What maximum temperature should be avoided in microwave-assisted fixation to prevent pyknotic, overstained nuclei?

• A. 40 C
• B. 55 C
• C. 65 C
• D. 75 C

Answer: (B)

In microwave-assisted fixation, temperature control matters because heat speeds how fixatives work, but too much heat can cause artifacts. Pyknosis refers to nuclei that become abnormally condensed and intensely stained, a sign of over-fixation or heat damage. Keeping the fixation temperature low enough helps preserve normal nuclear morphology and prevents overstaining. Around 55 C is the practical upper limit where you still get effective fixation without crossing into artifact risk. Higher temperatures—65 C or 75 C—significantly increase the chance of nuclear pyknosis and overstaining, so they should be avoided.