ALDEHYDE FIXATIVES Flashcards
• Gas produced by oxidation of methanol
Formaldehyde (Formalin)
Formaldehyde
Concentrations:
_____ gas form
______- stock concentration (causes overhardening of the external surfaces of tissues)
_____working solution; most commonly used
100%
• 37% to 40%
• 10%
Formaldehyde
Usually buffered to pH 7 with____
phosphate buffer
Advantages of formaldehyde
Cheap, readily available, and easy to prepare
Compatible with many stains
Penetrates tissue well
Allows natural tissue color to be restored
Disadvantages of formaldehyde
Irritating to the nose and eyes (allergic rhinitis); may cause Dermatitis
If unbuffered, may reduce both basophilic and eosinophilic staining
Prolonged fixation may cause bleaching of the specimen
Formaldehyde prolonged storage
EFFECTS
REMEDY
White formaldehyde ppt
- Filter
- Add 10% methanol (but dentures proteins thus unsuitable for EM)
Formaldehyde unbuffered
EFFECTS
REMEDY
Formation of Formic acid
Buffer or Methanol
- 10% formol saline + Mg** / Ca** carbonate in jar with marbles
Formaldehyde action of formic acid w excess blood
EFFECTS
REMEDY
Formalin pigments brown / black precipitate
- Saturated alcoholic picric acid
- 1% KOH in 80& ROH
blood - Kardasewitch’s Method
(70% ETOH & 28% ammonia H202 70%
ETOH & 28% ammonia water)
10% Formol-Saline
Saturated formaldehyde + 10% NaCl
Recommended for fixation of CNS tissues and general post mortem tissues for histochemical examination
10% Formol-Saline
Advantage: Ideal for Silver impregnation staining technique
Disadvantage: Slower; tissue shrinks during alcohol dehydration
10% Formol-Saline
10% Neutral Buffered Formalin (NBF)
Formaldehyde +
Na Dihydrogen Phosphate +
Disodium Hydrogen Phosphate
pH 7
Best general tissue fixative
10% Neutral Buffered Formalin (NBF)
Best for iron-containing pigments and elastic fibers which do not stain well after Susa, Zenker, or Chromate fixation
10% Neutral Buffered Formalin (NBF)
Disadvantage: Longer to prepare, inert to phospholipids and neutral fats
10% NBF