Which issue is described as mounting medium on top of the coverslip?

• A. Air bubbles beneath the coverslip
• B. Mounting medium on top of the coverslip
• C. Excessive dehydration
• D. Inadequate fixation

Answer: (B)

The important idea is how mounting medium should be applied for a proper mount. The mounting medium must fill the space between the slide and the coverslip so the tissue is in contact with a clear, refractive-index-matched medium. When mounting medium is placed on top of the coverslip, it no longer sits between the slide and the tissue. The tissue isn’t adequately preserved at the interface, and the medium can dry or crystallize on the top surface, creating uneven optics, glare, and a weak seal. This results in an image that looks hazy or imperfect and the coverslip can become unstable.

To avoid this, put a small drop of mounting medium on the slide (or apply it to the tissue side) and then place the coverslip so the medium is drawn into the space between slide and coverslip. Wipe away any excess from the edges after seating the coverslip. This distinction from issues like air bubbles beneath the coverslip (which is a different mounting artifact) helps ensure a clear, durable mount.