Embedding excellence.

Embedding tissue isn't just a routine step, it's an art. Whether you're a seasoned histotech or just starting out, embedding can have challenges that may impact an accurate diagnosis. Avoiding these common embedding missteps can improve workflow, reduce rework, and protect patient safety.

• Incorrect tissue orientation
  Proper orientation is essential. Wrong orientation can ruin the specimen(s) with the first cut. 
  • General rule: The side facing down in the cassette should face down in the mold – this is the surface to be sectioned. 
  • Large specimens: embed flat but slightly angled to allow gradual sectioning.
  • Tissues with a wall (e.g., gallbladder, GI tract cysts): Embed on edge so all layers are visible. 
  • Tubular structures (e.g., appendix, fallopian tube): Embed in cross section (on end) so that all layers are visible. 
  • Skin: Position the skin on edge so the epithelium or epidermis face one side of the mold - not up or down. 
  • When cassettes contain more than one specimen, avoid placing them randomly. Align all pieces in the same orientation, angle slightly, and are in a line or arranged according to your lab SOPs.
    
    
• Uneven embedding plane
  If tissue isn’t embedded evenly, extensive trimming may be needed, risking loss of important diagnostic material.
  • Use tissue tampers to gently press down on the tissue to ensure all parts of the tissue are even.
    
    
• Ignoring tissue-specific needs
  Different tissues require different embedding techniques. When in doubt, ask a colleague! Post a visual “cheat sheet” at embedding stations as a reminder.
  • Download and print this Embedding Best Practices sheet to post at your station!
    
    
• Air bubbles
  Air pockets can distort tissue sections and cause chatter.
  • Keep molds warm 
  • Use a tissue tamper to gently tap molds and cassettes to release trapped air. 
    
    
• Overcrowding in the mold
  • Center the tissue in the mold and leave a small margin around the tissue – specimens should never touch the edge of the mold - this can complicate sectioning. 
  • Don’t leave too much space; excess paraffin can make it difficult to fit on the slide.
  • Choose an appropriate size mold for each piece of tissue – it is not one size fits all. 
    
    
• Embedding multiple cassettes at once
  Embed one cassette at a time to prevent mix-ups. If interrupted, you’ll know exactly which tissue goes with which cassette.
  
  
• Embedding too quickly
  Speed can compromise quality. Mistakes like incorrect tissue orientation or uneven embedding often result from rushing. Take the time needed to embed correctly – patient care depends on it. 
  
  
• Inconsistent cooling of blocks
  Rapid, even cooling produces small paraffin crystals that support tissue better. Larger crystals can lead to microtomy issues and incomplete sections. 
  • Ensure your cold plate has a consistent and stable temperature and is cooled completely before using.
  • Avoid overcrowding blocks on the cold plate.
  • Let each block solidify completely before removing it from the mold. 
• Overheating paraffin 
  Embedding temperature should be only a few degrees above the melting point of the paraffin (typically 55-60°C). Overheating can cause:
  • Tissue shrinkage and tearing
  • Damaged cells or altered morphology
  • Tissue detachment if embedding temperature is not consistent with the tissue temperature
  • Breakdown of paraffin, making sectioning difficult
    
    
• Dirty forceps or wells
  Tissue fragments can transfer between blocks, risking misdiagnosis. Clean forceps after every block with gauze.