Scanning electron microscopy of intracellular structures

Low-voltage scanning electron microscopy

There are two reasons for the renewed interest in using the SEM at beam voltages, Vb, around 1kV (LVSEM). The most common one arises from applications in the semiconductor industry and emphasizes the reduction in charging artifacts and in subsurface beam damage. The second reason postulates that the increased contrast in the topographic component of the secondary electron signal will permit an important improvement in topographic spatial resolution if only a sufficiently small probe diameter can be obtained. We shall treat these two areas separately and then mention some of the strategies that have been adopted to make LVSEM work.

Surfaces are very important in the manufacture of modern semiconductor devices and the ability of the electron probe to induce current flow (EBIC), to detect variations in surface voltage, Vs, and to excite characteristic x rays in addition to its ability to image topography in an easily understandable way guaranteed the SEM a major role in programs of semiconductor development and failure analysis. There were two problems, however, charging and beam-induced damage to the specimen.

New insights on mitochondrial heterogeneity observed in prepared mitochondrial samples following a method for freeze-fracture and scanning electron microscopy

Mitochondria are dynamic intracellular organelles with diverse roles in tissue- and cell type-specific processes, extending beyond bioenergetics. In keeping with this array of functions, mitochondria are described as heterogeneous both between and within tissue types based on multiple parameters, including a broad spectrum of morphological features, and new research points toward a need for the evaluation of mitochondria as isolated organelles. Although transmission electron microscopy (TEM) is commonly used for the evaluation of mitochondria in tissues and renders mitochondrial structures in ultra-thin sections in two-dimensions, additional information regarding complex features within these organelles can be ascertained using scanning electron microscopy (SEM), which allows for analysis of phenotypic differences in three-dimensions. One challenge in producing mitochondrial images for evaluation of ultrastructure using SEM has been the ability to reliably visualize important intramitochondrial features, the inner membrane and cristae structures, on a large-scale (e.g. multiple mitochondria) within a sample preparation, as mitochondria are enclosed within a double membrane. This can be overcome using a 'freeze-fracture' technique in which mitochondrial preparations are snap-frozen followed by application of intense pressure to break open the organelles, revealing the intact components within. Previously published reports using freeze-fracture strategies for mitochondrial SEM have demonstrated feasibility in whole tissue specimens, but a detailed methodology for SEM analysis on isolated mitochondrial fractions has not been reported. By combining previously reported tissue freeze-fracture strategies, along with utilizing the depth of field created by SEM, herein we present a complete method reliant on the freeze-fracture of mitochondrial fractions prepared by differential centrifugation to produce a comprehensive and direct evaluation of three-dimensional mitochondrial ultrastructure by SEM. Image analysis of internal mitochondrial features demonstrates heterogeneity in mitochondrial ultrastructure from a single sample preparation.

Histological and ultrastructural analysis of the respiratory tracheae of Galeodes granti (Chelicerata: Solifugae)

Solifuges lack oxygen-carrying proteins like hemocyanins found in other chelicerates. For conduction of gases, therefore, we hypothesize that the tracheal system is divided into convective and respiratory parts, the latter having intimate association with respiring cells, tissues and organs. This hypothesis is supported by studies of tracheae in other arthropods. We used light, scanning, and transmission electron microscopy to examine the tracheae of Galeodes granti (Chelicerata, Solifugae). We studied tracheae in cheliceral and leg muscles, midgut and midgut diverticula, the heart and the supra- and subesophageal ganglia of the central nervous system. In all tissues examined, the tracheae penetrate into tissues and can be found between cells. Terminal branches with diameter less than one micron were found embedded into cell bodies of respiring tissue, thus reducing the diffusion distance. We suggest that the terminal branches are specialized for respiratory function due to their thin intima, with reduced endocuticle and exocuticle. Comparison with other tracheate chelicerates suggests parallel evolution of a common tracheal pattern based on homologous cellular building materials within the chelicerates and arthropods.

Ultrastructure of submandibular salivary glands of mouse: TEM and HRSEM observations

The fine structure of submandibular glands of mouse were analyzed using light microscopy (LM), high resolution scanning electron microscopy (HRSEM), and transmission electron microscopy (TEM) methods. For LM, the specimens were embedded in Spurr resin, stained by toluidin blue solutions. For TEM, the tissues of submandibular salivary glands were fixed with modified Karnovsky solution and postfixed with osmium tetroxide. For HRSEM, the tissues were fixed with 2% osmium tetroxide solution in 1/15M sodium phosphate buffer (pH 7.4). The samples were immersed successively in dymethylsulphoxide and freeze cracked. The maceration was made in diluted osmium tetroxide for 24-48 h. The samples were examined by high resolution scanning electron microscopy. The intracellular components of acinar and ductal cells revealed clearly the Golgi apparatus, rough endoplasmic reticulum, secretory granules, and mitochondria. The end bulbs of Golgi lamellae and flattened cisterns of rough endoplasmic reticulum showed the luminal surface. A few mitochondria were identified intermingling between the rough endoplasmic reticulum and the mitochondriales cristae in three-dimensional HRSEM images. Secretory granules were numerous and presented different sizes. Small granules of ribosomes were attached on cistern surface, measuring 20-25 nm in diameter. Numerous arranged microvilli were found on the luminal surface of secretory canaliculus. The contact surfaces of acinar cells revealed complicated interdigitations by cytoplasmic processes. The mitochondria of duct cells were disposed vertically and surrounded by basal infoldings of plasma membranes. Basement membrane showed a spongy-like structure having an irregular surface with various strands and meshes of fine collagen fibrils.

Strategies and results of field emission scanning electron microscopy (FE-SEM) in the study of parasitic protozoa

Field emission scanning electron microscopy (FE-SEM) provides a range of strategies for investigating the structural organization of biological systems, varying from isolated macromolecules to tissue organization and whole organisms. This review covers some of the results so far obtained using FE-SEM observation and various protocols of sample fixation to analyze the structural organization of parasitic protozoa and their interaction with host cells. The employment of FE-SEM can be broadened through the use of gold-labeled molecules or tracers, gradual extraction by detergents, and cleavage techniques. These analyses provide significant contributions to the characterization of these organisms concerning ultrastructure, cytoskeleton, motility and intracellular behavior.

Three-dimensional characteristics of submandibular salivary gland of ageing rats: an HRSEM study

The characteristics of the submandibular glands of ageing Wistar rats were studied using light and high-resolution scanning electron microscopy (HRSEM) methods. For light microscopy, the samples were fixed in Bouin solution and embedded in paraffin. Sections were stained with hematoxilin-eosin and Azo-Carmin. For conventional scanning electron microscopy, the tissues were fixed in modified Karnovsky solution, and treated with NaOH solution for 3-4 days. The O-D-O method was used for HRSEM. The samples were fixed in 2% osmium tetroxide, macerated in diluted osmium and dehydrated in an increasing series of ethanol. The samples were dried in a critical point dryer, coated with gold-palladium and examined in a Hitachi high-resolution scanning electron microscope, S-900. The results showed that submandibular glands with lobules are separated by connective tissue septum. The acinar formations and the ducts, revealing the serous and mucous cells were observed. After fracture in liquid nitrogen and treatment with NaOH solution to remove the cellular components, the original disposition of the collagen bundles fibers were revealed corresponding to the round, oval or irregular acinar and ductal structures. In the cytoplasm, organelles such as mitochondria, rough endoplasmic reticulum, Golgi apparatus and serous and mucous secretory granules were observed localized in the apical portion in three-dimensional HRSEM images. The serous secretory granules presented different sizes and shapes showing the modifications which occurred in the ageing rats. The striated duct cells revealed the presence of the secretory cells and mitochondria in parallel disposition. The mitochondrial cristae were noted in three-dimensional aspects. The lumen presented numerous cytoplasmic microprojections. The lumen of excretor ducts are covered by polygonal epithelial cells containing numerous microplicae.

3D analysis of chromosome architecture: advantages and limitations with SEM

Three-dimensional mitotic plant chromosome architecture can be investigated with the highest resolution with scanning electron microscopy compared to other microscopic techniques at present. Specific chromatin staining techniques making use of simultaneous detection of back-scattered electrons and secondary electrons have provided conclusive information on the distribution of DNA and protein in barley chromosomes through mitosis. Applied to investigate the structural effects of different preparative procedures, these techniques were the groundwork for the "dynamic matrix model" for chromosome condensation, which postulates an energy-dependent process of looping and bunching of chromatin coupled with attachment to a dynamic matrix of associated protein fibers. Data from SEM analysis shows basic higher order chromatin structures: chromomeres and matrix fibers. Visualization of nanogold-labeled phosphorylated histone H3 (ser10) with high resolution on chromomeres shows that functional modifications of chromatin can be located on structural elements in a 3D context.

Ultrastructural characterization of the isolated hydrogenosome in Tritrichomonas foetus

In the present study we show new aspects of the hydrogenosome ultrastructure as well alterations induced by the fractionation technique. The morphology of freshly isolated hydrogenosomes as well those found in whole cells of Tritrichomonas foetus were examined in thin-sections, in replicas of fast-freezing, and conventional freeze-fracture, freeze-etching, and by high resolution scanning electron microscopy (field emission in-lens scanning electron microscopy). The true surface as well the concave and convex fracture faces of the inner and outer membranes are shown. We showed that after fractionation procedures the hydrogenosome ultrastructure can be changed, since isolated hydrogenosomes present patchwork-like structures, rosettes and the inner hydrogenosomal membrane is displaced. The peripheral vesicle is seen as a distinct compartment, since its content and morphological appearance is quite different from the rest of the organelle. The peripheral vesicle shows a smooth surface but presenting pores with 20 nm in diameter with a density of 7/micron 2 when observed after freeze-etching. We report the existence of characteristic intramembrane particles distribution and density on hydrogenosome membranes of isolated and whole T. foetus, suggesting that this organelle can have its morphology changed as consequence of technical modifications or as expression of its metabolic state.

Field-emission scanning electron microscopy of the internal cellular organization of fungi

Internal viewing of the cellular organization of hyphae by scanning electron microscopy is an alternative to observing sectioned fungal material with a transmission electron microscope. To study cytoplasmic organelles in the hyphal cells of fungi by SEM, colonies were chemically fixed with glutaraldehyde and osmium tetroxide and then immersed in dimethyl sulfoxide. Following this procedure, the colonies were frozen and fractured on a liquid nitrogen-precooled metal block. Next, the fractured samples were macerated in diluted osmium tetroxide to remove the cytoplasmic matrix and subsequently dehydrated by freeze substitution in methanol. After critical point drying, mounting, and sputter coating, fractured cells of several basidiomycetes were imaged with field-emission SEM. This procedure produced clear images of elongated and spherical mitochondria, the nucleus, intravacuolar structures, tubular- and plate-like endoplasmic reticulum, and different types of septal pore caps. This method is a powerful approach for studying the intracellular ultrastructure of fungi by SEM.

Olfactory epithelium in young adult and aging rats as seen with high-resolution scanning electron microscopy

The present study uses mainly scanning electron microscopy to demonstrate the three-dimensional internal cell structures of rat olfactory epithelial cells. The aldehyde-prefixed osmium-DMSO-osmium (AODO) method devised by Tanaka and Mitsushima (1984) was applied to the present study to disclose intracellular structures such as endoplasmic reticulum, mitochondria, Golgi apparatus, and lysosomes. The spatial distribution pattern of these structures in olfactory and supporting cells is discussed, paying special attention to the formation of lipofuscin-like granules present in aged rats.

Changes in endoplasmic reticulum during spermiogenesis in the mouse

Changes in the endoplasmic reticulum of mouse spermatids during spermiogenesis were examined by scanning electron microscopy, applying the OsO4-DMSO-OsO4 method, which permits 3-dimensional observation of cell organelles. At the same time, the endoplasmic reticulum was stained selectively by the Ur-Pb-Cu method, and 0.5 micron-thick sections were prepared for observation by transmission electron microscopy. The results demonstrated stereoscopically the mode of disappearance of the endoplasmic reticulum. In spermatids of the early maturation phase, the endoplasmic reticulum was of uniform diameter, branched and anastomosed, forming a complicated three-dimensional network throughout the cytoplasm. A two-dimensional net was also noted to have formed just beneath the plasma membrane and about Sertoli cell processes invaginating the spermatid cytoplasm. As spermiogenesis progressed, the spread-out endoplasmic reticulum gradually aggregated to form a condensed, glomerulus-like structure consisting of a very thin endoplasmic reticulum connected to the surrounding endoplasmic reticulum. This structure corresponds to the so-called "radial body". Thus, the endoplasmic reticulum may aggregate, condense, be transformed into a radial body, and be removed from the cytoplasm. The two-dimensional endoplasmic reticulum-net, just beneath the plasma membrane and surrounding processes of Sertoli cells, disappeared in spaces where the three-dimensional endoplasmic reticulum network was scarce. Both the two-dimensional endoplasmic reticulum-net structure and the three-dimensional endoplasmic reticulum network disappeared at the same time, indicating that they may be closely related.

Application of an ultrahigh-resolution scanning electron microscope (UHS-T1) to biological specimens

In 1985 we developed an ultrahigh-resolution scanning electron microscope with a resolution of 0.5 nm. It is equipped with a field emission gun and an objective lens with a very short focal length. In this study we report a survey of some different preparation techniques and biological specimens using the new scanning electron microscope. Intracellular structures such as cell organelles were observed surprisingly sharper than those observed by ordinary scanning electron microscopes. However, at magnifications over 250,000 x, platinum particles could be discerned as scattered pebbles on the surface of all structures in coated materials. Using an uncoated but conductively stained specimen, we successfully observed ribosomes on a rough endoplasmic reticulum at a direct magnification of 1 million. In these images some protrusions were recognized on the ribosomes. Ferritin and immunoglobulin G were used as samples of biological macromolecules. These samples were observed without metal coating and conductive staining. The ferritin particles appeared as rounded bodies without any substructure on the surface and immunoglobulin G as complexes of three-unit bodies. In the latter the central body might correspond to the Fc fragment and two side ones to Fab fragments. We assume that ultrahigh-resolution scanning electron microscopy is an effective means for observation of the cell fine structures and biological macromolecules. It will open a new research field in biomedicine.

Dynamic changes in Sertoli cell processes invading spermatid cytoplasm during mouse spermiogenesis

Studies using thick sections stained by ATPase cytochemistry and scanning electron microscopy were carried out to determine three-dimensional ultrastructural alterations in Sertoli cell processes invading neighboring spermatids during mouse spermiogenesis. Sertoli cell processes start invading spermatid cytoplasm at the acrosomal phase of development and undergo considerable change at the maturation phase of development. At step 14, these processes elongate and begin to branch in the spermatid cytoplasm, and by step 15, they extend in various directions to form a complex of canals that the authors have designated the canal complex. The present observations also clarify that the complicated canal complex undergoes regional modification. At the late stages of maturation, the endoplasmic reticulum has gathered with other cell organelles to form aggregates of endoplasmic reticulum in the vicinity of which invading Sertoli cell processes extensively ramify further into thin tubules that intertwine with each other to form a region of thin tubules. In thin sections, each such region was a complex, consisting of small vesicles and endoplasmic reticulum, and corresponded to what has been defined as a mixed body by Morales and Clermont (Anat. Rec., 203:233-244, 1982). During the course of the formation of the region, the invading Sertoli cell processes are continuous at all times with the cell body of the surrounding Sertoli cell.

Tomographic three-dimensional reconstruction of cilia ultrastructure from thick sections

We have applied a computer-based tomographic technique to reconstruct the three-dimensional ultrastructure of newt lung cilia. Epon-embedded samples were cut into 0.25-micron-thick sections that were imaged at 1 MV with a high-voltage electron microscope. For the reconstruction shown, a tilt series of 53 micrographs was taken at tilt angles between -54 degrees and +50 degrees. The reconstruction was accomplished from these projections using a weighted back-projection algorithm. The 12-nm resolution of the reconstruction was sufficient to resolve the outer doublet and central pair microtubules, dynein arms, radial spokes, and central sheath structures. The reconstruction can be viewed from various angles and with appropriate parts cut away to reveal structural features of interest. The sense of depth in these views can be enhanced by stereo viewing of shaded surface images. From this reconstruction, we determined that newt lung cilia contain the more common triplet grouping of radial spokes.

Cytochemical and scanning electron-microscopic analysis of apoptotic cells and their phagocytosis in mucoid epithelium of the mouse stomach

The formation of apoptotic cells and their phagocytosis by viable neighbouring cells in the gastric epithelium of 2- to 6-day-old mice was analysed. In order to observe the topographic relationship between apoptotic and normal epithelial cells using scanning electron microscope, the critical-point dried tissues was cracked before coating with gold. Cytochemical methods for the identification of surface carbohydrates and different tracers for apical and lateral cell membranes were applied for the analysis using the transmission electron microscope. Apoptotic cells were found on apical and lateral surfaces; this indicates the presence of tight connections with viable cells at some points. Ruthenium red strongly stained all accessible surfaces of normal cells and of apoptotic bodies. The quantity of neutral mucosubstances, as revealed by staining with tannic acid-uranyl acetate, seemed to decrease in the glycocalyx of apoptotic cells. The scanning and transmission electron-microscopic results suggest that the phagocytotic vacuoles arise at the lateral side of the cells. The phagocytotic activity is not dependent upon a definite differentiation step of the mucoid cell.

Site-dependence of invasiveness of ECA109 human oesophageal carcinoma cells in nude mice

ECA109 human oesophageal carcinoma cells were injected either subcutaneously or intraperitoneally into BALB/CATc 1-nu/nu mice. After 23 weeks tumours were examined histologically and by scanning electron microscopy. Subcutaneous ECA109 tumours were well-delineated without signs of invasion. By contrast, intra-abdominal tumours invaded into the abdominal wall and abdominal organs. This result provides us with another example of site-dependence of invasion in vivo.

Low voltage scanning electron microscopy

The scanning electron microscope (SEM) is usually operated with a beam voltage, V0, in the range of 10-30 kV, even though many early workers had suggested the use of lower voltages to increase topographic contrast and to reduce specimen charging and beam damage. The chief reason for this contradiction is poor instrumental performance when V0 = 1-3 kV, The problems include low source brightness, greater defocusing due to chromatic aberration greater sensitivity to stray fields, and difficulty in collecting the secondary electron signal. Responding to the needs of the semiconductor industry, which uses low V0 to reduce beam damage, considerable efforts have been made to overcome these problems. The resulting equipment has greatly improved performance at low kV and substantially removes the practical deterrents to operation in this mode. This paper reviews the advantages of low voltage operation, recent progress in instrumentation and describes a prototype instrument designed and built for optimum performance at 1 kV. Other limitations to high resolution topographic imaging such as surface contamination, the de-localized nature of the inelastic scattering event and radiation damage are also discussed.

An internal view of the spherical body of Treponema macrodentium as revealed by scanning electron microscopy

The osmium-dimethyl sulfoxide-osmium method for clear visualization of intracellular structure was used to observe the detailed inner structure of the spherical bodies produced in vitro by a human oral treponeme. Scanning electron microscopy of the cracked spherical body revealed no morphological differences between the outer and inner surfaces of the spherical body membrane, and that multiple folded or somewhat linear main bodies adhere closely to the inner surface. In addition, axial flagella partially free from the main bodies spread widely within the body to make a network, and a number of blebs ranging from approximately 1 micrometer to 0.2 micrometer in diameter were located near the terminal or subterminal areas of the main bodies. The origin of the blebs and the mechanism of spherical body formation are discussed.

Photomechanical responses in crustacean retinula cells: the role of microtubules

Light and darkness promote opposite migrations of screening pigment granules in the retinal photoreceptors of the crayfish and other arthropods, and microtubules have been implicated in this part of the adaptation process. This report considers the topographic relationships of the migratory pigment granules with the microtubular system of the cells. Scanning electron microscopy was used to obtain a three-dimensional visualization of the internal organization of cells in different states of adaptation. Many pigment granules were found directly associated with the microtubular array, although most of them are not adjacent to microtubules irrespective of the adaptation conditions. However, the granules appear associated with each other and form extensive clusters or possibly a single conglomerate. Therefore, interaction of a limited number of granules with the microtubules could affect the movement of the whole granule population. When the pigment complex withdraws, many microtubules are still seen in the deserted regions. It is concluded that the microtubules might represent a supporting framework for the translocation of the pigment, which appears to move as a coherent mass rather than as independent particles.