Which statement is true?

• A. Acetone is sometimes used when a rapid acting fixative is needed.
• B. Formalin is a coagulative fixative and considered an excellent fixative for paraffin embedding.
• C. A fixative should penetrate slowly.
• D. Helly contains mercury.

Answer: (A)

Rapid action of fixatives is a key consideration in preserving cellular detail quickly and preventing autolysis. Acetone fixes tissues fast because it rapidly coagulates proteins and dehydrates the specimen, so it can stabilize morphology in a short period. This makes it useful when speed is essential—for example in certain cytology preparations or workflows where waiting for slower fixatives would compromise downstream procedures or antigenicity.

Keep in mind that while acetone’s speed is an advantage, it has drawbacks: it can extract lipids, cause tissue shrinkage, and make some specimens brittle, so it isn’t ideal for all tissues or for routine paraffin embedding. By contrast, formalin fixes mainly by cross-linking proteins, which preserves structure well and is excellent for paraffin embedding, but its mechanism and rate of penetration differ from a rapid coagulative fixative like acetone. It’s also worth noting that fixatives are chosen for their penetration rate, not for slow penetration, and some historic mercury-containing fixatives, such as Helly, do include mercury—though that detail doesn’t override the practical point about rapid fixation with acetone in appropriate scenarios.