A particular fixing agent may be better at preserving some tissue constituents than it is at preserving others. Fixative mixtures that contain this agent may be a better choice for fixation when this constituent is the target for examination.
An example would be acetic acid, which is excellent for nucleoprotein. Fixatives that contain acetic acid often give sharp and clear preservation of nuclear structure as a consequence. For that reason an acetic acid fixative would be a good choice when nuclear structure is important.
Fixatives are often classified according to whether they preserve structures in relation to each other, or whether they preserve individual cellular constituents.
Those fixatives that target overall tissue structure are called microanatomical fixatives. Those that target cellular components are called cytological fixatives. Of this group, some are nuclear preservatives in contrast to others which are far better at cytoplasmic preservation. Some other fixatives target particular chemicals in the tissue, including enzymes and antigens. These are the histochemical fixatives.
No classification system for fixation effects can be considered to be absolute as fixatives may be satisfactory for more than one purpose. As an example, formal sublimate is considered to be primarily a microanatomical fixative but it also preserves nuclear structure well, and formalin fixed tissues are often used for immunological staining.
Culling C.F.A., (1974)
Handbook of histopathological and histochemical techniques, Ed. 3
Butterworth, London, UK.