Tissue Processing and Embedding
The processing sequence
Processing moves tissue from hydrated to paraffin-infiltrated through graded dehydration (ascending alcohols), clearing (xylene or a substitute), and infiltration (paraffin). Each step has a purpose: alcohol removes water, clearing agents are miscible with both alcohol and paraffin, and molten paraffin replaces the clearing agent. Candidates should be able to explain why a processing schedule must be adjusted for tissue type and size-fatty tissue needs longer clearing and infiltration, while small biopsies need shorter times to avoid shrinkage and brittleness.
Embedding orientation
Embedding is where orientation errors are permanently locked in. The rule is to embed so the cut face is the diagnostic face and is parallel to the block surface. Skin should be on edge, tubular structures on cross-section, and multiple pieces aligned in one plane. Air bubbles or gaps under the tissue cause floating or torn sections.
Traps
Common question traps include: confusing dehydration with clearing; assuming xylene dehydrates (it clears); forgetting that incomplete dehydration leaves milky clearing fluid and poor sections; and over-processing small specimens until they crumble. Another trap is thinking paraffin infiltration is optional-without it, sections will not ribbon.
Worked example
A fatty breast specimen sections poorly with holes and chatter. The likely cause is inadequate clearing/infiltration because adipose tissue resists paraffin. The fix is a longer clearing and infiltration schedule (or a clearing agent better suited to fat), not a sharper blade.