Which type of fixatives act by creating a gel that impedes penetration by subsequent solutions and includes formaldehyde and glutaraldehyde?

• A. Noncoagulant fixatives
• B. Nonadditive fixatives
• C. Coagulant fixatives
• D. Michel solution

Answer: (A)

The key idea here is how fixation changes protein behavior in tissue. Coagulative fixatives denature and precipitate proteins, forming a gel-like network that can block or slow the movement of subsequently applied reagents, leading to uneven penetration. In contrast, fixatives like formaldehyde and glutaraldehyde fix by creating covalent cross-links between proteins, preserving structure without forming that obstructive gel barrier. Because formaldehyde and glutaraldehyde act by cross-linking rather than precipitating proteins into a gel, they are considered noncoagulant fixatives.