Histology Techniques - Tissue Fixation and Fixatives

Tissue fixation is used for several reasons, including prevention of putrefaction from bacteria, autolysis from enzyme degradation and loss of soluble substances. Most tissue fixation also helps enhance staining in later techniques like immunohistochemistry, if required and also routine haematoxylin and eosin staining.

Fixation preserves a sample of biological material (tissue or cells) as close to its natural state as possible in the process of preparing tissue for examination. To achieve this, several conditions usually must be met.

First, a fixative usually acts to prevent autolysis. This is self-destruction of the cell by the body own enzymes, which are released by lysosomes rupturing after death.  They cause more damage to soft tissue like brain than to harder tissue like collagen. Fixatives do this by disabling proteolytic enzymes and so prevent any damage occuring to the tissue from their activity.

Second, a fixative typically protects a sample from putrefaction. This is the destruction of the cell by bacteria which are normally commencals, but after death they spread quickly causing decomposition. Fixatives are toxic to most common microorganisms like bacteria, that may exist in a tissue sample or which might otherwise colonise the fixed tissue. In addition, many fixatives chemically alter the fixed material to make it either toxic or indigestable for opportunistic microorganisms, so helping to prevent putrefacation.

Finally, fixatives often alter the cells or tissues on a molecular level to increase their mechanical strength and stability. This increased strength and rigidity helps to preserve the morphology of the sample, which is important later on to acheive accurate diagnosis for patients.

Despite the care taken sometimes even the most careful fixation will effect the tissue and introduce unwanted artifical artifacts that can interfere with interpretation of cellular structure.

One very well known historical example of this phenomenon wass the bacterial mesosome which at first was believed to be organelle in gram-positive bacteria. However this was later shown by new techniques developed for electron microscope to be an chemical fixation artifact.

Standardization of fixation and other tissue processing procedures must be careful to account for artifacts by understanding what procedures introduce which kinds of artifacts. Researchers must know what types of artifacts to expect with each tissue type and processing technique to accurately interpret sections with artifacts, or idealy select techniques that will minimize artifacts production in the areas being analysed.

Here is a list of the main types of histology fixatives, as well as some of their advantages and disadvantages

Here is a list of further types of less common fixatives that can be used, as well as some of their advantages and disadvantages

Here is a list of the least common types of fixatives that can be used, as well as some of their advantages and disadvantages