The aim of tissue processing is to embed the tissue in a solid medium that is firm enough to give support to the tissue and enable thin sections to be cut with little damage to the tissue or to the knife. The most satisfactory material for routine histology is paraffin wax.
Before tissue can be processed to paraffin wax, it needs to be adequately fixed. Although dehydrating alcohols may complete the fixation of small fragments of tissue, larger pieces may not be fixed because of the slow rate of penetration of alcohol. Also, alcohols are less effective in dehydrating if called upon to act the dual role of dehydrant and fixative.
It is essential that the embedding material thoroughly permeates in fluid form, and that it solidifies without damaging the tissue. Paraffin wax is not miscible with water, and the fixed tissue must be gently but completely dehydrated to remove aqueous fixative and water, and cleared with a substance which is totally miscible with the dehydrant and embedding agent, such as xylene. 'Histoclear' is a nontoxic xylene substitute made from citrus fruit, and smells accordingly. The tissue is then embedded in paraffin wax in the correct orientation in metal moulds, and the wax cooled rapidly to minimize crystal size.
Tissue processing is carried out automatically in an Autotechnicon or similar instrument. This machine holds the 12 reagents required for processing and transports the tissue to each according to a programme controlled by a timing clock. Controlled heat, vacuum and vertical oscillation speed the exchange of fluids between tissues and reagents (Tables 13.2 and 13.3).
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