FIXATION
PREPARED BY MR. ABHIJIT DAS
DEFINITION
fixation refers to the process
of preserving biological tissues from decay. The fixation process stops autolysis (self-digestion by the tissue's own
enzymes) and putrefaction (decomposition by
bacteria), maintaining the tissue's structure and composition.
FIXATIVES
Fixatives are chemical agents used to preserve
biological tissues by preventing decay and maintaining cellular structure
CLASSIFICATION OF FIXATIVES
Fixatives can be classified into two main categories:
1. Aldehyde Fixatives:
- Formaldehyde
(formalin)
- Glutaraldehyde
2. Non-Aldehyde Fixatives:
- Alcohol
(ethanol, methanol)
- Acetic
acid
- Bouin's
solution
FEATURES OF AN IDEAL
FIXATIVE
· Rapid
Penetration: Quickly permeates tissues to prevent
autolysis and putrefaction.
· Preservation
of Cellular Structure: Maintains the tissue's cellular
architecture and morphology.
· Stability:
Maintains tissue preservation over time without degradation.
· Non-Toxicity
and Safety: Ideally, it should be safe to handle and have
minimal health hazards.
· Inexpensive
and Easy to Use: Economical and easy to prepare and
apply.
· No
Formation of Artifacts: Does not introduce artifacts that
could interfere with diagnostic interpretation.
DIFFERENT PREPARATIONS OF
FORMALIN FIXATIVES
1. 10% Neutral Buffered Formalin (NBF):
- Composition:
- 10%
formalin (which is a 4% formaldehyde solution)
- Buffering
agents like sodium phosphate (monobasic and dibasic) to maintain a
neutral pH (around 7.0).
- Advantages:
- Prevents
acidification, which could lead to artifacts and tissue damage.
2. Formalin-Acetic Acid-Alcohol (FAA):
- Composition:
- 10%
formalin
- Acetic
acid
- 50-70%
ethanol
- Advantages:
- Good
for preserving cellular detail and soft tissues.
3. Modified Bouin’s Solution:
- Composition:
- 10%
formalin
- Picric
acid
- Acetic
acid
- Use:
- Often
used for biopsy specimens of soft tissues like testes and
gastrointestinal tract tissues.
4. Davidson’s Fixative:
- Composition:
- 10%
formalin
- Acetic
acid
- Ethanol
- Tap
water
- Use:
- Specifically
used for fixing eyes and certain other delicate tissues.
- Advantages:
- Provides
good preservation of ocular tissues without excessive hardening.
5. Zinc Formalin:
- Composition:
- 10%
formalin
- Zinc
salts (e.g., zinc sulfate or zinc chloride)
6. Alcoholic Formalin:
- Composition:
- 10%
formalin
- Ethanol
(70%)
7. Formaldehyde Calcium:
- Composition:
- 10%
formalin
- Calcium
acetate
- Use:
- Used
for fixing glycogen and other carbohydrate-rich tissues.
8. 10% Formal Saline:
- Composition:
- 10%
formalin
- 0.9%
sodium chloride solution (normal saline)
- Use:
- Commonly
used for fixing whole organs or large specimens.