Introduction to the theory and practice of fixation of tissues

Many approaches to fixation and types of fixatives have been developed and tested over the last century. The mechanisms by which fixatives act to harden and preserve tissues fall into broad categories, including dehydrants, heat effects, cross-linkers, effects of acids, and combinations of these categories. Each fixative has advantages and disadvantages, including specific molecules retained within fixed tissues, swelling or shrinkage of fixed tissues, variations in the quality of histochemical and immunohistochemical staining, and varying capabilities to maintain the structures of cellular organelles. One of the major problems with formaldehyde type (cross-linking) fixatives has been the loss of antigen immunorecognition; correcting this usually requires sonic method of antigen recovery. Similarly, the extraction of mRNA and DNA from formalin fixed tissue in paraffin blocks is problematic. All widely used fixatives are selected by compromise-good aspects are balanced against less desirable features. This article discusses the basics of fixation and provides the formulas for the fixatives currently used in pathology, histology, and anatomy and discusses good and bad aspects of specific fixatives.