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How to prepare squash specimen samples for microscopic observation

Found in: Howto, Labwork

Squashing the specimens (instead of cutting them) is a fast and easy way to prepare specimens.

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Specimens have to be sufficiently thin and transparent to be viewed under the microscope. One can use a microtome to thinly section the material. These samples have to be sufficiently solid to be easily cut. Soft samples can not be easily cut, and must be dehydrated first in alcohol, which hardens them. There is also another possibility to prepare specimens. It is also possible to squash specimens between the coverslip and the slide.

  1. Place a drop of water on the slide and then a small piece of the specimen into the water.
  2. Carefully position the specimen in the center and place the cover glass on top, as if making a regular wet mount.
  3. Using a soft round object, such as an eraser, carefully press down on the coverslip without horizontal movement, which would introduce shearing forces. This is the testing stage, to check if the specimen is sufficiently soft. The cover glass may break otherwise. This is, why you should use an eraser, to protect yourself.
  4. If the sample is sufficiently soft, you can press down with more force. The sample should form a thin, almost transparent film between coverslip and slide. Again, do not introduce a horizontal movement.
  5. Excess water should be soaked up with tissue paper. If some of the specimen starts to appear from beneath the cover glass, then you used too much specimen.
  6. You can use any objective to observe under the microscope.

Samples that are too solid need to be softened first. Some plant material can be made softer by boiling, but this may not be enough to soften the cellulose of the cell walls. The cellulose of the cell walls can be made softer by heating with an acid, such as diluted HCl or acetate (careful, dangerous). Rinse the specimen after acid treatment with water and compress it.

Staining the samples should take place before squashing the specimen, as it is otherwise difficult for the stain to reach the cells.

Suitable specimens include soft fruits and fungi. Squashing may introduce artifacts. The cells are separated from each other and it is not possible to see the original place of the cells. If you want to see the arrangement of cells, as they occur naturally, then you need to resort to microtoming. The advantage of squashing is, that it is a fast and easy method to obtain very thin specimen samples.

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