Types of Fixatives in Histology

Cytological Fixatives

• Preserve specific parts and microscopic elements of the cell

Nuclear Fixatives

• Preserve nuclear structures (e.g., chromosomes)
• Typically contain glacial acetic acid as the primary component due to its affinity for nuclear chromatin
• pH of 4.6 or less
• Examples: Flemming's fluid, Carnoy's fluid, Bouin's fluid, Newcomer's fluid, Heidenhain's Susa

Cytoplasmic Fixatives

• Preserve cytoplasmic structures
• Must not contain glacial acetic acid, which destroys mitochondria and Golgi bodies
• pH of more than 4.6
• Examples: Flemming's fluid without acetic acid, Kelly's fluid, Formalin with "post-chroming", Regaud's fluid (Muller's fluid), Orth's fluid

Histochemical Fixatives

• Preserve chemical constituents of cells and tissues
• Examples: Formal Saline, 10% Absolute Ethyl Alcohol, Acetone, Newcomer's Fluid

Secondary Fixation

• Process of placing an already fixed tissue in a second fixative
• Facilitates and improves the demonstration of particular substances
• Makes special staining techniques possible
• Ensures further and complete hardening and preservation of tissues

Types of Fixatives

• Aldehydes (formaldehyde, glutaraldehyde): act by cross-linking proteins
• Oxidizing agents (osmium tetroxide, potassium permanganate): act by cross-linking proteins
• Alcohol-based fixatives (methyl alcohol, ethyl alcohol, acetic acid): protein-denaturing agents
• Metallic fixatives (mercuric chloride, picric acid): act by forming insoluble metallic precipitates

Classification of Fixatives

• According to composition:
  • Simple Fixatives (e.g., aldehydes, metallic fixatives, picric acid, acetic acid, acetone, alcohol, osmium tetroxide)
  • Compound Fixatives (e.g., combination of two or more fixatives)
• According to action:
  • Microanatomical Fixatives (e.g., 10% formal saline, 10% neutral buffered formalin, Heidenhain's Susa, Formal sublimate, Zenker's solution, Bouin's solution)