A comparison of microwaves and heat alone in the preparation of tissue for electron microscopy

Microwave-assisted processing and embedding for transmission electron microscopy

Microwave processors can provide a means of rapid processing and resin embedding for biological specimens that are to be sectioned and examined by transmission electron microscopy. This chapter describes a microwave-assisted protocol for processing, dehydrating and embedding biological material, taking them from living specimens to blocks embedded in sectionable resin in 4 h or less.

Structural and antigenic preservation of plant samples by microwave-enhanced fixation, using dedicated hardware, minimizing heat-related effects

We explored the use of microwave technology in fixation with the objective of achieving quicker fixation regimes, lower concentrations of toxic and volatile reagents, and enhanced antigen detection. We used a modified domestic microwave oven (900 W) and a low-power (5 W) microwave bench. The work was done on plant materials. The oven was supplemented with a cooling device, a stirring system, and a record of the sample temperature and the time of effective irradiation. The sample, immersed in a fixative solution of 1% paraformaldehyde (PFA) in PBS, was irradiated for only 10 minutes. The sample temperature did not exceed 37 degrees C. In these mild conditions, the quality of the (ultra)structural preservation of the samples, morphometrically assessed, was at the same level as obtained with the same fixative, using conventional methods. On the contrary, samples fixed in the same conditions without irradiation showed a poor structural preservation. The antigenic preservation of the irradiated samples was excellent, since the labeling levels of two nucleolar proteins, detected by immunogold, were three times higher than in conventionally fixed samples. In the so-called microwave bench, the pathway of microwaves is guided, so that low-power microwaves directly hit the sample and there is no dispersion of energy. Temperature of fixative did not increase after microwave irradiation. Fixation in the bench with either 4% PFA, or 1% PFA, for 20 minutes resulted in structural preservation of samples similar in quality as obtained with conventional fixation and in a similar or better level of antigen preservation. Therefore, controlling temperature and effective irradiation is crucial in order to obtain optimal structural and antigen preservation with microwave-enhanced fixation. The dramatic differences observed between microwave-irradiated samples and samples fixed in the same conditions without irradiation, strongly support the existence of specific effects of microwaves on fixation, independent from the mere heating of the samples.

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.

Microwave irradiation improvements in the silver staining of the nucleolar organizer (Ag-NOR) technique

The well-known technique of silver staining of the nucleolar organizer (Ag-NOR) is improved in contrast, selectivity and speed when performed with microwave irradiation. The Ag-NOR technique is a very useful tool for studies on the functional morphology and molecular architecture of the nucleolus, and is reputed to be one of the best techniques for diagnosis and prognosis of cancer lesions. To test the generality of the enhancing effects, our study has involved the use of both mammalian and plant cells. Two steps in the process are improved quantitatively by microwave irradiation: fixation and staining itself. Fixation with the ethanol-based reagent, Kryofix, for 3 min in the microwave oven, resulted in good structural preservation at the optical level, and enhanced the contrast and selectivity of silver staining. On the contrary, we found that neither glutaraldehyde fixation, nor a treatment of sections with Carnoy's solution, improved Ag-NOR staining. After an analysis of the effects of the different substances involved in sample preparation, we conclude that ethanol is an essential factor for fixation for nucleolar staining, particularly if aldehydes are eliminated from fixative solutions. The process of staining was performed with a drop of staining solution on a semithin section of plastic-embedded tissue in the microwave oven for 1 min. Staining under these conditions always improved the visualization of nucleoli, regardless of the fixation procedure. Therefore, microwave irradiation at both steps is recommended for giving the best results. Microwave irradiation probably enhances fixation by controlled heat, whereas the increase in reactivity of the staining solution is a direct effect by the microwaves on the silver ions themselves. We used this method to study nucleolar materials during mitosis in proliferating plant cells. Current applications of Ag-NOR staining can be improved with this technical modification.

Application of microwave fixation techniques in pathology to neuroscience studies: a review

The introduction of microwave energy into the scientist's repertoire of fixation modalities offers for the first time in relatively large specimens the potential for 'instantaneous' preservation of cellular structure for light and electron microscopy with minimal alteration of cellular biochemistry and antigenicity. Because of the rapid evolution of this new technology, we provide a classification system of newly generated microwave methods as applied to specimen preservation for microscopic analysis. With emphasis on neuronal tissue, we review qualitative and quantitative microscopy data of specimens fixed by two microwave methods in common use: (1) microwave stabilization and (2) fast and ultrafast, primary microwave-chemical fixation. In addition, we provide a table of neuropeptides or proteins in neuronal tissues that are preserved by various microwave fixation methods for histochemistry, immunohistochemistry, and immuno-electron microscopy studies. Commercial microwave ovens have limitations which can result in irreproducible fixation results. Therefore, we present a calibration protocol that is used to identify the best locations for fixation within large cavity (i.e., household) microwave ovens. We also provide a standardization protocol to improve the reproducibility of microwave fixation in calibrated, large-cavity microwave ovens.

Rapid cold fixation of tissue samples by microwave irradiation for use in electron microscopy

A cold microwave irradiation procedure was developed to fix rapidly and stain various tissues and monolayers for electron microscopy. Because microwave stimulation always produces some heat, melting ice was used to maintain the temperature of the tissue samples, the fixative, and the staining solution at 0 to 4 degrees C. The low temperature also reduced vapour formation, thus minimizing the risk of explosion. The microwave method shortened the total time of fixation and dehydration from the usual 3 h required by the conventional method to 65 min. After microwave fixation, the ultrastructural details of membranes and subcellular structures were excellent.