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Reagents Employed As Fixatives (Simple Fixatives)
Formaldehyde
Formaldehyde is a gas but it is soluble in water to the extent of 37-40%w/v. This
solution of formaldehyde in water is called formalin or full strength formalin. Formalin is
one of the commonly used fixative in all laboratories since it is cheap penetrates rapidly and
does not over harden the tissues.
It preserves the proteins by forming cross linkage with them and the tissue
component.
It denatures the proteins.
Glycogen is partially preserved hence formalin is not a fixative choice for
carbohydrates.
Some enzymes can be demonstrated in formalin fixed tissues.
It neither preserves nor destroys fat. Complex lipids are fixed but has no effect on
neutral fat. After formalin fixation fat may be demonstrated in frozen section. Pure
formalin is not a satisfactory fixative as it over hardens the tissue. A 10% dilution in water (tapor distilled) is satisfactory.
nce it oxidizes to formic acid if kept standing for long period so it shosl
neutralized by phosphates or calcium carbonate otherwise it tends to torm artifact; a bre
Pgment in tissues. To remove this pigment picric alcohol or saturated alcoholic sod
ydroxide may be used. Concentrated formalin should never be neutralized as thern
great danger of explosion. ere is a
The commercial formalin becomes cloudy on standing especially when stored in ae
Pde aue to formation of precipitate of paraformaldehyde which can be filtered. nacool
Formalin on prolonged exposure can cause either dermatitis its vapour may dama.
the nasal mucosa and cause sinusitis.
Time required for fixation:
Atroom temperature 12 hours
For small biopsies 4-6 hours
At 65°C fixation occurs in 2hours
Alcohol (Ethyl Alcohol)
Absolute alcohol alone has very little place in routine fixation for histopathology:
It acts as a reducing agents, become oxidized to acetaldehyde and then to acetie
acid.
It is slow to penetrate, hardens and shrinks the tissue.
Alcohol penetrates rapidly in presence of other fixative hence in combination eg.
Carnoy's fixative is used to increase the speed of tissue processing.
Ethanol preserves some proteins in relatively undenatured state so that it can be
used for immunofluorescence or some histochemical methods to detect certain
enzymes.
It is a fat solvent hence it dissolve fats and lipids.
Methyl alcohol is used for fixing blood and bone marrow smears.