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Histopathology (from the Greek histos (tissue) and pathos (suffering) refers to the microscopic examination of tissue in order to study the manifestations of disease (viral or not). Specifically, in clinical medicine, histopathology refers to the examination of a biopsy or surgical specimen by a pathologist, after the specimen has been processed and histological sections have been placed onto glass slides. This is the most important tool of the anatomical pathologist in routine clinical diagnosis of cancer and other diseases. Histopathological examination of tissues starts with surgery, biopsy, or autopsy. The tissue is removed from the body or plant, and then placed in a fixative which stabilizes the tissues to prevent decay. The most common fixative is formalin (10% formaldehyde in water). The tissue is then prepared using histology procedures for viewing under a microscope. Briefly, the samples are transferred to a cassette, a container designed to allow reagents to freely act on the tissue inside. This cassette is immersed in multiple baths of progressively more concentrated ethanol, to dehydrate the tissue, followed by toluene or xylene, and finally extremely hot liquid (usually paraffin). During this 12 to 16 hour process, paraffin will replace the water in the tissue, turning soft, moist tissues into a sample miscible with paraffin, a type of wax. This process is known as tissue processing. The processed tissue is then taken out of the cassette and set in a mold. Through this process of embedding, additional paraffin is added to create a paraffin block which is attached to the outside of the cassette.
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