Tissue samples of formaldehyde have been used for nearly 120 years. Its application to histology was quickly adopted and started success stories around the world. Even at this point, a 10% formaldehyde concentration is recommended, which corresponds to a final formaldehyde concentration of approx 4%.
This formalin concentration is still used today. The only modification after its introduction was the use of neutralizing phosphate buffers to dilute the formaldehyde stock solution, thereby reducing the chemical aggressiveness of formalin.
You can get more information about paraffin-embedded tissue via the web.
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Formalin reacts with protein and nucleic acid side groups and causes protein cross-linking and nucleic acid fragmentation. Nucleic acid fragmentation is caused by depurination, whereas protein cross-linking is mediated by reactive NH3 side groups.
The effect of formalin is further enhanced if the fixation is carried out (1) at a higher temperature, (2) in an unbuffered solution, or (3) for a longer period of time.
The use of formalin-fixed (and paraffin-embedded) tissue for protein detection or nucleic acid-based methods requires some consideration to avoid misinterpretation of the results. For breaking down protein networks, enzymatic degradation by proteinases such as proteinase K was initially the method of choice.
Since enzyme treatment is often associated with decreased morphological quality, pretreatment of formalin hot cutting by boiling for several minutes in a buffer with or without high pressure is the main approach for antigen recovery at present.
This heat pretreatment changes the protein's conformation and allows detection of the appropriate immunogenic epitopes, independent of the cross-linked protein.