Pathology Fixation
last authored: June 2010, Kimberly Ordinelli
last reviewed:
Introduction
Fixation is the most important step in Histology! This step preserves the tissue as life like as possible by stabilizing the tissue to avoid activity such as:
- decay
- putrefaction (bacterial attack)
- autolysis (enzyme attack)
Immediately after the removal of the tissue it is put into a fixative then transported to the lab to be processed. There are two types of fixation, physical and chemical.
Physical
Heat is used as a physical fixative. It denatures and stabilizes the proteins within the tissue. The tissue (2mm thick) is immersed in saline and heated by a microwave at a temperature between 50-68C. It is very important that the temperature remains within these limits. If the temperature rises above 68C, the tissue will show pyknotic, over stained nuclei. When temperature is below 50C poor fixation will occur.
Chemical
Chemical fixation is more commonly used in hospital labs today. There are different types - additive/nonadditive and coagulant/noncoagulant.
Additives change the electrical charge of the tissue by adding itself to the tissue. This change in charge is important to help maintain the shape of protein tertiary structure. Common additives are mercuric acid, picric acid, formaldehyde and others.
Nonadditives, such as alcohols and acetones, precipatate or coagulate the proteins instead of binding to them. Nonadditives disassociate bound water molecules from tissue protein groups. If overexposed, shrinkage and hardening can take place.
Coagulant forms a network within the tissue so solutions can penetrate deep inside the tissue. Some examples of these are zinc salts, mercuric chloride, and many more.
Noncoagulant forms a gel like substance within the tissue which makes the solutions entry more difficult.
Resources and References
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