<P> In both immersion and perfusion fixation processes, chemical fixatives are used to preserve structures in a state (both chemically and structurally) as close to living tissue as possible . This requires a chemical fixative . </P> <P> Crosslinking fixatives act by creating covalent chemical bonds between proteins in tissue . This anchors soluble proteins to the cytoskeleton, and lends additional rigidity to the tissue . </P> <P> By far the most commonly used fixative in histology is formaldehyde . It is usually used as a 10% neutral buffered formalin (NBF), that is approx . 3.7% - 4.0% formaldehyde in phosphate - buffered saline . Because formaldehyde is a gas at room temperature, formalin - formaldehyde gas dissolved in water (~ 37% w / v) is used when making the former fixative . Paraformaldehyde is a polymerized form of formaldehyde, usually obtained as a fine white powder, which depolymerises back to formalin when heated . Formaldehyde fixes tissue by cross-linking the proteins, primarily the residues of the basic amino acid lysine . Its effects are reversible by excess water and it avoids formalin pigmentation . Other benefits include: Long term storage and good tissue penetration . It is particularly good for immunohistochemistry techniques . Also the formaldehyde vapor can be used as a fixative for cell smears . </P> <P> Another popular aldehyde for fixation is glutaraldehyde . It operates in a similar way to formaldehyde by causing deformation of the alpha - helix structures in proteins . However glutaraldehyde is a larger molecule, and so its rate of diffusion across membranes is slower than formaldehyde . Consequently, glutaraldehyde fixation on thicker tissue samples may be hampered, but this problem can be overcome by reducing the size of the tissue sample . One of the advantages of glutaraldehyde fixation is that it may offer a more rigid or tightly linked fixed product--its greater length and two aldehyde groups allow it to' bridge' and link more distant pairs of protein molecules . It causes rapid and irreversible changes, fixes quickly, is well suited for electron microscopy, fixes well at 4 C, and gives best overall cytoplasmic and nuclear detail . However it is not ideal for immunohistochemistry staining . </P>

Commonly used as a preservative for tissue specimens