Marking the edges of tissue sections with tissue marking inks/dyes aids in the identification and correct orientation of the tissue pieces during embedding. This application also assists in adequately placing the intended surface toward the face of the block, which is the first surface to meet the microtome blade during microtomy.
Tissue marking inks/dyes must:
- Be insoluble in fixative solutions, processing solutions, and the embedding media
- Remain on the surface intended to be marked and not penetrate too far into the tissue interior
- Not react with histologic stains or be over-applied to obscure cells or tissue layers
- Be notably visible both macroscopically and microscopically
Some examples of tissue marking inks/dyes include India ink, silver nitrate, and various artist pigments. Different inks/dyes may be applied to mark anatomic or surgical landmarks. This will help orientation both during gross dissection and embedding and in the final histologic section.
- The ink/dyes may be applied to color the entire outside perimeter of the specimen or used selectively, using single or multiple colored inks/dyes.
- The application color and location are typically dictated in the gross description.
- It is recommended to try to avoid red ink/dye since it may be difficult to see. Black or bright blue ink/dye is often preferred.
Table 2. Tissue marking examples.| Tissue Type | Example |
| Small lesions present in a more extensive section of tissue | - The lesion may only be visible on one "face" or plane of the section. In this case, one side of the tissue can be inked, and specific instructions should be provided, such as "embed with inked tissue surface up."
- Note that most histologists will want to relate any directional instructions to the block face, which is the surface that will meet the microtome blade first.
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| Tubular structures | - It is sometimes helpful to submit the entire tube (vas deferens, temporal arteries, fallopian tubes) in the tissue cassette, with instructions to cut cross-sections before embedding.
- The lumen end can be dipped in tissue ink/dye to mark the opening for very tiny tubes more clearly. The lumen must be visible in the final section.
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| Multiple fragments | - The fragments may be inked on one edge but should be embedded so that all pieces are kept in the same plane or level in the block to obtain one complete, representative section.
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