Embedding

For mechanical support during the sectioning process, tissue must be infiltrated with an embedding medium. The usual embedding media are paraffin for light microscopy and an epoxy resin for EM samples. Paraffins are available that differ in melting point and hardness. Some products contain added plasticizers to make the blocks easier to cut. In general, the higher the melting point, the harder the wax. Waxes which melt at 55-58°C generally produce good sectioning results.

In paraffin embedding, the tissue is infiltrated with the paraffin and placed in a mold containing molten paraffin which is then allowed to cool. Often a vacuum is applied inside the tissue processor to assist penetration of the embedding agent. It is important to orient the tissue in the paraffin to facilitate sectioning along the desired axis.

Epoxy resins were introduced to provide the high strength, ultrathin, thermostable sections required by electron microscopy. Araldite, Epon and others are available from a number of sources. Each has a different profile of strength, permeation rate, convenience, etc. The user is referred to the suppliers of these materials for technical information.

Plastics require special reagents for dehydration and clearing that are expensive, limiting their use in light microscopy. The processing is usually done by hand and a special microtome is required for sectioning these blocks. Typically the technique is used for small biopsies, such as bone marrow or liver.

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