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Tissue fixation is the prevention of decay or putrefaction of biological tissues through physical or chemical means. It’s a process that works to terminate any ongoing biochemical reactions as well as increase tissue mechanical stability. The main objective of the tissue fixation process is to keep the histological specimen in a condition that allows for the preparation of thin stained sections.
However much tissue processing has been considered an irreversible process, some experiments have shown that tissue processing can be reverse mainly in immunohistochemistry particularly those involving formaldehyde though at the considerable quality of preservation.
Factors that affect tissue fixation
- Acidity or basity of the fixative
- Osmolarity potential of the fixative solution
- Size of the specimen and volume of fixative
- Duration of fixation
- Temperature during fixation
Types of tissue fixation processes
The type of fixation depends on the kind of specimen to be handled. there are three types namely Temperature fixation where the tissue is heated lightly or frozen to arrest tissue degrading processes such as autolysis, immersion where the specimen is immersed into a fixative fluid in a container and perfusion in which the fixative is passed through the blood vessels.
Principles of fixation
Cross-linkage: These are fixatives that act by creating covalent bonds between proteins in tissues making it firm. an example of fixatives that act like this includes formaldehyde and glutaraldehyde. This property is associated with aldehyde fixatives.
Precipitation: These fixatives reduce the solubility of protein molecules by disrupting hydrophobic interactions between them. This is the action property of alcohols such as methanol and acetone. They are used in combination with other fixatives because when used alone, cause considerable shrinkage and hardening or even swelling as seen when acetone is used alone.
Oxidation: These fixatives react with side chains of proteins and other bio-molecules allowing the formation of cross-links that harden the tissue. Examples of oxidizing fixatives include osmium tetraoxide, potassium dichromate, and potassium permanganate.