Histological examination of tissues starts with Surgery, Biopsy or Autopsy. The tissues are then fixed in a fixative, a process that stabilizes the tissues to prevent decay. The most common fixative is formalin (10% Formaldehyde in water). The samples are then immersed in multiple baths of progressively more concentrated Ethanol, to dehydrate the tissue, followed by a clearing agent such as Chloroform, Xylene or Histoclear, and finally hot molten paraffin wax (impregnation). During this 12 to 16 hour process, paraffin wax will replace the water: soft, moist tissues are turned into a hard paraffin block, which is then placed in a mould containing more molten wax (embedded) and allowed to cool and harden. This allows the sectioning of tissues into very thin (5 - 7 micrometer) sections using a Microtome. These slices, thinner than the average cell, are then placed on a glass slide for staining.
A 5 to 7 micrometer slice of most tissues is almost completely transparent with very little visible detail. To see the tissue under a Microscope, the sections are stained with one or more pigments. Hematoxylin and Eosin are among the most commonly used stains in histology and histopathology. Hematoxylin colors nuclei blue, eosin colors the Cytoplasm pink. Other compound used to color tissue sections include Saffron, silver salts and numerous natural and artificial dyes originally developed to stain cloth fibers. The science of tissue staining is called histochemistry.
Recently, antibodies are used to stain specific proteins: this is called immunohistochemistry. This technique has greatly increased the ability to identify categories of cells under a microscope. Other advanced techniques include in situ hybridization to identify specific DNA or RNA molecules, and confocal microscopy. Digital cameras are increasingly used to capture histological and histopathological images.
In the 19th Century, histology was an academic discipline in its own right. The 1906 Nobel Prize in Physiology or Medicine was awarded to two histologists, Camillo Golgi and Santiago Ramón y Cajal. They had dueling interpretations of the neural structure of the brain based in differing interpretations of the same images.
Note that tissues from plant, fungus and microorganisms can also be examined histologically. Their structure is very different from animal tissue.
2. Stedman's Medical Dictionaries (2005). Stedman's Online Medical Dictionary. Retrieved 2005-01-26.