Which fixative is considered the routine fixative?

• A. 10% NB Formalin
• B. Bouin's
• C. Osmium Tetroxide
• D. 70% Ethanol

Answer: (A)

The key idea is choosing a fixative that reliably preserves tissue structure for routine light microscopy and downstream processing. Neutral buffered formalin at about 10% is used as the standard because it rapidly penetrates tissue and forms stable cross-links between proteins, locking cellular architecture in place without causing excessive distortion. This preserves morphology well enough for routine hematoxylin and eosin staining and is compatible with paraffin embedding and many downstream procedures, making it economical and practical for daily use.

Bouin’s fixative, while it can enhance nuclear detail, contains picric acid and is hazardous; it also often causes color changes and compatibility issues with some stains and antigen detection, so it isn’t the routine choice. Osmium tetroxide excels for electron microscopy and lipid preservation but is expensive, toxic, and not suitable for routine light microscopy. Ethanol, at 70%, can fix some specimens but tends to cause shrinkage and hardening, and does not provide the broad, reliable preservation needed for routine paraffin processing.