Secondary fixation is the term used for the practice of initially fixing with 10% formalin, then refixing with another fixative. The second fiixative refixes the tissue so that some of its characteristics can be obtained. In this way it is possible to obtain fixation with different characteristics on different blocks from the same bulk tissue. One advantage of this procedure is that it can be applied to tissues that have been fixed with a formalin variant and stored in the fixative for some time.
The secondary fixative chosen is usually a strong precipitant type which partially overcomes the protective effect of formalin. It must be emphasised, however, that secondary fixation does not give the same results as would have been obtained if the secondary fixative had been applied to fresh, unfixed tissue. It must surely be self evident that only those structures and organelles preserved by the primary fixative can be affected by the secondary fixative. If the structure or organelle is no longer there, no procedure can restore it. Secondary fixation is therefore not a panacea for improper fixative selection initially.
The most popular secondary fixatives are often those that contain mercuric chloride, such as formal sublimate or Helly's solution, although the procedure is not confined to those. This makes the practice more difficult, since all materials must be collected and disposed of properly. Fixatives which do not contain mercury can still be used with reasonable effect.
Drury, R.A.B. and Wallington, E.A.,
Carleton's Histological Technique, Ed. 5,
Oxford University Press, Oxford, UK.