Preservatives are separate and distinct from fixatives, even though they may contain similar chemicals. The primary purpose of a fixative is to chemically alter the tissue proteins so that microscope preparations may be made. The primary purpose of a preservative is to keep, i.e. preserve, the tissue in the same state it was when first immersed in the preservative, that is without any deterioration. Some are used after the tissue has been fixed for long term storage or display, others with unfixed tissue to ensure viability of individual tissue components, still others as a coverslipping medium.
Preservatives used for coverslipping differ from mounting media because they do not solidify and require that the medium be sealed against evaporation and leakage by hardening media such as asphalt varnishes, paraffin wax or nail polish. Hardening mounting media are listed separately.
It should be noted that the method which preserves tissues the most effectively is to process them through paraffin. They will then be stable for very long periods of time, well over 100 years, as shown by the preservation in blocks processed at the end of the 19th century when histotechnology was in its infancy. Those blocks can still be sectioned and stained, the results being indistinguishable from those of well processed tissues from the present.
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Last updated January 2019