A smear is a thin layer of a specimen spread onto a microscope slide. This preparation step is performed prior to heat-fixing a sample to ensure the specimen is spread thinly on the slide before it is briefly heated over a heat source.

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In clinical settings, light microscopes are frequently used to observe microorganisms. Because most cells and microbes lack color and contrast in their natural state, specimens must be artificially treated to detect important cellular structures. The two basic types of preparation used to view specimens with a light microscope are wet mounts and fixed specimens.

Specimen Preparation

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A wet mount is the simplest type of specimen preparation for light microscopy, where the specimen is placed on a slide in a drop of liquid. Liquid specimens, like a drop of urine, can be applied directly with a dropper. Solid specimens, such as skin scrapings, are placed on the slide before liquid is added. Water is commonly used, but stains are often added to enhance contrast. A coverslip is then placed over the sample to keep it securely in place before microscopic examination.

Wet Mount

Smear

Samples for fluorescence and confocal microscopy are prepared similarly to those for light microscopy, but they use fluorochromes instead of standard dyes. These fluorochromes are often diluted in liquid before being applied to the slide. The dyes can be attached to antibodies to target specific proteins on specific cells (immunofluorescence), or they can attach to DNA molecules to stain cells based on specific DNA sequences (fluorescence in situ hybridization, or FISH).

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