Extraction of membrane lipids during fixation, dehydration and embedding of Acholeplasma laidlawii-cells for electron microscopy

Microglia in Human Postmortem Brain Samples: Quantitative Ultrastructural Analysis of Scanning Electron Microscopy Images

In this protocol, we describe the specific steps required to prepare human postmortem brain samples for ultrastructural microglial analysis. A detailed procedure is provided to improve the ultrastructural quality of the samples, using aldehyde fixatives followed by immunoperoxidase staining of allograft inflammatory factor 1 (AIF1, also known as IBA1), a marker of myeloid cells, and cluster of differentiation 68 (CD68), a marker of phagolysosomal activity. Additionally, we describe an osmium-thiocarbohydrazide-osmium (OTO) post-fixation method that preserves and increases the contrast of cellular membranes in human postmortem brain samples, as well as the steps necessary to acquire scanning electron microscopy (SEM) images of microglial cell bodies. In the last section, we cover the quantitative analysis of various microglial cytoplasmic organelles and their interactions with other parenchymal elements.

High-Resolution Field Emission Scanning Electron Microscopy (FESEM) Imaging of Cellulose Microfibril Organization in Plant Primary Cell Walls

We have used field emission scanning electron microscopy (FESEM) to study the high-resolution organization of cellulose microfibrils in onion epidermal cell walls. We frequently found that conventional "rule of thumb" conditions for imaging of biological samples did not yield high-resolution images of cellulose organization and often resulted in artifacts or distortions of cell wall structure. Here we detail our method of one-step fixation and dehydration with 100% ethanol, followed by critical point drying, ultrathin iridium (Ir) sputter coating (3 s), and FESEM imaging at a moderate accelerating voltage (10 kV) with an In-lens detector. We compare results obtained with our improved protocol with images obtained with samples processed by conventional aldehyde fixation, graded dehydration, sputter coating with Au, Au/Pd, or carbon, and low-voltage FESEM imaging. The results demonstrated that our protocol is simpler, causes little artifact, and is more suitable for high-resolution imaging of cell wall cellulose microfibrils whereas such imaging is very challenging by conventional methods.

Electron microscopy for ultrastructural analysis and protein localization in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a key model system for studying of a multitude of cellular processes because of its amenability to genetics, molecular biology and biochemical procedures. Ultrastructural examinations of this organism, though, are traditionally difficult because of the presence of a thick cell wall and the high density of cytoplasmic proteins. A series of recent methodological and technical developments, however, has revived interest in morphological analyses of yeast (e.g. 123). Here we present a review of established and new methods, from sample preparation to imaging, for the ultrastructural analysis of S. cerevisiae. We include information for the use of different fixation methods, embedding procedures, approaches for contrast enhancement, and sample visualization techniques, with references to successful examples. The goal of this review is to guide researchers that want to investigate a particular process at the ultrastructural level in yeast by aiding in the selection of the most appropriate approach to visualize a specific structure or subcellular compartment.

Self-assembled structures formed during lipid digestion: characterization and implications for oral lipid-based drug delivery systems

There is increasing interest in the use of lipid-based formulations for the delivery of poorly water-soluble drugs. After ingestion of the formulation, exposure to the gastrointestinal environment results in dispersion and digestion processes, leading to the production of amphiphilic digestion products that form self-assembled structures in the aqueous environment of the intestine. These structures are crucial for the maintenance of drug in a solubilized state prior to absorption. This review describes the structural techniques used to study such systems, the structures formed in assembled 'equilibrium' compositions where components are combined in expected ratios representative of the endpoint of digestion, structures formed using dynamic in vitro 'non-equilibrium' digestion models where the composition and hence structures present change over time and observations from ex vivo aspirated samples. Possible future directions towards an improved understanding of the structural aspects of lipid digestion are proposed.

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.

Dual fluorescence labeling of surface-exposed and internal proteins in erythrocytes infected with the malaria parasite Plasmodium falciparum

There is a need for improved methods for in situ localization of surface proteins on Plasmodium falciparum-infected erythrocytes to help understand how these antigens are trafficked to, and positioned within, the host cell membrane. This protocol for confocal immunofluorescence microscopy combines surface antigen labeling on live cells with subsequent fixation and permeabilization, which enables antibodies to penetrate the cell and label internal antigens. The key steps of the protocol are as follows: indirect labeling of the surface antigen using a fluorescently tagged secondary antibody; fixation and permeabilization; indirect labeling of the internal antigen using a secondary antibody tagged with a spectrally distinct fluorescent dye; and detection of the differentially labeled antigens using a laser scanning confocal microscope. The protocol can be completed in approximately 7 h. Although the protocol is discussed here in the context of malaria parasite-infected cells, it can also be modified to visualize the membrane and intracellular distribution of surface and internal proteins in other eukaryotic cells.

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.

A method for visualizing surface-exposed and internal PfEMP1 adhesion antigens in Plasmodium falciparum infected erythrocytes

Background

The insertion of parasite antigens into the host erythrocyte membrane and the structure and distribution of Plasmodium falciparum adhesion receptors on that membrane are poorly understood. Laser scanning confocal microscopy (LSCM) and a novel labelling and fixation method have been used to obtain high resolution immuno-fluorescent images of erythrocyte surface PfEMP1 and internal antigens which allow analysis of the accumulation of PfEMP1 on the erythrocyte membrane during asexual development.

Methods

A novel staining technique has been developed which permits distinction between erythrocyte surface PfEMP1 and intracellular PfEMP1, in parasites whose nuclear material is exceptionally well resolved. Primary antibody detection by fluorescence is carried out on the live parasitized erythrocyte. The surface labelled cells are then fixed using paraformaldehyde and permeabilized with a non-ionic detergent to permit access of antibodies to internal parasite antigens. Differentiation between surface and internal antigens is achieved using antibodies labelled with different fluorochromes and confocal microscopy

Results

Surface exposed PfEMP1 is first detectable by antibodies at the trophozoite stage of intracellular parasite development although the improved detection method indicates that there are differences between different laboratory isolates in the kinetics of accumulation of surface-exposed PfEMP1.

Conclusion

A sensitive method for labelling surface and internal PfEMP1 with up to three different fluorochromes has been developed for laser scanning confocal optical microscopy and the analysis of the developmental expression of malaria adhesion antigens.

Immuno-localization of Fas and FasL in rat hepatic endothelial cells: influence of different fixation protocols

Immunocytochemistry has been widely used to localize molecules involved in apoptosis. In this short report, we describe with the aid of confocal laser scanning microscopy the immunolocalization of Fas and FasL on liver sinusoidal endothelial cells, and show how the localization of these two molecules differ when the cells are fixed with different fixation protocols. Methanol fixation shows diffuse staining of Fas and FasL in the cytoplasm, as well as in the nucleus. In contrast, paraformaldehyde fixation reveals the presence of Fas and FasL polarized at one side of the cell and only in the cytoplasm. After fixation with a combination of paraformaldehyde and glutaraldehyde the polarization is still present although the fluorescence is concentrated and located as bright dots in the cytoplasm. In conclusion, paraformaldehyde preserves the (nuclear) membrane-associated structures better then methanol and results in a more accurate localization of Fas and FasL. Understanding the different outcome of these common used fixation protocols will assist investigators to select the most suitable method for visualizing membrane-bound Fas and FasL.

Elafin, a secretory protein, is cross-linked into the cornified cell envelopes from the inside of psoriatic keratinocytes

Elafin is a serine proteinase inhibitor highly expressed in psoriatic epidermal keratinocytes, but expressed scarcely, if at all, in normal skin. In addition to the proteinase inhibiting domain, elafin contains multiple transglutaminase substrate domains and has been identified as a constituent of the epidermal cornified cell envelope. It also contains a signal peptide sequence, and previous immunoelectron microscopy studies detected elafin in lamellar granules and also in the intercellular spaces. It has not been explained, however, how and when elafin molecules stored in the granules are cross-linked into the cell envelope. In order to elucidate this issue, we performed pre-embedding and postembedding immunoelectron microscopy of elafin and involucrin, another cell envelope constituent, using psoriatic epidermis. Postembedding double immunoelectron microscopy revealed that elafin was within the secretory (lamellar) granules and released into the intercellular spaces when the cell envelope was not formed. In the cells with involucrin-positive cell envelope, elafin immunolabels were localized diffusely within the cells and also along the cell envelope. Pre-embedding immunoelectron microscopy of purified cell envelope from psoriatic scale samples detected involucrin and elafin colocalizing on the cytoplasmic side of the cell envelope. These findings strongly suggest that elafin-containing granules are disintegrated upon the initiation of cell envelope formation, and that elafin is cross-linked on to the involucrin-positive cell envelope from the inside of keratinocytes. It seems that psoriatic keratinocytes utilize elafin as a major component of the cell envelope, consistent with the previously proposed "precursor availability hypothesis".

Ultrastructure of guinea pig stria vascularis processed by rapid freezing and freeze substitution

The rapid-freezing and freeze-substitution method fixes a specimen as if it were prepared before excision. We used this method to study the stria vascularis of guinea pigs using electron microscopy. Findings were essentially the same as those obtained with conventional chemical fixation, although freeze substitution made it possible to observe the membrane structures in a smoother and more linear manner. This method seems to be the procedure of choice for studying the instantaneous movement and behavior of cells.

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.

Effects of acetone, methanol, or paraformaldehyde on cellular structure, visualized by reflection contrast microscopy and transmission and scanning electron microscopy

The authors recently showed variable subcellular immunoreactivity of the Bcl-2 and Bax proteins after fixation of cell monolayers with acetone, methanol, or paraformaldehyde (PF) followed by methanol (PF/methanol). Here, the authors demonstrate by reflection contrast microscopy and transmission electron microscopy that acetone or methanol fixation result in complete loss of integrity of intracellular structures in contrast with PF or glutaraldehyde fixation. Scanning electron microscopy revealed poor preservation of plasma membrane integrity after fixation in acetone or methanol. Fixation with PF before methanol reduced damage to intracellular and plasma membranes. In addition, Western blot analysis demonstrated loss of Bcl-2 and Bax protein during acetone or methanol fixation, whereas PF fixation before methanol permeabilization markedly reduced this loss. For studies on the intracellular localization of soluble or unknown types of antigen, the authors discourage the use of acetone and methanol as single fixatives.

Localization of ceramide and glucosylceramide in human epidermis by immunogold electron microscopy

Ceramides and glucosylceramides are pivotal molecules in multiple biologic processes such as apoptosis, signal transduction, and mitogenesis. In addition, ceramides are major structural components of the epidermal permeability barrier. The barrier ceramides derive mainly from the enzymatic hydrolysis of glucosylceramides. Recently, anti-ceramide and anti-glucosylceramide anti-sera have become available that react specifically with several epidermal ceramides and glucosylceramides, respectively. Here we demonstrate the detection of two epidermal covalently bound omega-hydroxy ceramides and one covalently bound omega-hydroxy glucosylceramide species by thin-layer chromatography immunostaining. Moreover, we show the ultrastructural distribution of ceramides and glucosylceramides in human epidermis by immunoelectron microscopy on cryoprocessed skin samples. In basal epidermal cells and dermal fibroblasts ceramide was found: (i) at the nuclear envelope; (ii) at the inner and outer mitochondrial membrane; (iii) at the Golgi apparatus and the endoplasmic reticulum; and (iv) at the plasma membrane. The labeling density was highest in mitochondria and at the inner nuclear membrane, suggesting an important role for ceramides at these sites. In the upper epidermis, ceramides were localized: (i) in lamellar bodies; (ii) in trans-Golgi network-like structures; (iii) at the cornified envelope; and (viii) within the intercellular space of the stratum corneum, which is in line with the known analytical data. Glucosylceramides were detected within lamellar bodies and in trans-Golgi network-like structures of the stratum granulosum. The localization of glucosylceramides at the cornified envelope of the first corneocyte layer provides further proof for the existence of covalently bound glucosylceramides in normal human epidermis.

Enhanced resolution of membranes in cultured cells by cryoimmobilization and freeze-substitution

Investigations of cellular processes demand immediate arresting of the process at any given time and excellent retention of cellular material and excellent visibility of membranes. To achieve this goal we used cryofixation to arrest cellular processes instantly and tested diverse freeze-substitution protocols. Madin-Darby kidney cells and Vero cells were grown on carbon-coated sapphire disks. For cryofixation the sapphire disks covered with a cell monolayer were injected with the aid of a guillotine into liquid propane or ethane or a mixture of both cooled by liquid nitrogen. Freezing of the cryogen was prevented by using a partially insulated cylinder and by vigorous stirring that results in a substantial decrement of the freezing point of the cryogen. Cell monolayers can be cryofixed successfully using the guillotine in a safety hood at ambient temperature and humidity or at 37 degrees C and 45% humidity. The freezing unit can also be placed in a laminar flow for working under biohazard conditions. For visualizing cell membranes at high contrast and high resolution, cells were substituted in the presence of various concentrations of glutaraldehyde and osmium tetroxide and the temperature was raised to diverse final temperatures. Substitution for 4 hours at -90 degrees C in anhydrous acetone containing 0.25% anhydrous glutaraldehyde and 0.5% osmium tetroxide followed by a temperature rise of 5 degrees C/hour to 0 degrees C and final incubation for 1 hour at 0 degrees C resulted in high contrast and excellent visibility of subcellular components at the level of the membrane bilayer. The high spatial and temporal resolution makes this methodology an excellent tool for studying cell membrane-bound processes, such as virus-cell interactions.

High-pressure freezing provides new information on human epidermis: simultaneous protein antigen and lamellar lipid structure preservation. Study on human epidermis by cryoimmobilization

Current transmission electron microscopy techniques do not permit simultaneous visualization of skin ultrastructure and stratum corneum extracellular lipids. We developed a new procedure, which entails application of high-pressure freezing followed by freeze-substitution with acetone containing uranyl acetate, followed by low temperature embedding in HM20. Electrospray ionization mass spectrometry showed that the amount of lipids lost during preparation was minimal. The ultrastructure of cryoprocessed skin was compared with that of conventionally prepared skin samples. Cryoprocessing, but not conventional processing, enabled visualization of lipid stacks within epidermal lamellar bodies, as well as the extracellular lipid domains of the stratum corneum and the ultrastructure within keratinocytes. Anti-filaggrin immunocytochemistry also showed, e.g., excellent preservation of filaggrin on cryoprocessed samples. Additionally, the cytosol of keratinocytes appeared to be organized in "microdomain"-like areas. Finally, the stratum corneum appeared more compact with smaller intercellular spaces and hence tighter cell-cell interactions, after cryoprocessing, than after conventional tissue preparation for transmission electron microscopy. We conclude here that only cryoprocessing preserves skin in a close to native state.

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.

Heterogeneous expression of Gal alpha1-3Gal xenoantigen in pig kidney: a lectin and immunogold electron microscopic study

BACKGROUND

The Gal alpha1-3Gal antigen (Gal alpha) is the primary target for human natural anti-pig xenoantibodies. The presence of Gal alpha has been shown in porcine endothelial cells (ECs) using light microscopy, whereas the expression of Gal alpha in other cell structures in the porcine kidney is only partially characterized.

METHODS

Immunogold electron microscopy of pig kidney cryosections was performed using Griffonia simplicifolia isolectin B4 and affinity isolated human anti-Gal alpha1-3Gal antibodies.

RESULTS

The most intense expression of Gal alpha was found on the apical and basolateral portions of the plasma membrane of the proximal convoluted tubule segments 1 and 2 cells, whereas segment 3 and 4 cells were negative. A strong staining was found in peritubular capillary ECs and in the inner medullary and papillary collecting duct cells. Moderate labeling of ECs and subendothelium was observed in large blood vessels, whereas glomerular ECs reacted weakly. Additionally, glomerular parietal epithelial cells, connecting tubule cells, and some cortical collecting duct cells were labeled. Among interstitial cells, a part of type-1 cells and all type-2 cells were labeled, whereas others were negative.

CONCLUSIONS

By immune electron microscopy, a detailed information of the Gal alpha antigen distribution in porcine nephrons and blood vessels has been revealed, which clarifies conflicting data obtained by light microscopy. In addition, expression of the Gal alpha antigen in the renal interstitial cells was documented for the first time. These data are of importance for the understanding of xenoantibody-mediated hyperacute rejection, for interpretation of pig kidney xenograft biopsies, and for generating transgenic pigs lacking the Gal alpha epitope.

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.

Learning about truth and biases through experience: section surface corrugation, protein denaturation, and staining

By developing and investigating the use of thin resin sections for electron microscopy, we discovered that many of the current biases became invalid: 1) No evidence could be produced that the involved organic liquids produce the protein denaturation responsible for loss of antigenicity and lowered resolution. For immunolabeling, "water-soluble" resins are not per se to be preferred over less polar and more hydrophobic ones. 2) The relief formed by the corrugated section surface enables access to the antigenic sites. The depth of relief is determined by the strength of copolymerization and depends on temperature, the chemical nature of resin and biological matter, and, thus, also on surface modification through fixatives. The stronger the relief, the better the immunolabeling and the less the image resolution. Strong copolymerization favors flatter relief, but also a hiding of antigenic sites by thin layers of resin. 3) Heavy metal stain remains the main culprit for low resolution of sections when compared to results obtained with other preparation methods.

Preparation, use, and enlargement of ultrasmall gold particles in immunoelectron microscopy

The introduction of ultrasmall (approximately 1-3 nm) colloidal gold markers in immunoelectron microscopy (IEM) in 1989 has considerably improved the sensitivity of this marker system. Ultrasmall gold markers have opened the field of pre-embedding labeling studies to gold markers without the need of harsh permeabilizing steps. They are recommended for the detection of scarce antigens in ultrathin cryosections which may otherwise escape immunodetection. However, reports concerning the preparation of ultrasmall gold colloids, their conjugation to proteins, and their use in high-resolution studies (without an additional enlargement step) are very limited. Also, the available enlargement techniques necessary for the use of this marker in conventional electron microscopy require detailed discussion to clarify the large number of contradictory observations. The present review summarizes and discusses the findings accumulated within the last 10 years on the application of ultrasmall gold markers in IEM with regard to their merits, limitations, detection sensitivity, and suitability for different labeling techniques. It should provide practical hints for the use of ultrasmall gold colloids and discusses problems arising with enlargement techniques such as silver enhancement and gold toning procedures.

Radioautographic study of the incorporation of (3H)-choline into the phospholipids of secretory ameloblasts and enamel of normal and essential-fatty-acid-deficient rats

(3H)-choline, a precursor for phosphatidylcholine (PC) and sphingomyelin (SM), was injected into rats killed after 4, 24, 48, and 96 hrs. Radioautography carried out on malachite-green/aldehyde-fixed tissues demonstrated that labeled choline was incorporated into cells and further released into the extracellular matrix. In predentin, labeling decreased rapidly, whereas in dentin, silver grains formed a stable band. In contrast, labeling was still high at 48 and 96 hrs in secretory ameloblasts as well as in the forming enamel. This indicates that ameloblasts are actively involved in the synthesis of membranes. Membrane remnants of the ameloblasts could be released into the forming enamel. In rats fed with an essential fatty-acid-deficient (EFAD) diet for 42 days, (3H)-choline uptake was delayed and reduced in pulp cells and odontoblasts, and consequently the migration of labeled phospholipids into dentin. The influence of the EFAD diet on secretory ameloblasts was limited. No difference was detected between normally fed and EFAD-fed rats in the forming enamel.

Postembedding colloidal-gold immunocytochemistry of noncollagenous extracellular matrix proteins in mineralized tissues

Immunocytochemistry is a powerful tool for investigating protein secretion, extracellular matrix assembly, and cell-matrix and matrix-matrix/mineral relationships. When applied to the tissues of bones (bone and calcified cartilage) and teeth (dentin, cementum, and enamel), where calcium phosphate-containing extracellular matrices are the predominant structural component related to their weight-bearing and masticatory roles, respectively, data from immunocytochemical studies have been prominent in advancing our understanding of mineralized tissue modeling and remodeling. The present review on the application of postembedding, colloidal-gold immunocytochemistry to mineralized tissues focuses on the advantages of this approach and relates them to conceptual, theoretical, and experimental data currently available discussing matrix-mineral interactions and extracellular matrix formation and turnover in these tissues. More specifically, data are summarized regarding the distribution and role of noncollagenous proteins in different mineralized tissues, particularly in the context of how they interface with mineral, and how this relationship might be affected by the various tissue-processing steps and immunocytochemical strategies commonly implemented to examine the distribution and function of tissue proteins. Furthermore, a technical discussion is presented that outlines several different possibilities for epitope exposure in mineralized tissues during preparation of thin sections for transmission electron microscopy. Cell biological concepts of protein secretion by cells of the mineralized tissues, and subsequent extracellular matrix assembly and organization, are illustrated by examples of high-resolution, colloidal-gold immunolabeling for osteopontin, bone sialoprotein, and osteocalcin in the collagen-based mineralized tissues and for enamel protein (amelogenin) in enamel.

Lipids in predentine and dentine

Using two histochemical methods, malachite green-aldehyde and iodoplatinate, phospholipids were visualized in the predentine of rat incisors in the spaces located between collagen fibers and in dentine as needle-like structures located along individual or groups of mineralizing collagen fibers. The same staining pattern was seen with phospholipase A2-gold. Autoradiographic investigation using 3H choline as labelled precursor, visualized the incorporation of membrane-associated and extracellular choline-containing phosphatidyl choline and sphingomyelin. The cell and membrane-associated labelling decreased gradually between 24 and 4 days, whereas incorporation of the labelled precursor as stable extracellular matrix component was seen in dentine. In addition to these investigations, pharmacologically induced (suramine) and genetically (Krabbe's disease) lysosomal storage pathology was investigated. Defects due to lipid metabolism alterations were seen in predentine and/or in dentine. The major differences visualized here between the non-mineralized and mineralized compartments and interactions between phospholipids and proteoglycans, support the view that phospholipids as matrix components play an important role in the mechanisms of dentine formation and mineralization.

Unstained and in vivo fluorescently stained bacterial nucleoids and plasmolysis observed by a new specimen preparation method for high-power light microscopy of metabolically active cells

Microscope slides were coated with a layer of gelatin, the thickness of the gelatin increasing linearly along the long axis. The bacterial suspension is applied to the dried gelatin and covered by a coverslip. The medium is absorbed by the gelatin and thus the cells applied against the coverslip. By this method, cultures of concentrations below 10(8) cells/ml provide statistically relevant numbers for observation without prior concentration steps. It is easier to apply than the existing methods for the observation of bacterial nucleoids by phase contrast imaging. Because the cells are maintained in growing conditions the method is useful for the vital fluorescence DAPI-staining of various bacterial species and for observations of plasmolysis and its reversal at different physiological conditions and extracellular osmolalities. The previously generally assumed view that the plasmolytic changes of the cell morphology are immediate upon the hyperosmotic shock and are rapidly repaired when the cell is able to metabolize actively was confirmed; this is in contrast to some recent claims.

Ultrastructural studies and immunolocalization of enamel proteins in rodent secretory stage ameloblasts processed by various cryofixation methods

BACKGROUND

Cryofixation rapidly immobilizes cell and tissue components in their native state, thereby resulting in an ultrastructural preservation very close to the living situation. We have applied this approach to examine the morphology of secretory stage ameloblasts and the distribution of enamel proteins in these cells.

METHODS

Molar and incisor tooth germs from newborn mice and/or rats were quickly dissected and divided into segments. The segments were then rapidly frozen using slam, plunge or pressure freezing, freeze-substituted and embedded in Epon. In addition, incisors from older rats were chemically fixed by vascular perfusion and also dehydrated by freeze-substitution.

RESULTS

Well-preserved ameloblasts were obtained with all four tissue processing methods. However, slam freezing often showed mechanical damage to the ameloblasts, particularly at the level of the distal portion of Tomes' processes which appeared severed or distorted. Plunging into liquid nitrogen-cooled liquid propane resulted in comparatively less tissue distortion. High pressure freezing gave a relatively higher yield of well-preserved specimens, although displacement of organelles in ameloblasts was sometimes observed, probably resulting from hydrostatic pressure. Minimal ice crystal and mechanical damage was observed in chemically fixed tooth samples processed by freeze-substitution since such specimens are cryoprotected and their examination is not restricted to a surface layer. With all of the above cryopreparation methods, the ultrastructure of well-preserved ameloblasts was, in general, similar to that obtained following conventional chemical fixation, and immunocytochemistry with an anti-amelogenin antibody indicated no profound differences in the distribution of enamel proteins.

CONCLUSIONS

These results indicate that, despite some limitations, it is possible to adequately cryofix tooth organs while preserving the architecture of ameloblasts and permitting immunolocalization of enamel proteins. Furthermore, they confirm the general morphology of secretory stage ameloblasts as currently derived from conventional chemical tissue processing.

Iodoplatinate visualization of phospholipids in rat incisor predentine and dentine, compared with malachite green aldehyde

The iodoplatinate (IP) reaction, a selective method for visualization of phospholipids, was applied to the predentine and dentine of rat incisors and compared with malachite green aldehyde (MG) fixation/staining. Spot tests indicated (1) that IP specifically stains phospholipids, but not amino acids, displaying as do phospholipids, quaternary ammonium groups; and (2) phosphatidylserine and sphingomyelin were also stained by MGA. Although this reagent is known to interact with phosphorus, phosphoproteins remained unstained. In the rat incisor, an IP-positive network including granules and thin filaments was seen in predentine in the inter-collagen spaces, in many cases closely associated with collagen fibres and their periodic striations. In dentine, positively stained needle-like structures were located along individual collagen fibres, or at the surface of groups of collagen fibres. This staining pattern was unchanged on sections of material pretreated with acetone, whereas the staining was abolished or markedly reduced when the samples were treated either with chloroform/methanol or phospholipase C prior to the IP reaction. Pretreatment of the samples with hyaluronidase promoted subsequent diffusion of the staining. A very similar staining pattern was observed with MGA, in accordance with earlier reports. The present findings validate the histochemical results reported previously on the distribution and potential role(s) of phospholipids in dentine biomineralization.

Ultrastructure of mycobacterial surfaces by freeze-substitution

The global status of tuberculosis has recently received much public attention, particularly in some developed countries that are now reporting an increase in cases after several years of decline. A number of factors have contributed to the resurgence in tuberculosis and other mycobacterial infections, including homelessness, increased urban overcrowding among the poor, increased drug abuse and the AIDS epidemic. In addition, the intrinsic nature of mycobacterial impermeability to some antibiotics, and their ability to survive in host environments has been attributed to the unique chemistry and architecture of their walls. A better understanding of these surface-related properties could in turn lead to the design of more effective chemotherapeutic agents or potential vaccine candidates. Using freeze-substitution, we offer a revised perspective on mycobacterial wall design and discuss its significance.

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.

Electron spectroscopic study (ESI, EELS) of Nanoplast-embedded mammalian lung

The potential of Nanoplast melamine resin embedding for the study of mammalian lung parenchyma was examined by means of electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS). Samples were either fixed with glutaraldehyde-paraformaldehyde or glutaraldehyde-tannic acid, or were directly transferred to the embedding medium without prior fixation. Organic dehydrants, as well as fixatives containing heavy metals and stains, were omitted. A very high level of ultrastructural detail of chromatin, ribosomes, mitochondria and plasma membranes was achieved by ESI from the Nanoplast-embedded samples. The most prominent gain in ultrastructural detail was achieved when moving from an energy loss just below the L2,3 edge of phosphorus at 132 eV to an energy loss just beyond this edge. This reflects the prominent P L2,3 edge observed by EELS of Nanoplast-embedded samples in comparison with conventionally processed samples. Thus, taking into account possible sectioning artefacts, excellent heterochromatin images which rely on the phosphorus distribution can be obtained from Nanoplast-embedded samples by computer-assisted analysis of electron spectroscopic images. In this respect glutaraldehyde-paraformaldehyde fixation is preferable to glutaraldehyde-tannic acid fixation because the presence of silicon, revealed by EELS, in tannic-acid-fixed samples may introduce artefacts in phosphorus distribution images obtained by the three-window method because of the close proximity of the L2,3 edges of silicon and phosphorus.

Localization of malachite green positive lipids in the matrix of bone nodule formed in vitro

An electron histochemical study was carried out on bone nodules formed in vitro in collagenase-released calvarial cells in order to visualize the lipid components of the extracellular matrix (EM). The malachite green aldehyde fixative technique, which allows both preservation and staining of some phospholipids of the extracellular matrix, was used. Controls were performed on sections demineralized, and then submitted to lipid extraction with a chloroformmethanol mixture (2/1 v/v) and to glycosaminoglycans digestion with 0.5% bovine testicular hyaluronidase to verify specificity for lipid staining. This allowed us to visualize the lipids (1) in the osteoid as granules associated to ribbon-like structures connected to the collagen fibers, (2) as electrondense deposits seen as dots on the outer surface membrane of the matrix vesicles, and (3) in the mineralized matrix as roundish patches formed of needle-shaped materials and at the mineralization front as individual ones. This study demonstrated that at the EM level, the lipids are present in the osteoid at locations very similar to what have been observed for the glycosaminoglycans, and in the mineralized matrix as components of the crystal ghosts.

Medium temperature epoxy resin for immunocytochemistry: Quetol 651 with water

The addition of 1% water to the epoxy resin Quetol increased the labeling intensity of the sample. The significant decrease of the curing temperature of the epoxy resin may assist in preservation of antigens. Water may also reduce the cross-linkage of the resin allowing more antigen to be available to the antibodies. The modified Quetol resin is an option for use in immunocytochemistry studies.

A double surface membrane in plerocercoids of Ligula intestinalis (Cestoda: Pseudophyllidea)

The tegument of Ligula intestinalis plerocercoids is delimited by a membrane complex that in electron microscopy appears heptalaminate. We suggest that the plerocercoids are covered by two closely apposed lipid bilayers. Double membranes, which are well known in schistosomes, are thus not a unique feature of blood parasitic Digenea but could be documented for the first time in a cestode species that lives in the peritoneal cavity of its host. The surface-membrane complex of plerocercoids was lost for the most part after conventional preparation for electron microscopy but could be completely retained by improved fixation using osmium tetroxide plus potassium ferrocyanide. Furthermore, one type of vesicle in the tegumental syncytium of plerocercoids has been found to be enclosed by at least two membranes, which might indicate that these vesicles contribute to the renewal of the surface-membrane complex. Adult Ligula intestinalis removed from the gut of the final host Anas platyrhnychos or obtained by in vitro transformation exhibited a single surface membrane and lacked double membrane vesicles. The elongate electron-dense caps of the microtriches of plerocercoids were replaced by short caps in the course of worm maturation. Thus, the tegumental surface of this parasite is fundamentally altered following the change in its environment from the peritoneal cavity of the intermediate host to the intestine of the final host.

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.

Effects of monomeric acrylic embedding media on the antigenicity of two epitopes of the MIC2-encoded Ewing's sarcoma cell membrane antigen

Comparative electron microscopic studies of pre- and postembedding immunolabeling experiments have shown that the antigenicity of some epitopes is lost during acrylic resin embedding of the respective tissues. In the present investigation we have tested the sensitivities of two embedding-labile epitopes (HBA-71 and HBA-45) of the Ewing's sarcoma-associated MIC2-encoded E2 antigen to the effects of the different treatment steps, which are necessary for the preparation of ultrathin sections. The extent of antigenic retention was quantitated using flow cytometry and enzyme-linked immunosorbent assays (ELISA) of tumor cell lines, thymocytes and cell membrane extracts. Fixation, dehydration and high temperature treatment of MIC2-positive cells showed only minor effects on the reactivity with the HBA-71 and HBA-45 antibodies. However, exposure of the cells to the monomeric acrylic resins LR White (LRW), LR Gold (LRG) and Lowicryl K4M at 4 degrees C for 2-18 h resulted in a significant reduction of the HBA-71 and HBA-45 reactivities. In contrast, the antigenicity of both epitopes was maintained during treatment with the apolar Lowicryl HM20 embedding medium under these non-polymerizing conditions. The resins have no direct effect on the HBA-71/HBA-45 antigen, since it could be extracted in intact form from membranes of native, but not of fixed, tumour cells using LRW for membrane solubilization. These data indicate that the HBA-71/HBA-45 antigen remains in the cell membrane and is indirectly influenced by the extraction/modification of adjacent membrane constituents. The adverse effects of the polymerization process, in the case of embedding at low temperature in Lowicryl HM20, destroyed MIC2-antigenicity.(ABSTRACT TRUNCATED AT 250 WORDS)

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 of gram-negative eubacteria: general cell morphology and envelope profiles

Freeze-substitution was performed on strains of Escherichia coli, Pasteurella multocida, Campylobacter fetus, Vibrio cholerae, Pseudomonas aeruginosa, Pseudomonas putida, Aeromonas salmonicida, Proteus mirabilis, Haemophilus pleuropneumoniae, Caulobacter crescentus, and Leptothrix discophora with a substitution medium composed of 2% osmium tetroxide and 2% uranyl acetate in anhydrous acetone. A thick periplasmic gel ranging from 10.6 to 14.3 nm in width was displayed in E. coli K-12, K30, and His 1 (a K-12 derivative containing the K30 capsule genes), P. multocida, C. fetus, P. putida, A. salmonicida, H. pleuropneumoniae, and P. mirabilis. The other bacteria possessed translucent periplasms in which a thinner peptidoglycan layer was seen. Capsular polysaccharide, evident as electron-dense fibers radiating outward perpendicular to the cell surface, was observed on E. coli K30 and His 1 and P. mirabilis cells. A more random arrangement of fibers forming a netlike structure was apparent surrounding cells of H. pleuropneumoniae. For the first time a capsule, distinct from the sheath, was observed on L. discophora. In all instances, capsular polysaccharide was visualized in the absence of stabilizing agents such as homologous antisera or ruthenium red. Other distinct envelope structures were observed external to the outer membrane including the sheath of L. discophora and the S layers of A. salmonicida A450 and C. crescentus CB15A. We believe that the freeze-substitution technique presents a more accurate image of the structural organization of these cells and that it has revealed complex ultrastructural relationships between cell envelope constituents previously difficult to visualize by more conventional means of preparation.

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.

Effect of chemical fixatives on accurate preservation of Escherichia coli and Bacillus subtilis structure in cells prepared by freeze-substitution

Five chemical fixatives were evaluated for their ability to accurately preserve bacterial ultrastructure during freeze-substitution of select Escherichia coli and Bacillus subtilis strains. Radioisotopes were specifically incorporated into the peptidoglycan, lipopolysaccharide, and nucleic acids of E. coli SFK11 and W7 and into the peptidoglycan and RNA of B. subtilis 168 and W23. The ease of extraction of radiolabels, as assessed by liquid scintillation counting during all stages of processing for freeze-substitution, was used as an indicator of cell structural integrity and retention of cellular chemical composition. Subsequent visual examination by electron microscopy was used to confirm ultrastructural conformation. The fixatives used were: 2% (wt/vol) osmium tetroxide and 2% (wt/vol) uranyl acetate; 2% (vol/vol) glutaraldehyde and 2% (wt/vol) uranyl acetate; 2% (vol/vol) acrolein and 2% (wt/vol) uranyl acetate; 2% (wt/vol) gallic acid; and 2% (wt/vol) uranyl acetate. All fixatives were prepared in a substitution solvent of anhydrous acetone. Extraction of cellular constituents depended on the chemical fixative used. A combination of 2% osmium tetroxide-2% uranyl acetate or 2% gallic acid alone resulted in optimum fixation as ascertained by least extraction of radiolabels. In both gram-positive and gram-negative organisms, high levels of radiolabel were detected in the processing fluids in which 2% acrolein-2% uranyl acetate, 2% glutaraldehyde-2% uranyl acetate, or 2% uranyl acetate alone were used as fixatives. Ultrastructural variations were observed in cells freeze-substituted in the presence of different chemical fixatives. We recommend the use of osmium tetroxide and uranyl acetate in acetone for routine freeze-substitution of eubacteria, while gallic acid is recommended for use when microanalytical processing necessitates the omission of osmium.

Niemann-Pick disease

Results of the investigation carried out during this decade brought unambigous evidence of biochemical heterogeneity inside the complex of Niemann-Pick disease according to which two entirely different metabolic disorders can be recognized. 1. Niemann-Pick sphingomyelinosis, a clear-cut enzymopathy, the pivotal lesion of which is the deficiency of lysosomal spingomyelinase leading to widespread lysosomal deposition of sphingomyelin liquid crystals. Two main allelic variants are known. The first one, neuronopathic (former type A) known as infantile with rapid course, may also manifest considerably prolonged course or an atypical course with predominantly visceral symptomatology. Patients with the second, visceral, variant (former type B), display mainly slow clinical course and often reach adulthood. With rare exceptions the neuronopathic variant can be biochemically recognized from the visceral one by much lower values of the in vivo sphingomyelin degradation test in the former. 2. The rest of the complex comprising types C-D differs substantially from the sphingomyelinase deficiency group by the remarkable heterogeneity in the lysosomal stored lipid pattern given by differences among the affected cell populations. Sphingomyelin storage could be proved histochemically solely in the histiocytic population together with cholesterol, neutral glycosphingolipids and lysobisphosphatidic acid, whereas the brain neurons displayed only neutral glycosphingolipid storage. There is an increasing evidence of the crucial biochemical lesion in this group being an altered intracellular traffic of exogenously derived cholesterol caused probably by its deficient translocation from lysosomes to other intracellular membrane sites. This leads to decreased cholesterol esterification rate which is the basis of the newly developed diagnostic test. Inconstant depression of sphingomyelinase activity is considered to be a secondary phenomenon. The so-called lactosylceramidosis is a rare variant pertinent to this group. The biochemical nature of type E still awaits clarification. Both groups of Niemann-Pick disease display clinical and especially histochemical features which allows to establish diagnosis in a highly efficient way already at the clinicopathological level.

Improved chondrocyte morphology and glycogen retention in the secondary center of ossification following osmium-potassium ferrocyanide fixation

The use of osmium-potassium ferrocyanide as the secondary fixative greatly improved chondrocyte preservation and stabilized the cartilage matrix proteoglycan. The proteoglycan was similar in appearance to that seen following fixation in the presence of cationic dyes. Extensive glycogen preservation was noted in these cells, occupying the area prior to and during the formation of the secondary center of ossification. The volume and organization of the glycogen within the cell cytoplasm were greater than that following buffered osmium fixation, and the cellular vacuoles within were greatly reduced. The cells forming the secondary center prior to the onset of mineralization were of greatest interest, because other studies compared them with the primary growth plate and described them as showing signs of hypertrophy as early as 5 days postnatally, as is found in the primary growth plate. Our observations indicate that glycogen is present in these cells, and cellular enlargement was not present. The cells do not resemble the hypertrophic chondrocytes of the primary growth plate, as far as cytoplasmic content is concerned, and we suggest that they may contribute to the development of the secondary center in a different fashion.