Biological Fixation and Its Functions

Study Notes

Fixation stabilizes proteins in biological tissues, preventing decay through autolysis (self-digestion) or putrefaction (bacterial decomposition).
Fixatives are chemicals that halt biochemical reactions and increase tissue strength.
Fixatives kill tissue, preventing autolysis and putrefaction.
Fixatives maintain the relationship between cells and extracellular substances (e.g., collagen, reticulin, elastin).
Fixatives enhance tissue visibility by altering refractive indexes.
Fixation preserves biological samples as close to their natural state as possible for examination.

Fixatives increase tissue receptiveness to dyes, often acting as mordants to link dye to tissue.
Each fixative creates a unique morphological pattern and potential artifacts in nuclear chromatin, cell membranes, and staining intensity.
Physical fixation methods include heat (e.g., microwave) and desiccation (air drying), though these are less common.

Additive fixatives: Chemically bind to tissue, altering its structure. Examples include mercuric chloride, chromium trioxide, picric acid, formaldehyde, glutaraldehyde, glyoxal, osmium tetroxide, and zinc salts.
Nonadditive fixatives: Act on tissue without chemical binding, usually by dissociating water molecules from proteins. This can cause shrinkage and hardening. Examples include acetone and alcohols.
Coagulant fixatives: Form a network within tissue, improving solution penetration. Examples include zinc salts, mercuric chloride, cupric sulfate, ethyl alcohol, methyl alcohol, acetone, and picric acid (acetic acid is a coagulant for nucleic acids, but not cytoplasm).
Noncoagulant fixatives: Create a gel-like structure, hindering subsequent solution penetration. Examples include formaldehyde, glutaraldehyde, glyoxal, osmium tetroxide, potassium dichromate, and acetic acid.

Temperature: Affects tissue morphology.
Tissue Size: Thickness is crucial; ideally no more than 3mm thick to ensure reagent penetration.
Volume Ratio: Fixative volume should be 15-20 times greater than the tissue volume.
Time: Immediate placement in fixative is vital. Adequate fixation time (e.g., 6-8 hours for formalin, depending on tissue size) is necessary for morphology preservation and to prevent distortion during subsequent processing.