Methods of Microwave Fixation for Microscopy

Microwave-induced fast decalcification of rat bone for electron microscopic analysis: an ultrastructural and cytochemical study

Bone decalcification is a time-consuming process. It takes weeks and preservation of the tissue structure depends on the quality and velocity of the demineralization process. In the present study, a decalcification methodology was adapted using microwaving to accelerate the decalcification of rat bone for electron microscopic analysis. The ultrastructure of the bone decalcified by microwave energy was observed. Wistar rats were perfused with paraformaldehyde and maxillary segments were removed and fixed in glutaraldehyde. Half of specimens were decalcified by conventional treatment with immersion in Warshawsky solution at 4 degrees C during 45 days, and the other half of specimens were placed into the beaker with 20 mL of the Warshawsky solution in ice bath and thereafter submitted to irradiation in a domestic microwave oven (700 maximum power) during 20 s/350 W/+/-37 degrees C. In the first day, the specimens were irradiated 9 times and stored at 40 degrees C overnight. In the second day, the specimens were irradiated 20 times changing the solution and the ice after each bath. After decalcification, some specimens were postfixed in osmium tetroxide and others in osmium tetroxide and potassium pyroantimonate. The specimens were observed under transmission electron microscopy. The results showed an increase in the decalcification rate in the specimens activated by microwaving and a reduction of total experiment time from 45 days in the conventional method to 48 hours in the microwave-aided method.

Paleontological evidence for membrane fusion between a unit membrane and a half-unit membrane

Membrane fusion is of fundamental importance for many biological processes and has been a topic of intensive research in past decades with several models being proposed for it. Fossils had previously not been considered relevant to studies on membrane fusion. But here two different membrane fusion patterns are reported in the same well-preserved fossil plant from the Miocene (15-20 million years old) at Clarkia, Idaho, US. Scanning electron microscope, transmission electron microscope, and traditional studies reveal the vesicles in various states (even transient semi-fusion) of membrane fusion, and thus shed new light on their membrane structure and fusion during exocytoses. The new evidence suggests that vesicles in plant cells may have not only a unit membrane but also a half-unit membrane, and that a previously overlooked membrane fusion pattern exists in plant cells. This unexpected result from an unexpected material not only marks the first evidence of on-going physiological activities in fossil plants, but also raises questions on membrane fusion in recent plants.

Fixation of soft tissue surrounded by bone with microwave irradiation: electron microscopic observation of guinea pig inner ear

When electron microscopy is performed on organs such as the inner ear that cannot be removed immediately after decapitation of animals, it is necessary to fix the target organ or tissue by systemic or regional perfusion fixation. However, such methods of fixation can increase vascular pressure or perilymphatic pressure, making it difficult to perform precise morphological observation of the vascular endothelial cells and membranous labyrinth. We recently attempted fixation of the cochlea by microwave irradiation. Guinea pigs were decapitated. The bullas were then removed from each animal and fixed in a mixture of 2% paraformaldehyde and 0.5% glutaraldehyde. Microwave (300 W) irradiation was then applied to the specimen for 1 minute. The fixative was immediately replaced with new fixative (4 degrees C). This sequence of manipulations was repeated 10 times, for a cumulative microwave irradiation time of 10 minutes. During the microwave irradiation period, the fixative temperature was kept at about 30 degrees C. After the last round of irradiation, the specimens were kept immersed in the fixative for 1 hour. After a small slit was created in the bone on the lateral wall of the cochlea, the specimens were post-fixed in osmic acid and embedded in Epon 812. Each specimen was cut into halves along the plane containing the modiolus of the cochlea. After the bone on the lateral wall of the cochlea was cut off under a stereoscopic microscope, ultrathin sections were prepared for observation under a transmission electron microscope. With this technique, the stria vascularis and the organ of Corti were fixed to a degree comparable to or better than that achieved with the conventional method of fixation. Fixation with microwave irradiation is relatively simple and can solve the problems associated with perfusion fixation, and thus provides an excellent means of fixation. This technique appears to be particularly promising for fixation for soft tissue surrounded by bone.

Plant cytoplasm preserved by lightning

Usually only an organism with hard parts may be preserved in the fossil record. Cytoplasm, which is a physiologically active part of a plant, is rarely seen in the fossil record. Two Cretaceous plant fossils older than 100 million years with exceptional preservation of cytoplasm are reported here. Some cytoplasm is well preserved with subcellular details while other cytoplasm is highly hydrolyzed in the cortex of the same fossil even though both of preservations may be less than 2 microm away. The unique preservation pattern, sharp contrast of preservation in adjacent cells and the exceptional preservation of cytoplasm in the cortex suggest that lightning should play an important role in the preservation of cytoplasm and that cytoplasmic membranes may be more stable than the cell contents. Interpreting the preservation needs knowledge scattering in several formerly unrelated fields of science, including geophysics, botany, biophysics, cytology and microwave fixation technology. This new interpretation of fossilization will shed new light on preservation of cytoplasm and promote cytoplasm fossils from a position of rarity to a position of common research objects available for biological research. The importance of the identification of cytoplasm in fossil lies not in itself but in how much it influences the future research in paleobotany.

High-resolution immunocytochemistry of noncollagenous matrix proteins in rat mandibles processed with microwave irradiation

The mineral phase in calcified tissues represents an additional factor to be considered during their preservation for ultrastructural analyses. Microwave (MW) irradiation has been shown to facilitate fixative penetration and to improve structural preservation and immunolabeling in a variety of soft tissues. The aim of the present study was to determine whether MW processing could offer similar advantages for hard tissues. Rat hemimandibles were immersed in 4% formaldehyde + 0.1% glutaraldehyde buffered with 0.1 M sodium cacodylate, pH 7.2, and exposed to MWs for three periods of 5 min at temperatures not exceeding 37C. They were then decalcified in 4.13% EDTA, pH 7.2, for 15 hr, also under MW irradiation. Osmicated and non-osmicated samples were dehydrated in graded concentrations of ethanol and embedded in LR White resin. Sections of incisor, molars, and alveolar bone were processed for postembedding colloidal gold immunolabeling using antibodies against ameloblastin, amelogenin, bone sialoprotein, or osteopontin. Ultrastructural preservation of tissues was in most cases comparable to that obtained by perfusion-fixation, and there was no difference in distribution of labeling with those previously reported for the antibodies used. However, the immunoreactivities obtained were generally more intense, particularly at early stages of tooth formation. Amelogenin was abundant between differentiating ameloblasts and labeling for osteopontin appeared over the Golgi apparatus of odontoblasts after initiation of dentine mineralization. We conclude that MW irradiation represents a simple method that can accelerate the processing of calcified tissues while yielding good structural preservation and antigen retention. (J Histochem Cytochem 49:1099-1109, 2001)

Detection of covalent binding

Immunochemical detection of xenobiotics covalently bound to cellular proteins can provide information about toxic mechanism and is more specific than the alternative radiochemical studies. Both immunoblotting and immunohistochemical methods are used to pinpoint the target protein(s) and to identify the tissue targets. Also included in this unit are protocols for synthesizing artificial antigens, immunizing suitable host species, and using noncompetitive and competitive ELISA assays to characterize the antibodies produced.

Application of microwave technology to the processing and immunolabeling of plastic-embedded and cryosections

We have adapted existing microwave irradiation (MWI) protocols and applied them to the processing and immunoelectron microscopy of both plastic-embedded and frozen sections. Rat livers were fixed by rapid MW irradiation in a mild fixation solution. Fixed liver tissue was either cryosectioned or dehydrated and embedded in Spurr's, Unicryl, or LR White resin. Frozen sections and sections of acrylic-embedded tissue were immunolabeled in the MW oven with an anti-catalase antibody, followed by gold labeling. Controls were processed conventionally at room temperature (RT). The use of MWI greatly shortened the fixation, processing, and immunolabeling times without compromising the quality of ultrastructural preservation and the specificity of labeling. The higher immunogold labeling intensity was achieved after a 15-min incubation of primary antibody and gold markers under discontinued MWI at 37C. Quantification of the immunolabeling for catalase indicated a density increase of up to fourfold in the sections immunolabeled in the MW oven over that of samples immunolabeled at RT. These studies define the general conditions of fixation and immunolabeling for both acrylic resin-embedded material and frozen sections.

Initiating mechanisms involved in the pathobiology of traumatically induced axonal injury and interventions targeted at blunting their progression

To gain better insight into the initiating factors involved in traumatically induced axonal injury cats and rats were subjected to various forms of traumatic brain injury. Following injury at intervals ranging from 10 min. to 3 hours, the animals were sacrificed and prepared in accordance with multiple immunocytochemical strategies capable of detecting focal changes in the axolemma, the subaxolemmal spectrin network, the underlying cytoskeleton as well as any related abnormalities in axoplasmic transport. Through these approaches it was recognized that the most severe forms of injury resulted in focal abnormalities of axonal permeability which were observed together with calpain-mediated spectrin proteolysis in the subaxolemmal network. These events were associated with compaction of the underlying neurofilaments and some microtubular loss which occurred without any direct evidence of overt axoplasmic proteolysis with the exception of the most severely injured fibers. In addition to these severely injured axonal profiles, other injured axons did not manifest overt changes in axolemmal permeability or early calpain-mediated spectrin proteolysis but demonstrated dramatic neurofilament and microtubular misalignment and impaired axoplasmic transport. Lastly, other small caliber axons showed another form of intraaxonal change manifested in the local pooling of organelles in the nodal and paranodal regions, with the suggestion that some of these changes may be reversible. In relation to these axonal responses the efficacy of various therapeutic investigations were assessed. The use of calcium chelators showed a trend for protection in those axons manifesting altered axolemmal permeability. However, the use of early and delayed hypothermia demonstrated dramatic protection resulting in significant reduction in the number of damaged axonal profiles. These studies illustrate the diversity and complexity of those axonal responses evoked by traumatic brain injury, suggesting that multiple forms of therapy may be needed to blunt these multifaceted forms of progression.

Problems and artifacts of microwave accelerated procedures in neurohistotechnology and resolutions

Microwaving artifacts in histoprocessing and staining arise from the acceleration of diffusional and reactive processes. Because such accelerations provide the advantages of microwaving, and because microwave ovens cannot distinguish desirable from undesirable accelerations, artifacts are inevitable. Such microwaving problems can be categorized as follows: histoprocessing and staining reagents may be lost or altered; staining targets may move away from their in vivo sites, may be totally lost from the specimen, or may be altered; physical characteristics such as permeability of specimens or embedding resins may be changed; and staining processes themselves are sometimes different at elevated temperatures. The most general tips for detecting and/or avoiding such problems are to monitor and control the temperature of the reagents and the specimen, to standardize the procedures, and to observe the specimen and reagents carefully during microwaving when a new procedure is being introduced to the laboratory.

Microwave applications in neuromorphology and neurochemistry: safety precautions and techniques

In science, the introduction of a new method is never easy, not even if it concerns the use of a simple microwave oven. Most scientists do not realize the numerous applications of microwave techniques. This paper gives a broad overview of the application of microwave techniques in neuromorphology and neurochemistry, starting with a historical overview ranging from the introduction of microwave techniques as a scientific method in the 1970s to present. Organizations and publication rules are highlighted in the next part. The effect of microwave irradiation is discussed in two sections relating to microwave effects on the whole organism and on the neuron. The main body of the paper discusses the application of microwave techniques in the fields of neuromorphology and neuropathology. The paper then presents aspects of microwave irradiation as applied to ELISA techniques. In addition, cell fusion and cell reproduction under microwave irradiation are discussed.

Calibration and standardization of microwave ovens for fixation of brain and peripheral nerve tissue

Rapid and reproducible fixation of brain and peripheral nerve tissue for light and electron microscopy studies can be done in a microwave oven. In this review we report a standardized nomenclature for diverse fixation techniques that use microwave heating: (1) microwave stabilization, (2) fast and ultrafast primary microwave-chemical fixation, (3) microwave irradiation followed by chemical fixation, (4) primary chemical fixation followed by microwave irradiation, and (5) microwave fixation used in various combinations with freeze fixation. All of these methods are well suited to fix brain tissue for light microscopy. Fast primary microwave-chemical fixation is best for immunoelectron microscopy studies. We also review how the physical characteristics of the microwave frequency and the dimensions of microwave oven cavities can compromise microwave fixation results. A microwave oven can be calibrated for fixation when the following parameters are standardized: irradiation time; water load volume, initial temperature, and placement within the oven; fixative composition, volume, and initial temperature; and specimen container shape and placement within the oven. Using two recently developed calibration tools, the neon bulb array and the agar-saline-Giemsa tissue phantom, we report a simple calibration protocol that identifies regions within a microwave oven for uniform microwave fixation.

Rapid decalcification of temporal bones with preservation of ultrastructure

Decalcification of temporal bones, especially from primates, has routinely required long periods of time and has been a major deterrent to many types of morphological studies. In this investigation, temporal bones from the monkey, Macaca fuscata, were decalcified with ethylene diamine tetraacetic acid (EDTA) in a microwave oven. To isolate effects of microwaves on decalcification, tissue was fixed and embedded using routine methods; only decalcification was carried out in the microwave oven. The procedure is described in detail. Instead of months, decalcification was complete in two working days. Control procedures included decalcification at room temperature and use of a regular oven at a temperature equal to that reached in the microwave. The ultrastructure of cochlear tissue was equal to or better than that obtained with routine decalcification.

Four-hour processing of clinical/diagnostic specimens for electron microscopy using microwave technique

A protocol for routine 4-hour microwave tissue processing of clinical or other samples for electron microscopy was developed. Specimens are processed by using a temperature-restrictive probe that can be set to automatically cycle the magnetron to maintain any designated temperature restriction (temperature maximum). In addition, specimen processing during fixation is performed in 1.7-ml microcentrifuge tubes followed by subsequent processing in flow-through baskets. Quality control is made possible during each step through the addition of an RS232 port to the microwave, allowing direct connection of the microwave oven to any personal computer. The software provided with the temperature probe enables the user to monitor time and temperature on a real-time basis. Tissue specimens, goat placenta, mouse liver, mouse kidney, and deer esophagus were processed by conventional and microwave techniques in this study. In all instances, the results for the microwave-processed samples were equal to or better than those achieved by routine processing techniques.

Microwave fixation: understanding the variables to achieve rapid reproducible results

The use of microwave irradiation for rapid chemical fixation of tissues in electron microscopy is a subject of current interest. The effects of water load size and location, sample placement in the oven cavity (hot or cold spots), and time on tissue preservation were examined. The use of a microwave container (4 dram vial) encased in 60 ml of ice in a 100 ml polyethylene beaker and a 0% power setting between two 100% power settings (time interval) provided reliable control of temperature during microwave irradiation. High brightness neon lights provided a quick and easy method to identify and map hot and cold spots within the oven cavity. Using microwave irradiation for rapid glutaraldehyde and osmium tetroxide fixation of tissues (Pacific yew needle and mouse kidney and liver) for electron microscopy yielded preservation equal or better than routine immersion fixation when a time interval, a cold spot (as the sample location), and an ice-encased vial were used during microwave fixation. These adaptations provided reliable control of fixation conditions in an 800 watt laboratory microwave oven.