Histology is the study of tissue sectioned as a thin slice, using a microscope. It can be described as microscopic anatomy. Histology is an essential tool of biology. It is also an important tool of anatomical pathology since accurate diagnosis of tumors and other diseases usually requires histological examination of samples.
Histological examination of tissues starts with surgery, biopsy or autopsy. The tissues are then fixed in a fixative, a process that "cooks" the tissues to prevent decay. The most common fixative is formalin (10% formaldehyde in water). The samples are then immersed in multiple baths of ethanol, followed by toluene, finally hot paraffin. During this 12 to 16 hour process, paraffin will take the place of water. Soft, wet tissues are turned into a hard block. This allows the sectionning of tissues into very thin (5 micrometre) sections using a microtome. These slices, thinner than the average cell, are then layered on a glass slide for staining.
A 5 micrometre 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. Haematoxylin and eosin are among the most commonly used stains in histology. Haematoxylin colors nuclei blue, eosin colors the cytoplasm pink. Other compound used to color tissue sections include saffron, silver salts and numerous 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 images.
Histological classification of animal tissues
epithelium: the lining of glands, bowel, skin and some organs like the liver, lung, kidney,
endothelium: the lining of blood and lymphatic vessels,
mesothelium: the lining of pleural, peritoneal and pericardial spaces,
mesenchyme: the cells filling the spaces between the organs, including fat, muscle and tendon cells,
blood cells: the red and white blood cells, including those found in lymph nodes and spleen,
neurons: cells forming the brain, nerves and some glands like the pituitary and adrenal glands,
placenta: a specialized organ essential for the growth of the fetus in the mother's uterus, and
stem cells: cells able to turn into one or several of the above types.
Note that tissues from plant, fungus and microorganisms can also be examined histologically. Their structure is very different from the one above.
Cytology, the study of loose cells, for example cells taken from the cervix during a cervicovaginal smear or from the blood. The cells are directly spread on a glass slide and stained as described above.
Cell biology, the study of structures withing the cell itself (called organelles). It usually requires an electron microscope and biochemistry techniques.
Anatomy, which is at a level above tissues, studying organs visible by the naked eye; and
Morphology, which studies entire organisms.
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.
A histolgical artifact is a structure or feature of the sample that is absent in living tissues, but introduced during preparation or staining (1). There are many types of artifacts that can occur, and troubleshooting and minimizing artifacts is a major part of the discipline of histology.
1. Merck Source (2002). Dorland's Medical Dictionary. Retrieved 2005-01-26.
2. Stedman's Medical Dictionaries (2005). Stedman's Online Medical Dictionary. Retrieved 2005-01-26.
Last updated: 08-18-2005 15:24:36