A tissue fixative that protects macromolecules (DNA,RNA and protein) and histomorphology in clinical samples is called universal fixative.
Eg:- Formaline, Glutaraldehyde.
Advantage and
Disadvantage
It is a process of maintaining the tissue
in the same conditions as removed from the body by preventing autolysis by
keeping the tissue in a fixation solution.
Purpose of fixation:
Ø
To prevent autolysis and to preserve the cell.
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To harden the tissue.
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To conserve the consistency of cells.
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To after refractive index of cells which enable
and unstained components to be better visualized.
Fixation is done in
two stages:
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Primary fixation
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Secondary fixation
Primary fixation
is done in O.T where who organ or tissue is submerged in the fixative.
Secondary
fixation is done in Laboratory Large specimen are cut into slices of 1-2 cm
thickness and fixed in adequate fixative for 24 hours. Next day it is cut into
square. It is further fixed for 5-6 hour before tissue processing embedding is
done.
Chemical composition and mode of action of
different fixative: Adv/Disadv.
Formaline:- It is
a solution of formal dehyde gas in water
Commercial formaline-10ml
N/s-90ml
Advantages:- Normal
colour of tissue is retained. It is
best fixative for neurological tissue. It is easily prepared and compatible
with most stains. It penetrates the tissue rapidly.
Disadvantages:-
It causes excessive hardening of tissue.
Ø
Unpleasant vapour causes irritation in eyes.
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May cause allergic reaction to skin.
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Its acidic character may interfare with
substain.
Ø
Leads to formation of formaline pigment in
tissue having excessive blood at acidic PH.
Zenker’s fluid:-
Composition:-
Mercuric chloride:50gm
Patasium
chloride:25gm
Sodium
sulphate:10gm
D/W: upto
1000ml.
Mode of action:-
5 ml GAA is added to 95 ml of Zenker’s fluid just before use. This fixation is
used when tissue is to be stained by Masson’s Trichrome stain for connective
tissue.
Before staining tissue is washed for several hours in running water to
remove mercuric chloride pigmentation.
Ø
Mordanting is required to prevent fine grannules
of mercury.
Advantages:- Used
for 2°
fixation, brilliant staining of Nuclei of bone marrow, liver, spleen. It causes
excellent staining of connective tissue.
Disadvantags:-
Overnight washing of fixative tissue is required to remove yellow colouration
caused by dichromate.
Penetrating
power is poor, longer time is required.
Bovin’s fluid:-
It is compound fixtive. It has following composition.
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1.22% picric Acid- 750ml
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40% formalin – 250ml
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GAA – 50m
Advantages:- →Best
preservative for glycogen.
→Brilliant
staining by trichrome stain.
→Used for hematological and lymphoid tissue.
Disadvantages:- →Picric
acid is explosive.
→Yellow colour appearance due to picric Acid. Which makes the tissue
hard and brittle.
→It causes lysis of red cell.
Carnoy’s
fluid:-
Composition:- ①
Absolute alcohol- 300ml
② Chloroform- 150ml
③ GAA-10ml
Advantages:-
Ø
Used for cyhology of small piece of tissue.
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It has very good penetrating power.
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Dehydration is achieved during fixation.
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It has good for blood film and enzymes
demonstration.
Disadvantage:- Chloroform and alcohol are inflammable an
Evaporate.
Primary
fixatives:- It is initial fixation of whole tissue removed from operation.
Usual fixative used is done in formaline. Volume of fixative should be 10-20
times more than the volume of specimen. It should be kept in this solution for
6-24 hrs, minute specimen are wrapped in lens paper or special fine mesh
capsule before immersing it in fixative other fixatives that are used are
Hgcl2, K2CT2O7, picric acid, Ethylene oxide, Glutoralhyde, osmium tetraoxide
and alcohol.
Secondary
fixative:- It is also known as refixing, the tissue removed from are cut
into pieces of 3-5 mm thick 2-4cm length. Example are formaline, carnoy’s,
bovin’s zenker’s fluid etc.
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