Sections for the Congo red technique should be cut at approximately which thickness?

• A. 2-3 nm
• B. 10-12 μm
• C. 0.5-1 μm
• D. 50-100 nm

Answer: (A)

The important point here is that the Congo red technique for detecting amyloid relies on obtaining uniform, well-penetrated staining in a tissue section that is thin enough to be examined clearly under light microscopy with polarized light. If the section is too thick, dye penetration can be incomplete and the light path through the specimen becomes cluttered, making the apple-green birefringence difficult to observe. If it’s too thin, the section may be fragile or may not contain a representative amount of amyloid.

A thickness around 2–3 micrometers hits a practical balance: it’s thin enough to allow even dye penetration and clear optical viewing, yet thick enough to preserve tissue integrity and contain a representative sample of amyloid for accurate assessment. That’s why this range is favored for this technique.

In contrast, much thicker sections (like 10–12 μm) can hinder dye penetration and polarization visualization, while very thin sections (0.5–1 μm) may be fragile and not consistently representative, and extremely thin ultrathin sections (50–100 nm) are more appropriate for electron microscopy, not standard Congo red light microscopy staining.