To maintain the tissue in as lifelike a state as possible, tissue for analysis is usually placed directly into a fixative solution upon removal from the body. Fixation is normally carried out as soon as possible to prevent autolysis and to reduce possible infectivity. Several factors determine the choice of fixative for a given application. If morphological changes are known to take place rapidly after tissue collection—as with neural tissue—the speed of fixing action is extremely important.

For large tissue samples, the rate of penetration of the fixative into the sample must also be considered. If immunological detection methods are to be used, a fixative which preserves protein structure is required. For electron microscopy, fixatives which do not precipitate proteins avoid artifacts invisible to light microscopy. The price of a fixative may also be a factor. The low cost of formaldehyde fixatives is one reason for their popularity. A partial list of fixatives would include (grouped by chemical type):

Aldehydes

• Formaldehyde
• Glutaraldehyde
• Mirsky’s Fixative

Mercurials

• B-5
• Zenker’s

Alcohols

• Methanol
• Ethanol

Picrates

• Bouin’s solution

Oxidizing agents

• Osmium tetroxide
• Permangante fixatives (potassium permanganate)
• Dichromate fixatives (potassium dichromate)

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