How to Preserve, Stain & Mount Microscope Specimens

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  • 0:00 Why Use Slides?
  • 0:43 Dry vs. Wet Mount Slides
  • 2:05 Prepared Mount Slides
  • 3:14 Stains
  • 4:46 Preparing Specimens
  • 5:14 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
Microscopes are incredible tools for observing the tiniest of organisms. Here we will identify the types of slides that can be made and describe how to best preserve, stain, and mount specimens. At the end, you can test yourself with a brief quiz.

Why Use Slides?

Observing microscopic subjects, both living and non-living, is a fascinating and illuminating task. Perhaps you remember the first time you used a magnifying glass or microscope to get a closer look at your fingerprint, an ant, plant cell, or any other close observations. These specimens are placed onto thin, glass rectangles called slides and covered with a small cover slip for observation. Sometimes the specimen on the slide is a whole organism. Other times it is a thin slice of tissue, or a smear of fluid in which are cells to be observed. There are three basic types of slides that can be observed well under a light microscope, which we will now learn how to prepare!

Dry vs. Wet Mount Slides

The easiest slides to prepare are dry mount slides. These are essentially a dry piece of specimen placed directly on the slide with the cover slip on top of it. Simple to make! These work well for specimens such as hair, pollen, an insect wing, or dry skin. The one major disadvantage of these slides is that they are very temporary and will fall apart easily with no liquid holding the cover to the slide. A breeze can blow the specimen off the slide before the cover is applied! Handle these carefully, or be ready to prepare a new slide!

Simply adding liquid or oil to a specimen on a dry mount slide makes it a wet mount slide. This type of slide may or may not have a cover slip on top, and can also be effective in observing live specimens. The liquid could be water or a type of oil, and provides an effective medium for live specimens to move within, and also adds a magnifying effect that enhances observation. These slides are prepared by adding one or two drops of liquid to the slide first, then adding the specimen, followed by another drop or two of liquid. This way, the specimen is completely engulfed in the liquid. If a cover slip is applied, it must be done so very slowly as to prevent air bubbles from becoming trapped beneath it. Adding a cover will also have a slight flattening effect on the specimen, but it will also keep it from drying out as quickly, which is a disadvantage of open wet mount slides.

Prepared Mount Slides

The most challenging slides to make are prepared mount slides. These are permanent slides, usually with a stain applied to highlight specific parts of the specimen. It is used best on blood smears, bacteria cells, or tissue samples. Unlike a wet mount slide, there cannot be excessive amounts of water, and the specimen itself must be sliced very thin. This is one of the more challenging parts of making a prepared mount slide. Once the specimen is well-positioned and dry, with a stain applied if need be, a fixative is applied to quickly kill and preserve the specimen and freeze its positioning. Different types of chemicals can be used as fixatives, the easiest of which are are a blend of alcohol and acetic acid, the combination of which denatures proteins in the specimen. The fixative usually needs about a full day to sit, after which the specimen should be washed with water, removing extra fixative. At this point the specimen is ready to be preserved in a glycerin gel or hypoxy mounting medium. Only enough to cover the specimen should be added prior to placing the cover slip on top. Once dry, the slide is finished and permanent.

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