Paraffin processing is the common route to preparing slides for histological analysis. It is perfect for haematoxylin and eosin (H&E) staining as well as most special stains. Paraffin processing provides excellent resolution of cellular structures, if tissues are handled correctly at the fixation stage. Processing allows paraffin to infiltrate down to the cellular level, which gives the tissue enough even rigidity to be thinly sectioned and placed on a slide for further analysis.
Please adequately fix your samples and transfer them to 70% ethanol prior to bring them to the core. They can stay in 70% ethanol for an extended period of time without harming the sample.
Fixed tissue is first dehydrated through graded alcohols (70, 90, 95%), this is because wax and water do not mix. The tissues are then placed in xylene, which acts as a linking agent between the alcohols and wax. Finally the tissues are impregnated with paraffin wax at a temperature just above the melting point of the wax being used, and under vacuum. How much time and whether vacuum or heat are used, at each stage, is dependent on the type of tissues being processed. Human tissue often requires much longer in each stage plus the additional vacuum and heat however, if you use the vacuum and pressure on mouse tissues they often become very dry and difficult to section.
Tissues are then embedded in molten paraffin wax. This is a vital stage of tissue preparation, the positioning of the tissue in the mold is very important to getting what you want to see on the slide. Homogenous organs, like liver and spleen, are embedded on the flattest surface, more heterogenous tissues, like skin, hearts and brain can be placed in multiple orientations to ultimately section onto a slide the exact area of interest
Once the wax has cooled the resulting tissue 'blocks' can be sectioned on the microtome. Please see tissue sectioning for more details.
- Stains file, an all together good resource for histological technique: http://stainsfile.info/StainsFile/prepare/process/processing.htm
Dako processing chapter (written by John Kiernan): http://www.dako.com/08066_12may10_webchapter16.pdf