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Paraffin Infiltration

The final step of processing is infiltration of a cell-supporting medium. Paraffin wax is the most common infiltration and embedding medium. Paraffin wax is miscible with xylene as well as isopropanol, but is not miscible with alcohols or aqueous fixatives. A wide variety of infiltration media is available to suit every tissue type and application. During microtomy, factors such as wax hardness, stickiness, and brittleness should be considered with the technique and tissue in mind. As a general rule, the higher the melting point of paraffin, the harder the paraffin and the more difficult the ribboning. The lower the melting point, the softer/stickier the paraffin, and the easier it is to ribbon during microtomy. Most common paraffins have a melting point between 55-58° C and the paraffin is typically kept at 2-4° C above melting point on the processor. During this last processing step, heat and vacuum are both utilized for optimal infiltration of paraffin. However, too much heat and time in liquid paraffin may harden tissue and create microtomy problems. As with the previous processing steps, the optimal infiltration time for each tissue type/size must be determined for optimal results.