Aldehyde Fixatives in Histology

Aldehyde Fixatives

• Formaldehyde 10% is also known as Formalin 10%, Formol-saline 10%, Neutral buffered formalin, Formal-corrosive, Formol alcohol, Acetic alcohol formaldehyde, Formol ammonium bromide, 10% formol calcium, and Formol sucrose.

Formol-Corrosive/Formol-Sublimate

• Mercuric chloride is also known as corrosive sublimate.
• It is recommended for routine post-mortem tissues.
• It is valuable for secondary fixation following primary fixation in 10% formalin variants.
• Fixation time is 3-24 hours.
• Advantages: rapidly penetrates small pieces of tissue, produces minimum shrinkage and hardening, excellent for many staining procedures, brightens cytoplasmic and metachromatic stains, and preserves cytological structures and blood cells.
• Disadvantages: penetration is slow, forms mercuric chloride deposit, does not allow frozen tissue sections to be made, inhibits the determination of the extent of tissue decalcification, is intolerant to fixative, and corrodes metal.

10% Neutral Buffered Formalin/Phosphate-Buffered Formalin (pH 7)

• It is desirable compared to 10% formalin.
• It is recommended for preservation and storage of surgical, post-mortem, and research specimens.
• Buffer: phosphate.
• Fixation time: 4-24 hours.
• Advantages: similar to formol-saline, prevents precipitation of acid formalin pigments on post-mortem tissue, and best fixative for tissues with iron-pigments and for elastic fibers.
• Disadvantages: preparation is time-consuming, reduces positivity of mucin to PAS, reduces reactivity of myelin to Weigert's Iron Hematoxylin Stain, and is inert towards lipids.

10% Formol-Saline

• It is a simple microanatomical fixative composed of 10% formalin + NaCl (maintains osmolarity).
• It is recommended for fixation of central nervous tissue, general post-mortem tissues for histochemical exam, and fixation time is 24 hours at 35°C (95°F); 48 hours at 20-25°C (65-77°F).
• Advantages: penetrates and fixes tissues evenly, preserves microanatomic and cytologic details, large specimens may be fixed for a long time (must change the solution every 3 months), preserves enzymes and nucleoproteins, demonstrates fats and mucins, ideal for most staining techniques (silver impregnation), and + 70% alcohol = restoration of natural tissue color.
• Disadvantages: slow fixative (>24 hours), tissues tend to shrink during alcohol dehydration (solution: secondary fixation), and metachromatic reaction of amyloid is reduced.