Uneven nuclear staining and center-centered bubbling in a liver section fixed in 10% NBF are most likely due to what?

• A. Incomplete fixation before dehydration
• B. Over-fixation
• C. Prolonged clearing
• D. Rapid embedding

Answer: (A)

The main idea is that fixative needs to penetrate the tissue evenly to preserve structure for reliable staining. If fixation is incomplete before dehydration, the fixative has not fully crossed-links proteins throughout the section, so the outer regions fix faster while the center remains underfixed. This creates a diffusion or penetration gradient, leading to uneven nuclear preservation and staining, and can produce artifacts like vacuole- or bubble-like spaces centered in the specimen as dehydration and clearing proceed on inadequately fixed tissue.

Over-fixation tends to make tissue overly cross-linked and staining less uniform in a different way, often dulling nuclear detail but not specifically causing center-centered bubbling. Prolonged clearing can cause general tissue distortion or extraction artifacts, not the central bubbling pattern tied to lack of fixative penetration. Rapid embedding can introduce infiltration defects, but the hallmark center-focused bubbling tied to underfixation before processing aligns best with incomplete fixation prior to dehydration.