Post chromation may also be referred to as post chroming or post chromatisation. It is the practice of treating tissues fixed in a primary fixative, usually one containing formalin or potassium dichromate, with a simple solution of potassium dichromate for an extended period, usually days and sometimes weeks. The purpose is to improve the preservation of lipid and lipoprotein materials that would otherwise be removed by processing reagents. It is not absolute in its effect, but does improve the preservation of material such as mitochondria and myelin so they can be demonstrated in paraffin sections.
The process does not well preserve triglycerides and, for them, frozen sections or post osmication should be used.
Fixed tissues are placed into a 2.5% to 3% solution of potassium dichromate in distilled water and put into the dark. Potassium dichromate may be photosensitive and the absence of light is desirable. It is left for the required time, often a few days to a week, then washed with running tap water overnight to remove all remaining dichromate. The tissue may then be processed.
It will undoubtedly be found that the tissue is brittle. This is inherent in the process and the degree tends to increase as the duration of post chroming lengthens.
This process is not just a fixation step, but it may also mordant the tissues in the true sense of the term. Dichromate contains chromium which is an effective mordant for some dyes. It may be retained in conjunction with some lipid material and be used to demonstrate them. This is particularly so with myelin.
Culling points out that sections may also be post chromed by placing dewaxed sections into a dichromate solution for 12 to 24 hours, followed by washing and staining. Of course, material that is removed during processing will no longer be susceptible to post chroming, although surprising amounts of protein bound lipids do resist extraction and are still present.
Drury, R.A.B. and Wallington, E.A.,
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Oxford University Press, Oxford, UK.
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