Advantages of fast-freeze fixation followed by freeze-substitution for the preservation of cell integrity

Preserving elemental content in adherent mammalian cells for analysis by synchrotron-based x-ray fluorescence microscopy

Trace metals play important roles in biological function, and x-ray fluorescence microscopy (XFM) provides a way to quantitatively image their distribution within cells. The faithfulness of these measurements is dependent on proper sample preparation. Using mouse embryonic fibroblast NIH/3T3 cells as an example, we compare various approaches to the preparation of adherent mammalian cells for XFM imaging under ambient temperature. Direct side-by-side comparison shows that plunge-freezing-based cryoimmobilization provides more faithful preservation than conventional chemical fixation for most biologically important elements including P, S, Cl, K, Fe, Cu, Zn and possibly Ca in adherent mammalian cells. Although cells rinsed with fresh media had a great deal of extracellular background signal for Cl and Ca, this approach maintained cells at the best possible physiological status before rapid freezing and it does not interfere with XFM analysis of other elements. If chemical fixation has to be chosen, the combination of 3% paraformaldehyde and 1.5 % glutaraldehyde preserves S, Fe, Cu and Zn better than either fixative alone. When chemically fixed cells were subjected to a variety of dehydration processes, air drying was proved to be more suitable than other drying methods such as graded ethanol dehydration and freeze drying. This first detailed comparison for x-ray fluorescence microscopy shows how detailed quantitative conclusions can be affected by the choice of cell preparation method.

A simple cryo-holder facilitates specimen observation under a conventional scanning electron microscope

A pre-cryogenic holder (cryo-holder) facilitating cryo-specimen observation under a conventional scanning electron microscope (SEM) is described. This cryo-holder includes a specimen-holding unit (the stub) and a cryogenic energy-storing unit (a composite of three cylinders assembled with a screw). After cooling, the cryo-holder can continue supplying cryogenic energy to extend the observation time for the specimen in a conventional SEM. Moreover, the cryogenic energy-storing unit could retain appropriate liquid nitrogen that can evaporate to prevent frost deposition on the surface of the specimen. This device is proved feasible for various tissues and cells, and can be applied to the fields of both biology and material science. We have employed this novel cryo-holder for observation of yeast cells, trichome, and epidermal cells in the leaf of Arabidopsis thaliana, compound eyes of insects, red blood cells, filiform papillae on the surface of rat tongue, agar medium, water molecules, penicillium, etc. All results suggested that the newly designed cryo-holder is applicable for cryo-specimen observation under a conventional SEM without cooling system. Most importantly, the design of this cryo-holder is simple and easy to operate and could adapt a conventional SEM to a plain type cryo-SEM affordable for most laboratories.

The fine structure of the Acanthamoeba polyphaga cyst wall

Members of the genus Acanthamoeba are present in diverse environments, from freshwater to soil, and also in humans, causing serious brain and corneal infections. Their life cycle presents two stages: the dividing trophozoite and the quiescent cyst. The structures of these life stages have been studied for many years, and structural data have been used for taxonomy. The ultrastructural work on Acanthamoeba cysts was carried out previously by routine transmission electron microscopy (TEM), a process that requires the use of chemical fixation, a procedure that can cause serious artifacts in the ultrastructure of the studied material. In order to prevent fixation artifacts, we processed Acanthamoeba polyphaga cysts by ultrarapid freezing, followed by freeze-fracturing and deep-etching, in order to obtain a 3D visualization of the arrangements of the cyst wall. The exocyst presented an irregular surface, with vesicles located within or near this layer. The endocyst, instead, showed a biphasic arrangement with a more compact district in its innermost part, and a more loosened outer layer. For this reason, it was difficult to distinguish the filaments present in the intercyst space from those forming the endocyst. Surprisingly, the intercyst space was thinner when compared with samples processed by conventional TEM, evidencing the possible damage consequent to the use of chemical fixation.

Preparation techniques for transmission electron microscopy of Hydra

Hydra is a classical model organism in developmental and cell biology with a simple body plan reminiscent of a gastrula with one body axis and a limited number of cell types. This rather simple organism exhibits a regeneration capacity that is unique among all eumetazoans and is largely dependent on the stem cell properties of its epithelial stem cell population. Molecular work in the past few years has revealed an unexpected genetic complexity of these simple animals, making them an interesting model for studying the generation of animal form and regeneration. In addition, Hydra has an interstitial stem cell system with a unique population of nematocytes, neuronal cells that are characterized by an explosive exocytotic discharge. Here, we compare classical and modern transmission electron microscopy (TEM) fixation protocols including protocols for TEM immunocytochemistry (post-embedding immunogold labeling). We presume that TEM studies will become an important tool to analyze cell-cell interactions as well as cell matrix interrelationships in Hydra in the future.

Chapter 6: cubic membranes the missing dimension of cell membrane organization

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between “inside” and “outside”, survival, and death. Intracellular compartmentalization—governed by biomembranes as well—is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into “unusual”, highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Methods for studying the nuclei and chromosomes of dinoflagellates

Dinoflagellates are unicellular eukaryotic organisms whose nuclear structure, chromosome architecture, chromatin organization, DNA composition, and mitosis show original features. It has been necessary to adapt techniques and to create innovative methods for growing cells, isolating nuclei, and studies of their chromosomes by transmission electron microscope (TEM). Among these are innovative squash and whole-mount preparations for light and TEM observations of chromosome architecture and the spatial organization of nucleofilaments. Particular attention was given to adapt high-pressure freezing (fast-freeze fixation) techniques for the best preservation of delicate antigenic sites, and good immunodetection. The study of DNA replication with or without incorporation of bromodeoxyuridine (BrdU) was also refined to use confocal laser scanning microscopy. In this chapter, we describe methods that we have invented and/or improved from existing techniques in order to better understand this fragile chromosome architecture and the mechanisms intervening during mitosis and the cell cycle. These methods allowed us to detect specific DNA-binding proteins and the distribution of B-and Z-DNA in chromosomes during the cell cycle and mitosis, and to focus on the indissoluble link between chromosome structure and function.

Spatial profiling invertebrate ganglia using MALDI MS

The ability of MALDI TOF MS to spatially map peptides and proteins directly from a tissue is an exciting advance to imaging mass spectrometry. Recent advances in instrumentation for MS have resulted in instruments capable of achieving several micron spatial resolution while acquiring high-resolution mass spectra. Currently, the ability to obtain high quality mass spectrometric images depends on sample preparation protocols that often result in limited spatial resolution. A number of sample preparation and matrix deposition protocols are evaluated for spatial profiling of Aplysia californica exocrine gland and neuronal tissues. Such samples are different from mammalian tissues, but make good targets for method optimization because of the wealth of biochemical information available on neuropeptide processing and distribution. Electrospray matrix deposition and a variety of freezing methods have been found to be optimum for these invertebrate tissues, with the exact protocols being tissue dependent.

Immunohistochemical identification of multipotent adult progenitor cells from human bone marrow after transplantation into the rat brain

In the immunohistochemical analysis of the brain, tissue preparation and fixation are critical steps. During our studies of transplanting human bone marrow multipotent adult progenitor cells (hMAPCs) into the rat brain, we noticed that various methods of brain tissue preparation and fixation differentially influenced antigenic preservation in the transplants and in the host brain. Here, we report a simple, effective and reproducible method of tissue preparation and fixation that results in the immunohistochemical labeling of transplanted and host cells.

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.

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.

Contribution of cryofixation and freeze-substitution to analytical microscopy: a study of Tritrichomonas foetus hydrogenosomes

The hydrogenosome, an organelle that produces molecular hydrogen and ATP from the oxidation of pyruvate or malate under anaerobic conditions, presents some characteristics common to mitochondria. It is found in several trichomonad species, protists living in oxygen-poor environments, as well as certain free-living ciliates, rumen ciliates, and some fungi. We performed a comparative microanalytical study (energy dispersive X-ray analysis and electron spectroscopic imaging) of different fixation methods for electron microscopy analyzing hydrogenosomes of the bovine parasite Tritrichomonas foetus. The study included the elemental composition and the mapping of calcium, phosphorus, and oxygen. A preparation of T. foetus cells, based on cryoimmobilization by high-pressure freezing and freeze-substitution, was compared to a second preparation based on chemical fixation followed by dehydration and routine processing. The ultrastructural preservation achieved by the cryotechnique was far superior to the chemical fixation, since it allowed the successful cryoimmobilization of intracellular ion contents. The detection of several cations (Al, Mg, Co, Ca, Fe) by X-ray microanalysis inside the peripheral vesicle of the hydrogenosome was only possible in cryofixed cells. The presence of aluminum and cobalt ion in the hydrogenosomal vesicle was established for the first time. Electron-spectroscopic images of calcium showed that this element, in addition to the vesicle compartment, is present in the hydrogenosome's membrane in varying concentrations, which might reflect changes in the physiology of this organelle.

Characterization of p80, a novel nuclear and cytoplasmic protein in dinoflagellates

The presence of myosin in dinoflagellates was tested using an anti-Acanthamoeba castellanii myosin II polyclonal antibody on the heterotrophic dinoflagellate Crypthecodinium cohnii Seligo. Western blots revealed the presence of a unique band of 80 kDa in total protein extracts and after immunoprecipitation. Expression of this 80 kDa protein appeared constant during the different phases of the cell cycle. In protein extracts from various other dinoflagellates, this 80 kDa protein was detected only in the autotrophic species Prorocentrum micans Ehr. Screening of a C. cohnii cDNA expression library with this antibody revealed a cDNA coding for an amino acid sequence without homology in the databases. However, particular regions were detected: - a polyglutamine repeat domain in the N-terminal part of the protein, - four peptide sequences associated with GTP-binding sites, - a sequence with slight homology to the rod tail of Caenorhabditis elegans myosin II, -a sequence with homology to a human kinesin motor domain. Immunocytolocalization performed on C. cohnii thin sections with a polyclonal antibody raised against the recombinant protein showed p80 to be present both within the nucleus and in the cytoplasm. Labelling was widespread in the nucleoplasm and more concentrated at the periphery of the permanently condensed chromosomes. In the cytoplasm, labelling appeared in a punctate region close to the nucleus and in the flagellum. Potential functions of this novel protein are discussed.

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.

Scanning electron microscopic study of the renal glomerulus by an in vivo cryotechnique combined with freeze-substitution

The 3-dimensional ultrastructure of mouse renal glomeruli under normal haemodynamic conditions was studied by scanning electron microscopy using an in vivo cryotechnique followed by freeze-substitution, and compared with glomeruli prepared by conventional fixation methods. Mouse kidneys were frozen with a cryoknife apparatus and a liquid isopentane-propane mixture (-193 degrees C). Surface areas of the frozen tissues were freeze-fractured with a scalpel in liquid nitrogen. The specimens were routinely freeze-substituted, freeze-dried, ion-sputtered, and then observed in a scanning electron microscope at an accelerating voltage of 5 kV. Renal glomeruli showed good ultrastructural preservation of the surface tissues. Podocytes with interdigitating foot processes covering capillary loops exhibited smooth surface contours and their cell surfaces were arranged more tightly than those seen by the conventional fixation method. Filtration slits between foot processes were found to be narrow. The internal structure of the glomerular tuft was seen in the freeze-fracture faces. The capillary lumen with variously shaped erythrocytes was kept open in frozen glomeruli under normal blood circulation conditions. The ultrastructure of renal glomeruli, as revealed by the in vivo cryotechnique with freeze-substitution, appears to be closer to that of the living state.

The spindle pole body of Schizosaccharomyces pombe enters and leaves the nuclear envelope as the cell cycle proceeds

The cycle of spindle pole body (SPB) duplication, differentiation, and segregation in Schizosaccharomyces pombe is different from that in some other yeasts. Like the centrosome of vertebrate cells, the SPB of S. pombe spends most of interphase in the cytoplasm, immediately next to the nuclear envelope. Some gamma-tubulin is localized on the SPB, suggesting that it plays a role in the organization of interphase microtubules (MTs), and serial sections demonstrate that some interphase MTs end on or very near to the SPB. gamma-Tubulin is also found on osmiophilic material that lies near the inner surface of the nuclear envelope, immediately adjacent to the SPB, even though there are no MTs in the interphase nucleus. Apparently, the MT initiation activities of gamma-tubulin in S. pombe are regulated. The SPB duplicates in the cytoplasm during late G2 phase, and the two resulting structures are connected by a darkly staining bridge until the mitotic spindle forms. As the cell enters mitosis, the nuclear envelope invaginates beside the SPB, forming a pocket of cytoplasm that accumulates dark amorphous material. The nuclear envelope then opens to form a fenestra, and the duplicated SPB settles into it. Each part of the SPB initiates intranuclear MTs, and then the two structures separate to lie in distinct fenestrae as a bipolar spindle forms. Through metaphase, the SPBs remain in their fenestrae, bound to the polar ends of spindle MTs; at about this time, a small bundle of cytoplasmic MTs forms in association with each SPB. These MTs are situated with one end near to, but not on, the SPBs, and they project into the cytoplasm at an orientation that is oblique to the simple axis. As anaphase proceeds, the nuclear fenestrae close, and the SPBs are extruded back into the cytoplasm. These observations define new fields of enquiry about the control of SPB duplication and the dynamics of the nuclear envelope.

Ultrastructure of quick-frozen and freeze-substituted chick osteoclasts

For comparison with chemically fixed osteoclasts, we prepared chick osteoclasts by quick freezing followed by freeze-substitution. In spite of technical difficulties this demonstrated that osteoclasts can be satisfactorily frozen in situ by the metal contact method. Ultrastructural differences were revealed between conventional fixation and quick freezing. Compared with conventional fixation, the quick freezing method appeared to improve preservation: (1) a discrete trilaminar plasma membrane and other intracellular membranes showed a smooth profile without undulation or rupture; (2) cytoskeletal components appeared to be clearer, straighter, and more numerous; (3) the interior of the ruffled finger contained interconnected lattice structures whereas highly organised microfilaments were seen in the clear zone; (4) well developed tubulovesicular structures (TVSs) that branched or anastomosed with each other were revealed in the cytoplasm; (5) the contents of intracellular membrane systems including the nuclear envelope, endoplasmic reticulum, and Golgi complex were stained to a various extent; (6) vesicles and vacuoles were much smaller, round and well-defined with electron-dense contents; (7) crystalline structures were seen at the extracellular channels of the ruffled border, in the lumen of TVSs, and in vesicles; (8) in some instances mitochondrial granules were visible; (9) within the resorptive lacuna, osteoclasts adhered to the degraded bone matrix without any intervening empty space.

In situ localization of two fibrillar collagens in two compact connective tissues by immunoelectron microscopy after cryotechnical processing

Two fibrillar collagens, the worm cuticular collagen and the vertebrate Type I fish scale collagen, both organized in a compact tissue, were localized by immunogold electron microscopy in resin sections after freeze-fixation and freeze-substitution. Identification of these two fibrillar collagens failed with the use of postembedding labelling after conventional electron microscopic processing. Positive labeling of the Type I collagen was observed in sections of fish scales freeze-fixed by either slam-freezing or high-pressure freezing, freeze-substituted in acetone with or without osmium tetroxide, and embedded in LR White. The worm cuticular collagen was detected in sections of cuticle that were freeze-fixed, freeze-substituted (necessarily with osmium tetroxide added to acetone), and embedded in either LR White or Epon. It was also detected in specimens pre-fixed by aldehydes before freeze-fixation. The Type I fish scale collagen appears to be more sensitive than the fibrillar cuticular collagen of worms to the procedures employed for postembedding immunoelectron microscopy. Our results have shown that freeze-fixation and freeze-substitution preserved the antigenicity of the fibrillar collagens organized in a compact three-dimensional network, whereas immunolabeling failed after conventional electron microscopic procedures. These cryostabilization techniques appear to be of value to improve the immunolocalization of collagens.

Comparison of high-pressure freezing/freeze substitution and chemical fixation of catfish barbel taste buds

The barbel taste buds of catfish are widely used as a model system for investigating the structure and function of vertebrate taste buds. We have examined the ultrastructure of the taste buds of the channel catfish, Ictalurus punctatus, as part of a comparative study of the morphology of taste buds in various mammalian and non-mammalian vertebrates. Since conventional chemical fixation methods have limited usefulness for certain kinds of ultrastructural studies (i.e., localization of diffusible substances or labeling techniques requiring retention of biological activity), we have developed methods for fixing catfish barbel taste buds by high-pressure freezing followed by freeze-substitution (HPF/FS) and have compared the ultrastructure of taste buds fixed by this technique and by chemical fixation procedures. The morphological details of the cells within taste buds are significantly affected by the method of fixation employed. In general, membrane contours are smoother and intracellular organelles more regular in shape in HPF/FS samples as compared with the chemically fixed specimens. Absolute and relative electron-densities of various tissue components are also affected by the fixation method employed. Certain ultrastructural features are more clearly visualized by one or the other of the fixation procedures. Fixation of barbel taste buds by HPF/FS not only provides an alternative view of the ultrastructure of taste bud cells but also offers a method of tissue preparation that may prove to be preferable to chemical methods for ultrastructural investigations involving procedures such as immunohistochemical labeling.

Cryofixation, cryosubstitution, and immunoelectron microscopy: potential role in diagnostic pathology

In immunoelectron microscopic investigations, retention of antigenic sites is crucial. Methods for preparing samples for conventional electron microscopy involve chemical fixatives followed by dehydration in organic solvents and embedding in plastic resins, all procedures potentially detrimental to antigenicity. Cryomethods provide a physical fixation alternative for the preparation of biological samples for ultrastructural, immunocytochemical, and microanalysis studies without the use of any chemicals. This can be particularly useful in diagnostic pathology, providing an alternative to conventional fixation methods which sometimes destroy the antigen in question. The recent development of a portable cryofixation device, the PS1000 Portable Metal Mirror Ultra-Rapid Cryofixation Unit (Delaware Diamond Knives, Inc., Wilmington, DE, USA), provides an opportunity to freeze tissue immediately after procurement for use in diagnostic immunocytochemistry studies. This feasibility study examined the quality of tissue preservation with this device, in terms of both preservation of cellular ultrastructure and immunolabeling. Human tonsil and thymus tissue was slam frozen and, after cryosubstitution in Lowicryl K11M, was examined by immunoelectron microscopy. Good ultrastructural preservation was obtained and reasonable immunolabeling with antibodies to AE1/AE3 keratin filaments was also observed.

Dynamic structure of glomerular capillary loop as revealed by an in vivo cryotechnique

Morphological studies using immersion or perfusion fixation methods do not reveal the ultrastructure of functioning kidneys with normal circulation. A simple apparatus was developed for freezing the kidneys in vivo without stopping the blood supply, and the ultrastructure of the glomerular capillary loops was examined under different haemodynamic conditions. Mouse kidneys were frozen under normal blood flow conditions; others were frozen in the same way after ligation of the abdominal aorta at a point caudal to the renal arteries. They were then processed for the freeze-substitution or deep-etching method. Good ultrastructural preservation was obtained within about 5 microM depth from the frozen tissue surface. Functioning glomeruli with normal blood flow possessed open capillary lumens, different shapes of foot processes and atypical basement membranes with low density. Moreover, heterogeneity in width between foot processes was identified on the replica membranes. Under the acute conditions used to increase blood supply into the kidneys, the spaces between the flat foot processes became more widely dilated and the basement membrane was seen to be three-layered. The ultrastructure of glomeruli in functioning kidneys has been demonstrated for the first time by this "in vivo cryotechnique."

Cell biology of kidney glomerulus

It has been accepted that some artifacts are inevitably produced by the conventional preparation steps for electron microscopy, including fixation, dehydration, embedding, ultrathin sectioning, and staining. Therefore, conventional ultrastructural findings on kidney glomeruli are hardly thought to be correlated with the physiological functions of kidneys in vivo. In this chapter, two preparation techniques, the quick-freezing and deep-etching (QF-DE) method or the quick-freezing and freeze-substitution (QF-FS) method, are presented and shown to be useful for clarifying the ultrastructures of kidney glomeruli more closely to structures in vivo with fewer artifacts. Moreover, the ultrastructures of glomerular capillary loops have been demonstrated by a new "in vivo cryotechnique," that shows that hemodynamic factors should be considered in the morphological study of glomerular functions.

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.

Freeze-fracture study of Bodo sp. (Kinetoplastida: Bodonidae)

Freeze-fracture technique was used to analyse the structure of conventionally fixed and quickly frozen Bodo sp., a free-living kinetoplastid. In the former method, chemically fixed and cryopreserved cells presented a corrugated membrane pattern in the flagella and cell body surfaces. In the latter, however, replicas from quickly frozen unfixed flagellates showed membranes with a smoother aspect, allowing the observation of intramembranous particles (IMPs) on the fracture faces, hardly detectable in previously fixed samples. The IMPs were randomly distributed throughout the cell surface, except in the sparsely seen short IMP rows.

Fine structure of Tritrichomonas foetus as seen using cryotechniques

Tritrichomonas foetus was studied using different physical and chemical fixation methods such as fast-freezing (by high pressure, "slam-freezing," and jet-propane), freeze-substitution, conventional freeze-fracture and deep-etching, cryoultramicrotomy, and routine preparation for transmission electron microscopy. The use of fast-freezing fixation (FFF) proved to be superior in terms of structural preservation due to the rapidity of this fixation compared to that obtained using conventional chemical fixation. The low temperature techniques used here were useful to confirm data already obtained by conventional freeze-fracture using chemical fixation and cryoprotection, such as the presence of flagellar rosettes and costa structure. Cryoultramicrotomy and slam-freezing also demonstrated the presence of hair-like structures projecting out from the protozoan surface. New aspects of organelles of T. foetus were demonstrated.

Cryofixation combined with physical dehydration for quantitative immunoelectron cytochemistry

Common methods for preparing samples for immunoelectron microscopy involve glutaraldehyde fixation (GA) followed by chemical dehydration (CD) or cryofixation (CF) succeeded by physical dehydration, i.e., freeze drying (FD) or freeze substitution (FS). The effects of these techniques have been evaluated with regard to the sizes of epoxy resin embedded rat somatotrophic secretory granules as well as the immunolabeling densities over these granules. The measurements were performed by computerized image analysis using electron microscopy in transmission (TEM) and scanning transmission (STEM) modes, which allowed us to define the immunolabeling in detail. The embedded secretory granules showed the same diameters after GA (2 hr) with CD and GA (15 min) with CF and FS, but were smaller after CF-FS, and smallest after GA (15 min) with CF and FD. The highest labeling density appeared after GA (15 min) and physical dehydration, in particular after freeze substitution. Based on our STEM pictures a new factor for evaluating and interpreting immunolabeling of granules is introduced; the "accessible immunogold labeling surface." It defines the fraction of the epoxy resin surface that is labeled and varies with the preparation methods. By using this factor, an order of labeling densities/micron 2 over the accessible areas could be established for the different techniques: GA-CF-FS > CF-FS > GA-CF-FD > GA-CD. The high labeling after GA-CF-FS may be due to the combination of a large accessible area and accurate preservation of the antigenicity of the hormones in the granules.

Ultrastructure of quick-frozen secretory ameloblasts of the rat molar tooth

Ameloblasts in molar tooth germs from 4 to 12-day-old neonatal rats were quick-frozen and then freeze-substituted in acetone-tannic acid and acetone-osmium tetroxide. Following quick freezing and freeze-substitution, the preservation of ameloblasts was principally the same as seen in conventionally-fixed ones. Several differences, however, became apparent between two methods. Concerning enamel formation, the quick-freezing procedure greatly improved the preservation including that of the extrusion of secretory granules into the extracellular space. The exocytosis is a type of merocrine secretion. The precursor stippled material was demonstrated at the front of Tomes' processes and in the intercellular space of the distal terminal junction. In the more advanced stage, the stippled material tended to decrease gradually in amount. Prior to the formation of the typical Tomes' process, large secretory granules fused directly with the distal plasma membrane, which plays a role in the formation of prismless enamel. After the typical Tomes' process had formed, secretory granules containing electron-dense material instead of the large secretory granules accumulated in the distal portion of the Tomes' process. These latter secretory granules had two different destinations; i.e. the granules discharged their contents into the infoldings of the distal part of Tomes' process and into the infoldings of the lateral cell membrane. These types of secretion seem to have a functional role in the rod or inter-rod enamel formation. The ultrastructure of the stippled material consisted of 3-dimensionally interconnected strands after quick freezing, which is different from the appearance of granules embedded in amorphous material after conventional fixation. The stippled material was seen in the intercrystal space among the newly formed crystals. Also, an extensive tubulo-vesicular structure was preserved in both the cell body and Tomes' process. The present data suggest that the membrane of this structure showed a smooth contour opening to the extracellular space, and also an intimate relationship with the secretory granules.

Cryofixation, cryosubstitution, cryoembedding for ultrastructural, immunocytochemical and microanalytical studies

Cryofixation, cryosubstitution and cryoembedding are a set of low-temperature methods for immunocytochemical and microanalytical ultrastructural studies. This review covers the theoretical and practical aspects of these cryomethods, simple, low-cost, safe devices that provide reproducible results and a summary of recent results. Sections prepared by these three cryomethods can be used to determine elemental composition, molecular composition, functions and 3-D ultrastructure. The information obtained can be treated by multivariate statistical methods. Thus, each cellular compartment can be identified by its morphology, molecular and elemental composition and function and changes in these data during physiological and pathological processes can be monitored.

A variant of a slam freezing device for electron microscopy

A home-made slam freezing device is presented that allows reproducible results in freezing various unfixed tissues. The heart of the device is an aluminum socket, which harbors a plunger that is set in motion by a spring. At the end of the plunger there is an electromagnet which holds the sample on a sheet metal planchette. During stop freezing the electrical contacts are interrupted and the plunger can be withdrawn leaving the specimen on the cooled copper block. This guarantees freezing of not only solid tissues, but also cell suspensions, such as blood or bone marrow.

Characterisation of an epithelium-like layer of cells in the multicellular Dictyostelium discoideum slug

Ultrarapid freezing (RF) followed by freeze-substitution (FS) provide superior preservation of the Dictyostelium discoideum multicellular slug tissue over conventional methods of chemical fixation at room temperature. The peripheral cells of slugs prepared by RF and FS form a tight layer of flattened cells. This cell layer resembles epithelia of other multicellular organisms in that it has close junctional contact between cells associated with the extracellular matrix (ECM, slime sheath). This is the first report that clearly demonstrates the existence of such peripheral cellular specialisation in this otherwise well-studied model system. Junctional contacts between adjacent cells mean that there is no intercellular space evident between apical membranes of apposing cells, and basally the intermembraneous space between peripheral cells is less than 10 nm. By contrast, the intercellular space between internal cells is approximately 10–25 nm. The shape of the peripheral cells varies with their location around the slug. In the posterior prespore zone, the peripheral cells are squamous and exhibit polarity along their antero-posterior axis. In the anterior prestalk zone, peripheral cells are less flattened, project irregular filipodia between internal cells, and are polarised along their apical-basal axis. Colloidal gold immunocytochemistry with the markers MUD1, MUD50 and MUD62 demonstrated that the peripheral layer is formed of prestalk cells in the anterior region and ventrum, and mostly prespore cells along the dorsum. Thus, the peripheral layer, while having specific cell classes in different regions, is not differentiation-specific. Rather, it appears that the structure of these epithelium-like cells is influenced by interaction with molecules of the ECM (sheath).

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.

Three-dimensional reconstruction and analysis of mitotic spindles from the yeast, Schizosaccharomyces pombe

Mitotic spindles of Schizosaccharomyces pombe have been studied by EM, using serial cross sections to reconstruct 12 spindles from cells that were ultrarapidly frozen and fixed by freeze substitution. The resulting distributions of microtubules (MTs) have been analyzed by computer. Short spindles contain two kinds of MTs: continuous ones that run from pole to pole and MTs that originate at one pole and end in the body of the spindle. Among the latter there are three pairs of MT bundles that end on fibrous, darkly staining structures that we interpret as kinetochores. The number of MTs ending at each putative kinetochore ranges from two to four; all kinetochore-associated MTs disappear as the spindle elongates from 3-6 microns. At this and greater spindle lengths, there are no continuous MTs, only polar MTs that interdigitate at the spindle midzone, but the spindle continues to elongate. An analysis of the density of neighboring MTs at the midzone of long spindles shows that their most common spacing is approximately 40 nm, center to center, and that there is a preferred angular separation of 90 degrees. Only hints of such square-packing are found at the midzone of short spindles, and near the poles there is no apparent order at any mitotic stage. Our data suggest that the kinetochore MTs (KMTs) do not interact directly with nonkinetochore MTs, but that interdigitating MTs from the two spindle poles do interact to form a mechanically stable bundle that connects the poles. As the spindle elongates, the number of MTs decreases while the mean length of the MTs that remain increases. We conclude that the chromosomes of S. pombe become attached to the spindle by kinetochore MTs, that these MTs disappear as the chromosomes segregate, that increased separation of daughter nuclei is accompanied by a sliding apart of anti-parallel MTs, and that the mitotic processes of S. pombe are much like those in other eukaryotic cells.

Differential localization of the cystic fibrosis transmembrane conductance regulator in normal and cystic fibrosis airway epithelium

Deletion of the amino acid residue Phe 508 of the cystic fibrosis transmembrane conductance regulator (CFTR) protein represents the most common mutation identified in cystic fibrosis (CF) patients. A monoclonal and a polyclonal antibody directed against different regions of CFTR were used to localize the CFTR protein in normal and CF airway epithelium derived from polyps of non-CF and CF subjects homozygous for the delta Phe 508 CFTR mutation. To identify the cellular and subcellular localization of CFTR, immunofluorescent light microscopy, confocal scanning microscopy, and immunogold transmission electron microscopy were performed on cryofixed tissue. A markedly different subcellular distribution was identified between normal and CF airway epithelial cells. In normal epithelium, labeling was restricted to the surface apical compartment of the ciliated cells. In contrast, in the epithelium from homozygous delta Phe 508 CF patients, CFTR markedly accumulated in the cytosol of all the epithelial cells. These findings are consistent with the concept that the CFTR delta Phe 508 mutation modifies the intracellular maturation and trafficking of the protein, leading to an altered subcellular distribution of the delta Phe 508 mutant CFTR.

The ultrastructure of the chromosome periphery in human cell lines. An in situ study using cryomethods in electron microscopy

We studied the chromosome periphery in human HeLa and TG cells using cryomethods in electron microscopy. A contrasted layer of peripheral chromosomal material (PCM) was visible in cryo-ultrathin sections of mitotic cells. This PCM was composed of closely packed fibrils associated with granules. The PCM did not cover the entire chromosome surface but was found around most of the chromosomes and even between two chromatids. The organization of the PCM was not affected by colchicine treatment of mitotic cells. In cells prepared by quick-freezing, the PCM appeared to be a fibrous material at the chromosome periphery, and was also associated with granules that resembled inter-chromatin granules in size and shape. At higher magnification, direct contacts between the chromosomes and the fibrils of the PCM were observed. The cryotechniques used are known to preserve the native organization of cells. Therefore, the architecture of the perichromosomal region analysed presumably corresponds to that in vivo during mitosis. These observations show that in HeLa and TG cells, a particular structure present at the chromosome periphery in the form of PCM is persistent and ubiquitous. In addition, we showed by immunolabelling that the PCM is the specific site of accumulation of nucleolar antigens during mitosis. These two results, i.e. the identification of specific morphological structures and the compartmentation of proteins, indicate that this layer is a specific region of mitotic cells.