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Tissue Dehydration

Dehydration is simply the removal of water from aqueous-fixed tissue. Since most fixatives are aqueous, this step is necessary to prepare the tissue for embedding in non-aqueous media like paraffin. Alcohols are most commonly used in the laboratory for tissue dehydration, since they are miscible with aqueous fixatives like 10% formalin. In this step, the alcohol penetrates tissue quickly and the water is replaced with alcohol. Since alcohols act rapidly and may shrink and harden tissue too much, care must be taken when calculating the amount of time needed in the dehydration step. This step is performed at room temperature. Ethyl alcohol and isopropyl alcohol are used most often, with methanol and butanol being used to some degree in special techniques. Since most alcohols (with the exception of isopropyl alcohol) and paraffin are NOT miscible, another step, known as clearing, is introduced.