2.1-FIXATIVES

Question 1

What type of fixatives create covalent bonds between proteins (especially lysine residues)?

Answer 1

Crosslinking fixatives

Question 2

Why are crosslinking fixatives ideal for immunohistochemistry?

Answer 2

Antigenicity is not lost

Question 3

What fixative prevents autolysis and preserves tissue morphology?

Answer 3

Buffered formalin

Question 4

What are the advantages of buffered formalin (10 total)?

Answer 4

Cheap+readily available+compatible with many stains+preserves fat/mucin/glycogen+does not over-harden tissues+penetrates well+allows frozen sections+restores natural tissue colors+preserves enzymes+no need for frequent washing

Question 5

What are the disadvantages of buffered formalin (6 total)?

Answer 5

Irritating fumes+skin irritation+tissue shrinkage+soft fixative for cytoplasmic structures+forms pigments in unbuffered solutions+prolonged fixation causes bleaching/dispersal of fat/glycogen

Question 6

What factors influence formalin fixation?

Answer 6

Post-mortem interval+composition+volume+time+temperature+tissue thickness+post-fixation storage

Question 7

What is the fixation time for 10% formal-saline?

Answer 7

12-24 hours

Question 8

What tissues are fixed with 10% formal-saline?

Answer 8

Routine histopathology+CNS tissues+histochemical exam tissues+lipids/phospholipids

Question 9

What fixative replaces mercuric chloride and improves IHC?

Answer 9

Zinc formalin

Question 10

What fixative brightens cytoplasmic/metachromatic stains and fixes lipids?

Answer 10

Formol-corrosive (formol-sublimate)

Question 11

What polymerized formaldehyde is used for antigen detection?

Answer 11

Paraformaldehyde

Question 12

What fixative combines paraformaldehyde and glutaraldehyde for light/electron microscopy?

Answer 12

Karnovsky’s fixative

Question 13

What fixative preserves ultrastructure but deforms alpha-helix proteins?

Answer 13

Glutaraldehyde (2% buffered)

Question 14

What is the storage condition for stable glutaraldehyde?

Answer 14

4°C at pH 5

Question 15

What fixative is ideal for electron microscopy due to rapid irreversible crosslinking?

Answer 15

Glutaraldehyde

Question 16

What fixative is not routinely used for tissues but is good for cytologic smears?

Answer 16

Precipitating (denaturing) fixatives

Question 17

What is the mechanism of action of precipitating fixatives?

Answer 17

Rapidly denatures and precipitates proteins by destroying hydrogen bonds

Question 18

What are the advantages of absolute alcohol as a fixative?

Answer 18

Fixes and preserves glycogen+pigments+blood+tissue films+smears

Question 19

What are the disadvantages of absolute alcohol as a fixative?

Answer 19

Instantly coagulates proteins but causes distortion

Question 20

What fixative is used for dry and wet smears

Answer 20

blood smears

Question 21

What fixative is used for touch preparations?

Answer 21

Isopropyl alcohol (95%)

Question 22

What fixative is used for frozen sections and smears

Answer 22

preserves nucleic acids but extracts lipids?

Question 23

What fixative completes fixation during processing and clears lipids in large fatty specimens?

Answer 23

Alcoholic formalin

Question 24

What fixative acts faster than alcoholic formalin due to acetic acid and is used for diagnostic cryostat sections?

Answer 24

Formol-acetic alcohol

Question 25

What fixative is used for rapid diagnosis

Answer 25

preservation of glycogen

Question 26

What fixative is recommended for mucopolysaccharides and nuclear proteins?

Answer 26

Newcomer’s fluid

Question 28

What metallic fixative is a protein precipitant but causes tissue shrinkage and requires iodine treatment to remove brown color?

Answer 28

Mercuric chloride

Question 29

What metallic fixative contains mercuric chloride

Answer 29

potassium dichromate

Question 30

What metallic fixative combines Zenker’s solution with formalin for better preservation of cytoplasmic details?

Answer 30

Zenker-Formol (Helly’s solution)

Question 31

What metallic fixative is used for lymphoid tissues and provides excellent nuclear detail?

Answer 31

Lillie’s B-5 fixative

Question 32

What metallic fixative preserves glycogen well but dissolves lipids and requires proper washing to avoid artifacts?

Answer 32

Heidenhain’s Susa solution

Question 34

What oxidizing agent reacts with protein side chains to form crosslinks and is mainly used as a secondary fixative?

Answer 34

Oxidizing agents (e.g.

Question 35

What oxidizing agent provides excellent preservation of cellular details and is commonly used in electron microscopy?

Answer 35

Osmium tetroxide (osmic acid)

Question 36

What oxidizing agent is part of Flemming’s solution and preserves lipids well while providing good nuclear detail?

Answer 36

Flemming’s solution with or without acetic acid

Question 37

What chromate fixative uses 1-2% aqueous solution to precipitate proteins and preserve carbohydrates?

Answer 37

Chromic acid

Question 38

What must be added to chromic acid to prevent decomposition?

Answer 38

Strong reducing agent

Question 39

What is the concentration of potassium dichromate used to preserve lipids and mitochondria?

Answer 39

3% aqueous solution

Question 40

What chromate fixative demonstrates chromatin

Answer 40

mitochondria

Question 41

What chromate fixative is used to study early degenerative processes

Answer 41

tissue necrosis

Question 42

What is the primary use of Orth’s fluid in microbiology?

Answer 42

Demonstrates rickettsiae and other bacteria

Question 43

What fixative contains picric acid in saturated aqueous solution (1%) to precipitate proteins and form crystalline picrates?

Answer 43

Picric acid fixatives

Question 44

What are the advantages of picric acid fixatives?

Answer 44

Penetrates tissue well+produces brighter staining+preserves connective tissue+glycogen+lipids

Question 45

What are the disadvantages of picric acid fixatives?

Answer 45

Low pH may hydrolyze nucleic acids+explosive in dry form

Question 46

What fixative is used for embryos

Answer 46

pituitary biopsies

Question 47

What effect does Bouin’s solution have on tissue color?

Answer 47

Stains tissue bright yellow

Question 48

What fixative is preferred for GIT and endocrine tissues with reduced lysis?

Answer 48

Hollande’s solution

Question 49

What alcoholic Bouin’s variant minimizes microanatomical distortion and is ideal for Masson’s trichrome staining?

Answer 49

Gendre’s solution

Question 50

What compound precipitates DNA and is added to fixatives to prevent nucleic acid loss?

Answer 50

Glacial acetic acid

Question 51

Why is glacial acetic acid contraindicated for cytoplasmic fixation?

Answer 51

Damages cytoplasmic organelles

Question 52

What fixatives are recommended for acid mucopolysaccharides and connective tissue mucin?

Answer 52

Lead fixatives

Question 53

What fixative penetrates hydrophobic protein domains and acts as a weak decalcifying agent?

Answer 53

Trichloroacetic acid

Question 54

What fixative is used for cryostat sections

Answer 54

enzyme studies (phosphatases/lipases)

Question 55

What solution preserves fresh tissues for transport in immunofluorescence studies (not a fixative)?

Answer 55

Michel’s solution

Question 56

What is the maximum recommended time for storing tissues in Michel’s solution?

Answer 56

24 hours