Formaldehyde and glutaraldehyde are the most commonly used aldehyde fixatives. They work by forming cross-links both within and between proteins, particularly between lysine residues. Damage to the tertiary structure of the proteins occurs on a limited basis. Formalin (37% aqueous formaldehyde) is normally diluted 10 fold and neutrally buffered to make a working fixative solution consisting of 4% formaldehyde. The buffer prevents autolysis that occurs under acid conditions and also prevents the development of coloration of the tissue caused by formalin pigment.

Aldehyde fixatives form crosslinks between proteins.

Glutaraldehyde (normally used as a 2% buffered solution) causes deformation of alpha-helix structures in proteins, which limits its usefulness as a fixative for immunological stains. Glutaraldehyde is nonetheless a rapid fixative. As a result, it has become the standard for electron microscopy. Recently glutaraldehyde has become identified as a powerful allergen. A maximum exposure limit of 0.05mg/m³ has been imposed on laboratories in the United Kingdom, sharply limiting the use of this fixative there.

National Diagnostics' Mirsky's Fixative is an aldehyde based formula which is exceptionally good at preserving fine tissue structure and protein conformations. This fixative is an excellent replacement for Glutaraldehyde in EM applications. For light microscopy, Mirsky's Fixative is especially recommended for immunoperoxidase techniques where formaldehyde can lead to the loss of epitopes. The use of microwave enhancement can speed the action of Mirsky's Fixative greatly, allowing tissues to be embedded within a few hours.

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