Fixatives

Question 1

Formaldehyde/Formalin

Answer 1

• non-coagulant
• additive
• forms methylene bridges by binding to amino group
• traps glycogen
• fixes lipids but doesn’t make them insoluble
• enhances staining with basic and cationic dyes
• preserves some antigens and enzymes
• forms AFH at pH below 6.1

Question 2

Acetic Acid

Answer 2

• does not fix carb or lipids
• penetrates quickly and doesn’t harden tissues
• never used alone
• excellent fixative for nucleoproteins
• causes swelling

Question 3

Acetone

Answer 3

• coagulant
• non-additive
• rapid
• dissolves lipids
• overhardens and shrinks tissues
• good demonstration of enzymes
• used for brain if suspected of rabies

Question 4

Ethanol/Methanol

Answer 4

• coagulant
• non-additive
• rapid
• dissolves lipids
• not used alone
• overhardens and shrinks tissues
• methanol for touch preps and blood smears
• ethanol preserves glycogen
• used alone for cytology or slide fixative

Question 5

Glutaraldehyde

Answer 5

• non-coagulant
• additive
• also forms methylene bridges
• cannot be used for staining techniques that bind with aldehydes (Periodic Acid Schiff’s)
• slow
• primary fixative for e- microscopy (2 hours)

Question 6

Mercuric Chloride

Answer 6

• coagulant
• additive
• binds to sulfhydryl groups
• never used alone, can be secondary
• good quality staining but no longer used
• leaves a black pigment (remove with dezenkerization: alcoholic iodine followed by sodium thiosulfate)

Question 7

Osmium tetroxide

Answer 7

• non-coagulant
• additive
• secondary fixative for e- microscopy
• preserves lipids (appear black)
• tissues must be thin
• can fix cornea and nasal mucosa (use fume hood)
• expensive

Question 8

Picric acid

Answer 8

• strong coagulant for nuclear proteins
• leaves DNA soluble
• additive
• keeps tissue soft but shrinks them
• not used alone
• demonstration of glycogen
• enhance trichrome staining
• lyses RBCs
• colours tissues yellow (remove w/ 70% alcohol)

Question 9

Zinc Salts (ZnSO4)

Answer 9

• coagulant
• additive
• acceptable replacement for mercury
• usually used with formaldehyde
• enhances nuclear detail
• often used for lymph node and bone marrow biopsies
• good for immunohistochemistry

Question 10

B5

Answer 10

• HgCl2, concentrated formaldehyde, sodium acetate
• additive
• enhances both acid and basic stains
• used for Bx: renal, GI, lymph node
• nuclear detail
• antigen preservation
• radio-opaque
• never used alone
• always get mercury pigment

Question 11

Bouin’s

Answer 11

• picric acid, 40% formaldehyde, acetic acid
• additive
• poor for nucleoproteins
• preserves glycogen
• enhances acid dyes
• enhances trichrome staining
• excellent tissue morphology
• lyses RBCs
• stains tissues yellow
• Bx: GI, endocrine
• may form formalin pigments

Question 12

Zinc Formalin

Answer 12

• zinc sulfate, formaldehyde, distilled water/alcohol
• quicker than formalin alone
• additive
• preserves glycogen
• great for immunohistochemistry
• enhances basic dyes
• used for Bx
• may get AHF pigments
• substitute for Hg fixatives

Question 13

Zenkers

Answer 13

Mercuric Chloride, potassium dichromate, glacial acetic acid (extremely toxic)

Question 14

Helly’s

Answer 14

Mercuric Chloride, potassium dichromate, conc. Formalin (extremely toxic)

Question 15

Carnoy’s

Answer 15

Ethanol, chloroform, glacial Acetic acid (preserves glycogen, rapid acting)

Question 16

Orth

Answer 16

Potassium dichromate, sodium sulfate, conc. formalin (used to demonstrate Chromaffin cells)