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Formaldehyde (methanal) is the simplest of aldehydes (chemical formula CHCO). As it is a small molecule, it vapourises easily and is gaseous at room temperature. It is a toxic and irritant substance.




LogoKeyPointsBox.pngWhen formalin is used to fix tissue for histopathological analysis, conventional teaching is that 10 times the volume of formaldehyde is required relative to the specimen and that fixation occurs at about 1mm per hour (although penetration increases with increasing temperature). While this is a good rule of thumb, the situation is more complex in that 'fixation' is a process of formaldehyde penetration, binding and cross-linking which do not occur at the same rate and vary with tissue type.[1]

Formaldehyde is often supplied as a 37% w/v solution. To prevent polymerisation of formaldehyde, methanol is added. The resulting solution is often referred to as formalin. This can be further diluted and buffered to produce 10% formalin (equivalent to 4% formaldehyde). This concentration is often used as a fixative and preservative for histological specimens. It gives tissues a firmer texture, making subsequent cut-up easier and also stops autolysis and necrosis of the tissue by inactivating micro-organisms and proteolytic enzymes. Poorly fixed tissue stains unreliably and certain features may be lost. Mitotic figures, for instance, are easily lost in poorly fixed tissue. Formalin is also used to preserve prosected and dissected cadavers.

Formalin cross-links sections of the peptide chains of proteins. In the case of immunohistochemistry, this process can alter epitopes which means that antibodies designed for fresh tissue may not work with fixed tissue. The cross-linking process can be partially reversed with various methods of antigen retrieval.

Molecular Biology

The same cross-linking process can be helpful in techniques such as chromatin immunoprecipitation (ChIP). This is method of establishing a physical interactions between a protein and particular area of DNA within the chromatin structure of a chromosome. Formaldehyde cross-links the DNA and protein, preserving the interaction for subsequent analysis.


Rarely used in clinical setting due to toxicity. Used occasionally in laboratory fume cupboard fumigation. A chemically similar compound called glutaraldehyde (propanal) is sometimes used to disinfect endoscopy equipment.

See Also