commonly used as a preservative for tissue specimens
Fixation (histology) 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.