Why low temperature embedding for X-ray microanalytical investigations? A comparison of recently used preparation methods

Resins for combined light and electron microscopy: a half century of development

The last fifty years have seen enormous improvements in the way biological specimens are prepared for microscopy. The Fifties produced the essential groundwork upon which many of our current methodologies are based. Acrylic resin embedding was introduced in 1949, with subsequent publications seeking improvements to resin formulations, embedding protocols, and modes of polymerisation. Procedures for progressive lowering of temperature processing, cryosubstitution, freeze-drying and polymerisation by ultra-violet light at low temperatures, all had their genesis in this decade of great innovation. The Sixties marked the period when the acrylics were eclipsed by the more stable and reliable epoxy resins, and much of our present-day understanding of ultrastructure was elucidated. The Seventies carried on this work with advances in technical developments concerned mainly with freezing methodologies. The beginning of the Eighties saw a resurrection of the acrylic resins, with new formulations of these resins giving reliable and stable embeddings. The low temperature and freezing methodologies pioneered in the Fifties, backed up by recent improvements to low temperature technologies, were used to further our understanding of ultrastructure and breathe new life into the science of immunocytochemistry. The remainder of the Eighties and Nineties has seen the ever increasing application of these various microscopical techniques to a wide range of biological studies. The flexibility offered by the acrylic resins in choosing between different processing, embedding and polymerisation methods has provided the impetus for detailed studies to bring to the attention of microscopists the underlying trends governing specimen preparation. Therefore, looking forward to the new Millennium, this has allowed for a more reasoned choice in organising a strategy to deal with a variety of microscopical requirements and for planning an appropriate protocol.

Use of freeze-substitution and molecular distillation drying in the preparation of Dunaliella parva for ion localization studies by X-ray microanalysis

The determination of ion concentrations within cells and sub-cellular compartments remains a difficult procedure, as the volumes to be analyzed are rather small. X-ray microanalysis is sufficiently sensitive, and has adequate resolution, to measure these concentrations. The major difficulties are related to the preparation of material for analysis. We have compared the measurement of sodium, potassium, and chloride contents in a salt tolerant unicellular alga, Dunaliella parva, following either freeze-substitution (using two different resins) or molecular distillation drying. All three procedures gave similar results: after freeze substitution, ion contents were marginally (but not significantly) higher following embedding in Nanoplast MUV 116 resin than in Spurr resin. Since the Nanoplast can be polymerised at low temperatures, it has advantages over the Spurr resin.

Suggestions for the quantitative X-ray microanalysis of thin sections of frozen-dried and embedded biological tissues

When a microregion in a thin section of frozen-dried and embedded tissue is analysed by the conventional electron-probe X-ray continuum-normalization method, the measured quantity is in mmol of element per kg of embedded specimen. As each microregion contains an unknown amount of embedding medium, this quantity generally lies indeterminately somewhere within the wide range between mmol of element per kg of hydrated tissue and mmol of element per kg of dehydrated tissue. However, if a 'tag' element is incorporated in the embedding medium, the contribution of the medium to the local continuum count in each probed field should be measureable, and the X-ray data may then unambiguously yield mmol of element per kg of dehydrated tissue. This result should not be affected by shrinkage on freeze-drying or by incomplete replacement of water by embedding medium. The same X-ray data can additionally provide estimates of mmol of element per unit volume, mmol of element per kg of hydrated tissue and local dry-mass fraction. However, these estimates are subject to errors due to tissue shrinkage, incomplete replacement of water and beam damage.

Morphology of rat exocrine pancreas prepared by anhydrous cryo-procedures

This paper describes and compares the morphology of a relatively complex tissue, the exocrine pancreas, prepared by state-of-the-art anhydrous cryoprocedures. Cryopreparative procedures are being used increasingly for a wide range of applications, for example, electron-probe x-ray microanalysis and immunocytochemical localization of antigenic molecules, because they preserve the composition of the specimen better than procedures involving aqueous media. Some doubts have remained concerning the morphology of cryosections and the precise identification of subcellular structures. We show that thin and sufficiently large cryosections of fresh biological tissues can be produced using commercially available hardware. The freeze-dried cryosections display high intrinsic contrast, are stable under the beam, and allow identification of intracellular fine structure.

Low temperature techniques for X-ray microanalysis in pathology: alternatives to cryoultramicrotomy

Many diseases are associated with a change in the distribution of diffusible ions at the cell or tissue level. These diseases can profitably be studied by X-ray microanalysis. This technique for the study of ion distribution requires the use of cryoprepared specimens. Analysis at low or medium resolution can be carried out on thick or semi-thick cryosections, or on frozen-hydrated or freeze-dried embedded bulk samples. Such analyses are particularly useful in the initial stages of an investigation or when data from a large number of samples have to be acquired. Also X-ray microanalysis of cultured or single cells prepared by freeze-drying can be used to rapidly collect information on a large number of cells. Analysis at high resolution has to be carried out on thin sections: Cryosections or sections of freeze-substituted or freeze-dried embedded tissue. For the latter type of specimens, the use of low-temperature embedding methods may have important advantages.

Capabilities and limitations of probe methods for the microanalysis of chemical elements in biology: a brief introduction

This article reviews some of the technical problems involved in achieving high-resolution artefact-free quantitative microprobe analyses of biological specimens, and points to some of the techniques now available to overcome these problems.

Elemental changes associated with chondrocyte differentiation in rat rib growth plate

Quantitative X-ray microanalysis was under-taken to follow the elemental changes that occur in the process of chondrocyte differentiation. For analysis at the cellular level, semi-thick freeze-dried cryosections of rat rib growth plate cartilage were used. For evaluation of the elemental concentrations at the subcellular level, thin sections of freeze-dried and low temperature vacuum embedded cartilage were analyzed. Levels of Na, P, S, Cl, K, and Ca were determined in the cells and extracellular matrix in different zones of the cartilage--resting, proliferative, and hypertrophic. Proliferative cells had a sodium concentration that was twice that of resting cells, suggesting that Na may play an important role in the regulation of DNA- and protein-synthesis in chondrocytes, A concomitant rise in Na and S concentration occurred between resting zone and proliferative zone cartilage matrix. The high concentrations of Na and K in the matrix are probably due to the high amount of sulfate in proteoglycans which may bind these cations.