Embedding Orientation

Correct orientation of tissue in a mold is the most important step in embedding. Incorrect placement of tissues may result in diagnostically important tissue elements being missed or damaged during microtomy. In circumstances where precise orientation is essential, tissue should be marked (notched or inked) or agar double embedded. Usually tissues are embedded with the surface to be cut facing down in the mold.

Some general considerations are as follows:

• Elongated tissues are placed across the block
• Tissue with lumen (i.e. Fallopian tubes, vas deferens, vessels, appendix, etc.) are embedded on end so that the lumen is visible.
• Tissue from “hollow” organs (i.e. bowel, stomach, esophagus) should be embedded on edge so as to show the various layers.
• Tissues with an epithelial surface such as skin, are embedded to provide sections in a plane at right angles to the surface (hairy or keratinized epithelia are cut last to minimize the damage to the knife surface and avoid scratches in the deeper layers)
• Hard tissues (bone, uterus) will section more easily if it is embedded diagonally (dense/hard areas should be the last to be cut by the knife)
• Multiple tissue pieces are aligned across the long axis of the mold, and not placed at random.
• Fragments are clustered toward the center of the block

Troubleshooting Embedding

• Soft mushy tissue may be due to insufficient processing. Tissue may have been cut too thick during grossing or spent insufficient time in processing solutions. Solution: cut thinner and reprocess.
• Incorrect orientation is usually caught at the microtomy stage when the technologist inspects the blocks. Solution: melt down block and re-embed.
• To prevent incorrect orientation, mark the tissue with ink at grossing or place embedding instructions on the grossing sheet
• Tissue carry over prevented by wiping forceps with clean paper towel or gauze in between samples. Smooth forceps (i.e. without teeth) should be used for filtered specimens and those that may have abundant free cells to avoid tissue being trapped in the teeth of the forceps.
• Tissue not embedded at the same level (plane) can cause portions of the tissue to be missing on the slide. This can be prevented by pressing down on all edges of the tissue in the wax during orientation and working quickly to prevent hardening of wax. Visual inspection of blocks at the microtomy level can ensure all portions look like they are on the same plane. Solution: melt down and re-embed if necessary
• Pieces of tissue missing from the block can be prevented by recording the expected number of pieces of tissue in each cassette during grossing. The embedding technologist must consult the grossing log to be aware of the expected number of pieces. Also careful inspection of the cassettes (including lid) before discarding will ensure no tissue remains inside.