Although various fixatives are available and the information about each fixative is extensive, our focus will be on formalin fixation. The MOST common and widely accepted fixative is 10% neutral buffered formalin (NBF). This aqueous solution is considered a “universal fixative” since it can be used for a wide variety of stains and techniques.
The properties of formalin are as follows:
- Penetrates tissue quickly, but fixes slowly because cross-linking to tissue proteins takes a long time
- Causes less tissue shrinkage than other fixatives
- Hardens tissue, but less than alcohols and acetone
- Relatively inexpensive and stable
- Lipids are preserved, but not made insoluble
- May result in formalin pigment in bloody tissue, but this is prevented by neutralizing the formalin
Although biopsy specimens may only require several hours to fix properly, larger surgical samples require eight or more hours of fixation prior to the subsequent processing steps. Most small specimens are fixed at room temperature prior to processing, but larger samples will finish fixing in the first step on the tissue processor. Since formalin and paraffin are NOT miscible (capable of mixing), formalin-fixed tissue must be taken through several intermediate steps before it is placed in paraffin.