CHAPTER 7: CHEMICAL FIXATIVES PART 2

Question 1

Act by reducing the solubility of protein molecules and often by disrupting the hydrophobic interactions that give many proteins their tertiary structures..

Answer 1

Precipitating / Denaturing (Alcoholic) Fixatives

Question 2

Methanol, ethanol, and acetone are protein denaturants, rarely used alone for fixing blocks unless studying nucleic acids. However, they are poorly used for cytologic smear.

a. TRUE/FALSE
b. FALSE/TRUE
c. BOTH TRUE
d. BOTH FALSE

Answer 2

a. TRUE/FALSE

Question 3

Suitable for fixing thin layer preparations such as blood films or cell cultures

Answer 3

Methanol

Question 4

• For solid specimens taken from patients with gout
• Fast and cheap
• For subsequent histochemical detection of sodium urate crystals
• Not good for EM due to tissue shrinkage

Answer 4

95% Ethanol

Question 5

Most useable DNA fragments for PCR

Answer 5

Ethanol

Question 6

Used to fix and preserve glycogen, pigments, blood, tissue films and smears

Answer 6

Absolute Alcohol

Question 7

Absolute alcohol is combined w/ ______ to bring out some of the original color of the specimen.

Answer 7

GLYCERIN

Question 8

• It is excellent for fixing dry and wet smears, blood smears and bone marrow tissues.
• It fixes and dehydrates at the same time

Answer 8

Methyl Alcohol 100%

Question 9

For fixing touch preparations, although some touch preparations are air dried and not fixed, for certain special staining procedures such as Wright-Giemsa

Answer 9

Isopropyl Alcohol 95%

Question 10

The following are true about Ethyl Alcohol (70%-100%), except:

a. 1-3 hours fixation time
b. If the lower concentrations are used, the RBC’s become hemolyzed and WBC’s are inadequately preserved.
c. Hemosiderin preservation is less than in buffered formaldehyde
d. It may be used as a simple fixative.

Answer 10

a. (correct answer: 18-24 hours)

Question 11

Advantages of Ethyl Alcohol (70%-100%)

Answer 11

1. It preserves but does not fix glycogen.
2. It fixes blood, tissue films and smears
3. It preserves nucleoproteins and nucleic acids,
   hence, is used for histochemistry, especially for enzyme studies.
4. It fixes tissue pigments fairly well.
5. It is ideal for small tissue fragments
6. It may be used both as a fixative and dehydrating agent

Question 12

Considered to be the most rapid fixative and may be used for urgent biopsy specimens for paraffin processing within 5 hours

Answer 12

Carnoy’s Fixative

Question 13

It preserves Nissl granules and cytoplasmic granules well

Answer 13

Carnoy’s Fixative

Question 14

It is also used to fix brain tissue for the diagnosis of rabies

Answer 14

Carnoy’s Fixative

Question 15

Disadvantages of Carnoy’s Fixative

Answer 15

1. It produces RBC hemolysis, dissolves lipids and can produce excessive hardening and shrinkage
2. It is suitable only for small pieces of tissues due to slow penetration.
3. It dissolves fat, lipids, and myelin.
4. It leads to polarization unless very cold temperatures (-70°C) are used
5. It dissolves acid-soluble cell granules and pigments

Question 16

• 3-4 hours fixation time
• For frozen sections and smear
• Can produce fair results after conventional processing if fixation time is kept very short
• Preserves nucleic acids but extracts lipids

Answer 16

Clarke’s Solution

Question 17

• 12-24 hours fixation time
• 0.5 calcium acetate can be added to ensure neutrality
• Sometimes used during processing to complete fixation following incomplete primary formalin fixation
• Can be used for fixation or post-fixation of large fatty specimens (breast)
• Used for primary fixation specimens, it can be placed directly into 95% ethanol for processing

Answer 17

Alcoholic Formalin

Question 18

• 1-6 hours fixation time
• Faster acting agent than alcoholic formalin due to the presence of acetic acid
• Sometimes used to fix diagnostic cryostat sections
• Used for primary fixation specimens, it can be placed directly into 95% ethanol for processing

Answer 18

Formol-acetic Alcohol

Question 18

• 1-6 hours fixation time
• Faster acting agent than alcoholic formalin due to the presence of acetic acid
• Sometimes used to fix diagnostic cryostat sections
• Used for primary fixation specimens, it can be placed directly into 95% ethanol for processing

Answer 18

Formol-acetic Alcohol

Question 19

• It is used to fix sputum, since it coagulates mucus
• It is good for preservation of glycogen and for micro-incineration technique (the burning of a minute tissue specimen for identification of mineral
  elements from the ashes).

Answer 19

Gendre’s Fixative (w/ glacial acetic acid)

Question 20

Disadvantages of Gendre’s Fixative (w/ glacial acetic acid)

Answer 20

1. It produces gross hardening of tissues
2. It causes partial lysis of RBC.
3. Preservation of iron-containing pigments is poor
4. Formaldehyde does not give as good a morphological picture as glutaraldehyde.
5. Formaldehyde causes little cross-linking under usual fixation conditions where low concentrations of proteins are used, while glutaraldehyde is most effective at cross-linking.

Question 21

• 12-18 hours at 3oC
• It is recommended for fixing mucopolysaccharides and nuclear proteins.
• It produces better reaction in Feulgen stain than Carnoy’s fluid
• It acts both as a nuclear and histochemical fixative.

Answer 21

Newcomer’s Fluid