Many approaches to fixation and types of fixatives have been developed and
tested over the last century. The mechanisms by which fixatives act to hard
en and preserve tissues fall into broad categories, including dehydrants, h
eat effects, cross-linkers, effects of acids, and combinations of these cat
egories. 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 extracti
on of mRNA and DNA from formalin fixed tissue in paraffin blocks is problem
atic. All widely used fixatives are selected by compromise-good aspects are
balanced against less desirable features. This article discusses the basic
s of fixation and provides the formulas for the fixatives currently used in
pathology, histology, and anatomy and discusses good and bad aspects of sp
ecific fixatives.