and breathe new life into the science of immunocytochemistry. The remainder of the Eighties and Nineties has seen the ever increasing application of these various microscopical techniques to a wide range of biological studies. The flexibility offered by the acrylic resins in choosing between different processing, embedding and polymerisation methods has provided the impetus for detailed studies to bring to the attention of microscopists the underlying trends governing specimen preparation. Therefore, looking forward to the new Millennium, this has allowed for a more reasoned choice in organising a strategy to deal with a variety of microscopical requirements and for planning an appropriate protocol.

Immunocytochemistry of lipids: chemical fixatives have dramatic effects on the preservation of tissue lipids

We report here the effects of chemical fixatives on lipids studied under conditions simulating the immunogold labelling of phosphatidylserine. Using anti-phosphatidylserine antibodies, it is shown that the labelling intensity of a phosphatidylserine/phosphatidylcholine coating depends largely on the conditions of fixation. In fact, the usual aldehydic fixatives washed out most of the phostphatidylserine, thus preventing the binding of anti-phosphatidylserine antibodies. This was confirmed on biological samples such as rat liver and brain by measuring the loss of radiolabelled lipids during the fixation procedure. Furthermore, the complete procedure of tissue preparation for electron microscopical observation was investigated. The loss of (radiolabelled) lipids was studied in tissue samples during fixation and resin embedding. The results showed that the classical procedure (glutaraldehyde fixation followed by epoxy resin embedding) results in the loss of 73-91% of the tissue lipids whereas in unfixed, freeze-substituted samples, more than 76% of the tissue lipids are preserved.

Novel entry pathway of bovine herpesvirus 1 and 5

Herpesviruses enter cells by a yet poorly understood mechanism. We visualized the crucial steps of the entry pathway of bovine herpesvirus 1 (BHV-1) and BHV-5 by transmission and scanning electron microscopy, employing cryotechniques that include time monitoring, ultrarapid freezing, and freeze substitution of cultured cells inoculated with virus. A key step in the entry pathway of both BHV-1 and BHV-5 is a unique fusion of the outer phospholipid layer of the viral envelope with the inner layer of the plasma membrane and vice versa resulting in "crossing" of the fused membranes and in partial insertion of the viral envelope into the plasma membrane. The fusion area is proposed to function as an axis for driving the virus particle into an invagination that is concomitantly formed close to the fusion site. The virus particle enters the cytoplasm through the opened tip of the invagination, and the viral envelope defuses from the plasma membrane. There is strong evidence that the intact virus particle is then transported to the nuclear region.

Immunocytochemical localisation of actin and profilin in the generative cell of angiosperm pollen: TEM studies on high-pressure frozen and freeze-substituted Ledebouria socialis Roth (Hyacinthaceae)

Actin was demonstrated for the first time at the EM level in the generative cell of mature angiosperm pollen by using immuno-gold labelling of high-pressure frozen and freeze-substituted Ledebouria socialis Roth anthers. In addition, profilin, an actin-monomer binding protein, is shown to coexist in the generative cell. We attribute the detection of actin and profilin to the applied cryomethods which yield a much better preservation of ultrastructure and antigenicity of delicate cytoskeletal constituents than conventional fixation techniques. Actin labelling was observed within the cytoplasm of the generative cell and became especially clear in close vicinity to microtubular bundles. Filamentous structures congruent with the actin labelling patterns do occur, but are not a frequent feature. Profilin was localised throughout the cytoplasm.

Freeze-fracture, deep-etch, and freeze-substitution studies of olfactory epithelia, with special emphasis on immunocytochemical variables

Freeze-fracturing and deep-etching are a well-suited set of methods to study membrane and cytoplasmic features. Various approaches are available. Possible variables include tissue preparation, fracturing only or fracturing followed by etching, modes and materials of replication, and various ways of combining freeze-fracturing and/or deep-etching with (immuno)cytochemistry. Freeze-substitution, in particular combined with embedding in methacrylate resins such as the Lowicryls, is becoming rather widely accepted for purposes of ultrastructural (immuno)cytochemistry. Most investigators active in this field agree that this combination yields superior results compared to (immuno)cytochemistry combined with more conventional means of thin section transmission electron microscopy. Yet relatively little information is available on the variations that can occur with different approaches of freeze-substitution immunocytochemistry. This review deals with some of the variations in freeze-fracturing, freeze-etching, and freeze-substitution as applied to olfactory epithelial structures and with the effectiveness of observations obtained by application of the above sets of methods in relating the special morphology of olfactory epithelial cellular structures with those obtained by other approaches. Indeed, the data obtained continue to provide an integral image in which that morphology can be related to the special biochemistry, cell and molecular biology, and electrophysiology of olfactory epithelial structures.

Immunogold labelling of fatty acyl chains

For the first time, antibodies against a hydrophobic hapten have been used for immunogold labelling of a lipid antigen (BSA-C18:1 conjugate) coated on polystyrene. The labelling was visualised either directly in transmission electron microscopy or in light microscopy after silver enhancement. Good recognition of the fatty acyl chain was obtained even after treatment of the antigen coat with various cross-linking fixatives used for electron microscopy, i.e. formaldehyde, glutaraldehyde and osmium tetroxide.

Significance of plasmolysis spaces as markers for periseptal annuli and adhesion sites

During hyperosmotic shock, the protoplast and stretched-out peptidoglycan layer first shrink together until the turgor pressure in the cell is relieved. Being non-compressible, the outer and inner membranes must fold their superfluous surfaces. While the protoplast contracts further, the inner membrane rearranges into plasmolysis spaces visible by phase-contrast microscopy. Two opposing theories predict a similar positioning of spaces in dividing cells and filaments: the 'periseptal annulus model', based on adhesion zones, involved in the predetermination of the division site; and a 'restricted, random model', based on physical properties of the protoplast. Strong osmotic shock causes retraction of the inner membrane over almost the entire surface forming the so-called 'Bayer bridges'. These tubular adhesion sites are preserved by chemical fixation, and can be destroyed by cryofixation and freeze-substitution of unfixed cells. Both the regular positioning of the plasmolysis spaces and the occurrence of tubular adhesion sites can be explained on the basis of physical properties of the membrane which necessitate rearrangements by membrane flow during shrinkage of the protoplast.

Freeze substitution after fast-freeze fixation in preparation for immunocytochemistry

As compared to classical chemical fixation, the physical immobilization of ultrastructures by fast-freeze fixation (FFF) and the subsequent exchange of water in its solid state by freeze substitution (FS) improve the preparation procedure for immunogold labeling (IGL). FFF-FS results in a morphological preservation of unchallenged quality, as well as in a better preservation of antigenic reactivity, thus allowing remarkable precision of labeling on sections. However, FFF, particularly over a cooled metal plate, requires a heavy and expensive machine. It is not suitable for all biological specimens and in the best conditions, which remain difficult to standardize, the thickness of the well-preserved portion of the specimen does not exceed a few microns for compact tissues, and exceptionally 30-40 microns for isolated cells. The FS procedure is long and must be adjusted empirically for every new specimen and antigenic detection. The preservation of a given antigen's reactivity in the presence of fixative agents and embedding resins remains unpredictable. The action of fixative agents is different and milder in FS than when they are used classically in chemical fixation. By chance, one of the best FS procedures for the preservation of both ultrastructure and antigenicity appears to be by using acetone alone, together with a molecular sieve to improve the water exchange process. A large choice of embedding resins usually allows us to find a compromise between ultrastructural and antigenic preservation.

Impact of freeze substitution on biological electron microscopy

Considering the increasing necessity for improved preparation techniques in biological electron microscopy as a basis for the identification and localization of cellular substances within the compartments of the cell, this review is focussed on the method of freeze substitution as an important link between the cryofixation (ultrarapid freezing) and resin embedding of biological specimens. The theory and practice of freeze substitution is summarized with particular interest in the physical and thermodynamic as well as in the chemical basis of this technique. A survey of practical aspects of the technical process of freeze substitution concerning the equipment and various protocols successfully applied in biological systems is also given. The main advantage of freeze substitution versus conventional chemical fixation is seen in the maintenance of the hydration shell of molecules and macromolecular structures. This results in an improved fine structural preservation, superior retention of the antigenicity of proteins and decreased loss of unbound, diffusible cellular components. Examples of excellent visualization of the ultrastructure of macromolecular complexes (nucleic acids, extracellular material, membranes etc.), small organisms (bacteria, algae, cyanobacteria and fungi) and large biological samples such as plant and animal tissue as well as the plant-pathogen (fungus) interface and infection structures are presented. Recent data on the molecular characterization of freeze-substituted biological tissue are exemplified with special emphasis on the subcellular detection of soluble components (elements, lipids, proteins and drugs) and the inter-/intracellular localization of proteins including foreign proteins in transgenic plants. The molecular analysis of freeze-substituted specimens is achieved by the combination of low temperature preparation techniques in biological electron microscopy with various detection methods such as X-ray microanalysis, immunocytochemistry and high resolution autoradiography.

Reevaluation of envelope profiles and cytoplasmic ultrastructure of mycobacteria processed by conventional embedding and freeze-substitution protocols

The cell envelope architectures and cytoplasmic structures of Mycobacterium aurum CIPT 1210005, M. fortuitum, M. phlei 425, and M. thermoresistible ATCC 19527 were compared by conventional embedding and freeze-substitution methods. To ascertain the integrity of cells during each stage of the processing regimens, [1-14C]acetate was incorporated into the mycolic acids of mycobacterial walls, and the extraction of labeled mycolic acids was monitored by liquid scintillation counting. Radiolabeled mycolic acids were extracted by both processing methods; however, freeze-substitution resulted in the extraction of markedly less radiolabel. During conventional processing of cells, most of the radiolabel was extracted during the dehydration stage, whereas postsubstitution washes in acetone yielded the greatest loss of radiolabel during freeze-substitution. Conventional embedding frequently produced cells with condensed fibrous nucleoids and occasional mesosomes. Their cell walls were relatively thick (approximately 25 nm) but lacked substance. Freeze-substituted cells appeared more robust, with well-dispersed nucleoids and ribosomes. The walls of all species were much thinner than those of their conventionally processed counterparts, but these stained well, which was an indication of more wall substance; the fabric of these walls, in particular the plasma membrane, appeared highly condensed and tightly apposed to the peptidoglycan. Some species possessed a thick, irregular outer layer that was readily visualized in the absence of exogenous stabilizing agents by freeze-substitution. Since freeze-substituted mycobacteria retained a greater percentage of mycolic acids in their walls, and probably other labile wall and cytoplasmic constituents, we believe that freeze-substitution provides a more accurate image of structural organization in mycobacteria than that achieved by conventional procedures.

An appraisal of low-temperature embedding by progressive lowering of temperature into Lowicryl HM20 for immunocytochemical studies

The progressive lowering of temperature (PLT) method of embedding for electron microscope immunolabelling has been examined with the objective of formulating a standardized protocol which can be applied to a wide variety of samples. The methods described cover fixation, processing of samples by the PLT method, embedding in Lowicryl HM20 and subsequent immunolabelling. Each of the steps in the fixation and embedding protocol has been assessed for its potential to retain both morphology and antigenicity. Comparison of samples embedded in Lowicryl K4M and HM20 at -25 degrees C indicate an increased membrane contrast in HM20 sections, and a further improvement in morphology when samples were embedded in HM20 at -50 degrees C rather than at -25 degrees C. The results of applying the methods described are demonstrated in a range of samples of both mammalian and botanical origin, which includes solid tissues, cells in suspension, and cells grown in vitro as a monolayer culture and embedded in situ. Samples processed by this method have been immunolabelled using a wide range of antibodies recognizing nuclear, cytoplasmic, cell membrane and extracellular matrix antigens.

Freeze-substitution studies of bacteria

Typically, models of bacterial structure combine biochemical data obtained from bulk analyses of cell populations with electron microscopic observation of individual cells. Recent development of a battery of cryotechniques specific for biological electron microscopy have begun to supercede routine procedures such as conventional thin sectioning. One of these cryotechniques, freeze-substitution, combines the advantages of ultrarapid freezing with standard microtomy methods. This technique is particularly well suited to the examination of bacterial structure and has yielded additional ultrastructural information consistent with biochemical data but often challenging models of cell structure obtained from conventional microscopical methods. In addition to retaining more accurately the spatial distribution of cell components, freeze-substitution has been successfully combined with immunochemical labelling techniques and has enabled identification and localization of specific molecules both within the cell and on the cell surface. In this review, I describe current ideas on bacterial ultrastructure, modified in accordance with new data obtained from recent freeze-substitution studies.

Subcellular localization of Forssman glycolipid in epithelial MDCK cells by immuno-electronmicroscopy after freeze-substitution

Forssman antigen, a neutral glycosphingolipid carrying five monosaccharides, was localized in epithelial MDCK cells by the immunogold technique. Labeling with a well defined mAb and protein A-gold after freeze-substitution and low temperature embedding in Lowicryl HM20 of aldehyde-fixed and cryoprotected cells, resulted in high levels of specific labeling and excellent retention of cellular ultrastructure compared to ultra-thin cryosections. No Forssman glycolipid was lost from the cells during freeze-substitution as measured by radio-immunostaining of lipid extracts. Redistribution of the glycolipid between membranes did not occur. Forssman glycolipid, abundantly expressed on the surface of MDCK II cells, did not move to neighboring cell surfaces in cocultures with Forssman negative MDCK I cells, even though they were connected by tight junctions. The labeling density on the apical plasma membrane was 1.4-1.6 times higher than basolateral. Roughly two-thirds of the gold particles were found intracellularly. The Golgi complex was labeled for Forssman as were endosomes, identified by endocytosed albumin-gold, and lysosomes, defined by double labeling for cathepsin D. In most cases, the nuclear envelope was Forssman positive, but the labeling density was 10-fold less than on the plasma membrane. Mitochondria and peroxisomes, the latter identified by catalase, remained free of label, consistent with the notion that they do not receive transport vesicles carrying glycosphingolipids. The present method of lipid immunolabeling holds great potential for the localization of other antigenic lipids.

Antigenic differences between Coxiella burnetii cells revealed by postembedding immunoelectron microscopy and immunoblotting

The aim of this study was to investigate the antigenic structures of the morphologically distinct cells of the Coxiella burnetii developmental cycle. Postembedding immunoelectron microscopy with polyclonal antibodies produced in rabbits to (i) phase I cells, (ii) a chloroform-methanol residue fraction of cells, (iii) the cell walls (CW) of large and small cells and small dense cells (SDC), and (iv) the peptidoglycan-protein complexes of small cells and SDC labelled the continuum of morphologically distinct cells. But these antibodies did not distinguish between the antigenic structures of the various cells. Monoclonal antibodies to the phase I lipopolysaccharide labelled the CW of a majority of the smaller cells, but there was diminished reactivity to the larger cells. Although monoclonal antibodies to a 29.5-kDa outer membrane protein labelled the CW of the large mother cells, the large cells, and the small cells, a minority of the SDC with compact CW were not labelled. The endogenous spore within the mother cell was not labelled by the polyclonal or monoclonal antibodies to cellular components. A selected population of SDC was prepared by osmotic lysis of large cells, differential centrifugation, Renografin step-gradient fractionation, and breakage of the small cells in a French press at 20,000 lb/in2. The pressure-resistant SDC collected as fraction CL did not contain the 29.5-kDa protein, as evidenced by the lack of (i) Coomassie brilliant blue staining of protein in the 29.5-kDa region of sodium dodecyl sulfate-polyacrylamide gels and (ii) reactivity of the 29.5-kDa protein antigenic epitopes in immunoblotting with monoclonal antibodies to the protein. In contrast, CW of the pressure-sensitive small cells contained the 29.5-kDa protein. Therefore, the observed ultrastructural differences between large and small cells and SDC reflect differences in sensitivity to breakage by pressure treatment and in cell-associated antigens. Although the process of differentiation in C. burnetii remains an enigma, we have taken steps toward identifying cellular antigens as markers of differentiation. The pressure-resistant SDC in fraction CL that are devoid of the 29.5-kDa protein may be useful for answering questions about the physiological events required for triggering outgrowth and sequential regulation of the Coxiella developmental cycle.

The use of freeze-substitution and LR gold in the study of rye grass (Lolium perenne) pollen

Pollen grains of Lolium perenne (rye grass) were prepared for transmission electron microscopy by rapid freezing in liquid propane, substitution in acetone, methanol or diethyl ether, and embedment in the acrylic resin London Resin gold. These were compared to pollen chemically fixed (CF) in aldehyde/osmium tetroxide and embedded in the epoxy resin Quetol 651. Ultrastructural preservation was superior in freeze-substituted (FS) pollen, particularly with the use of acetone or methanol. Optimally preserved FS pollen displayed a homogeneous aspect of the cytoplasm and nucleoplasm, and smooth, uninterrupted contour or organelles. A striking difference was also seen in the preservation of inclusions in the intine. Varied forms and sizes of intine inclusions were evident in FS pollen but these were not discernible in the CF image. The FS scheme studied here presents enormous potential for both ultrastructural and immunolabelling studies in rye grass pollen. Problems discussed include artifacts associated with each of the substitution solvents used, and a gradient of freezing damage observed within the pollen grain.

Improved preservation of phospholipid-rich multilamellar bodies in conventionally embedded mammalian lung tissue--an electron spectroscopic study

Different conventional methods of tissue processing were studied to determine the extent to which phospholipid-rich multilamellar bodies of pulmonary alveolar epithelial type II cells of the pig were preserved. Prolonged treatment with half-saturated aqueous uranyl acetate yielded excellent results on the stabilization of the multilamellar substructure, irrespective of whether glutaraldehyde-paraformaldehyde or glutaraldehyde-tannic acid was used as a primary fixative. The lamellar periodicities were observed to be 5.5-6.1 nm. Differences in the phosphorus distribution among several types of lipid bodies of alveolar epithelial type II cells were studied by means of electron spectroscopic imaging and electron energy-loss spectroscopy. Multilamellar bodies gave phosphorus signals which were significantly higher than those obtained from granular regions of composite bodies, whereas homogeneous bodies gave phosphorus signals which were even lower than those obtained from mitochondria, endoplasmic reticulum membranes or ribosomes.

Freeze-substitution and the preservation of diffusible ions

Freeze-substitution of biological material in pure acetone followed by low-temperature embedding in the Lowicryls K11M and HM23 yields stable preparations well suited for sectioning and subsequent morphological and microanalytical studies. Transmission electron microscopy of dry-cut sections shows that diffusible cellular thallium ions (T1+) of T1(+)-loaded muscle are localized at similar protein sites in freeze-substituted as in frozen-hydrated preparations. A comparison of X-ray microanalytical data obtained from freeze-dried cryosections and sections of freeze-substituted normal (potassium-containing) muscle shows that K+ ion retention in the freeze-substituted sample is highly dependent on the freeze-substitution procedure used so far, in the best case, about 67% of the cellular K+ is retained after freeze-substitution in pure acetone and low-temperature embedding. It is concluded that the macromolecules during the preparative steps and that ion retention may be increased by further optimizing freeze-substitution and low-temperature embedding.

New aspects of bacterial ultrastructure as revealed by modern acrylics for electron microscopy

Modern acrylics can be used over a wide temperature range (+60 degrees C to -80 degrees C) for infiltration, embedding, and polymerization. They can be used in procedures involving chemical fixation or rapid freezing. This flexibility allows for studies to be carried out upon the effects that different parameters involved in preparing biological tissue for microscopy have upon structure and retention of immunoreactivity. With most preparative methods contributions have been made to our knowledge on bacterial structure in gram-negative and gram-positive cells. The future should lie in integrating the advantages of the various methods for the purpose of advancing our understanding of bacterial structure/function.

The 'Bayer bridges' confronted with results from improved electron microscopy methods

In electron micrographs of conventionally prepared thin sections of Escherichia coli one observes (i) a wavy appearance of the two membranes showing frequent appositions (named adhesion sites) and (ii) intermembrane bridges after plasmolysis which, it is claimed, occur at the adhesion sites and are related to intermembrane protein transport (transmigration). When chemical fixation is replaced by cryofixation, the observations are very different. (a) The two membranes are equally spaced and no contacts, adhesions or other sorts of connections are visible. (b) After plasmolysis the protoplast is shrunken, but the typical bridges are no longer produced. (c) In addition, when peptidoglycan is stained on conventionally prepared sections, it is revealed as a 7-nm-thick sacculus which is not interrupted at the sites of apposition. In view of the new observations, the structural concepts derived from conventionally prepared material must be revised. It is proposed that the intermembrane space is entirely filled by a gel, the outer part of which is the 7 nm thick, very stable, chemically resistant peptidoglycan (or murein). The inner part is much less stable and is proposed to undergo rapid autolytic changes upon cell death. The large 'Bayer bridges' might then tentatively be explained as an artificial post-mortem enhancement of either a stream of proteins transmigrating across the periplasm or of a pre-existing, but not yet resolved, structure. This enhancement probably occurs during the 7-10 min between plasmolysis and fixation that are prescribed for the procedure necessary for revealing 'Bayer bridges'.

Evaluation of freeze-substitution and conventional embedding protocols for routine electron microscopic processing of eubacteria

Freeze-substitution and more conventional embedding protocols were evaluated for their accurate preservation of eubacterial ultrastructure. Radioisotopes were specifically incorporated into the RNA, DNA, peptidoglycan, and lipopolysaccharide of two isogenic derivatives of Escherichia coli K-12 as representative gram-negative eubacteria and into the RNA and peptidoglycan of Bacillus subtilis strains 168 and W23 as representative gram-positive eubacteria. Radiolabeled bacteria were processed for electron microscopy by conventional methods with glutaraldehyde fixation, osmium tetroxide postfixation, dehydration in either a graded acetone or ethanol series, and infiltration in either Spurr or Epon 812 resin. A second set of cells were simultaneously freeze-substituted by plunge-freezing in liquid propane, substituting in anhydrous acetone containing 2% (wt/vol) osmium tetroxide, and 2% (wt/vol) uranyl acetate, and infiltrating in Epon 812. Extraction of radiolabeled cell components was monitored by liquid scintillation counting at all stages of processing to indicate retention of cell labels. Electron microscopy was also used to visually confirm ultrastructural integrity. Radiolabeled nucleic acid and wall components were extracted by both methods. In conventionally embedded specimens, dehydration was particularly damaging, with ethanol-dehydrated cells losing significantly more radiolabeled material during dehydration and subsequent infiltration than acetone-treated cells. For freeze-substituted specimens, postsubstitution washes in acetone were the most deleterious step for gram-negative cells, while infiltration was more damaging for gram-positive cells. Autoradiographs of specimens collected during freeze-substitution were scanned with an optical densitometer to provide an indication of freezing damage; the majority of label lost from freeze-substituted cells was a result of poor freezing to approximately one-half of the cell population, thus accounting for the relatively high levels of radiolabel detected in the processing fluids. These experiments revealed that gram-positive and gram-negative cells respond differently to freezing; these differences are discussed with reference to wall structure. It was apparent that the cells frozen first (ie., the first to contact the cryogen) retained the highest percentage of all radioisotopes, and the highest level of cellular infrastructure, indicative of better preservation. The preservation of these select cells was far superior to that obtained by more conventional techniques.