Histology

Image:Emphysema H and E.jpg Histology is the study of tissuesectioned as a thin slice, using a microscope. It can be described as microscopic anatomy. Histology is an essential tool of biology.

Histopathology, the microscopic study of diseased tissue, is an important tool of anatomical pathologysince accurate diagnosis of cancerand other diseases usually requires histopathological examination of samples.

The trained technicians who perform the preparation of histological sections are Histotechnicians, Histology Technicians (HT) or Histology Technologists (HTL). Their field of study is called histotechnology.

Source of tissue

Histological examination of tissues starts with surgery, biopsyor autopsy.

Technical procedure

Fixation

The tissues are fixed in a fixative, a process that stabilizes the tissues to prevent decay. The most common fixative is formalin (10% formaldehydein water).

Embedding

The most common technique is wax embedding. The samples are immersed in multiple baths of progressively more concentrated ethanolto dehydrate the tissue, followed by a clearing agent such as chloroform, xyleneor 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.

Sectioning

The tissue is then sectionned 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.

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 stainedwith one or more pigments. Hematoxylin and eosin (abbreviated H&E) are among the most commonly used stains in histology and histopathology. Hematoxylin colors nucleiblue, eosin colors the cytoplasmpink. Other compound used to color tissue sections include saffron, silver salts and numerous natural and artificial dyesoriginally developed to stain cloth fibers. The science of tissue staining is called histochemistry.

Recently, antibodiesare 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.

Alternative techniques

Alternative techniques include cryosection. The tissue is frozen and cut using a cryostat. They are stained in simular ways to that of wax sections Plastic embedding is commonly used in the preparation of material for electron microscopy. Tissues are imbedded in epoxyresin. Very thin sections (less than 0.1 micrometers) are cut using diamond or glass knives . The sections are stained with electron dense stains (uranium and lead) so that they can be seen with the electron microscope.

History

In the 19th Century, histology was an academic discipline in its own right. The 1906 Nobel Prizein Physiology or Medicine was awarded to two histologists, Camillo Golgiand Santiago Ramón y Cajal. They had dueling interpretations of the neural structure of the brain based in differing interpretations of the same 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, bone, cartilage and tendon cells,
• bloodcells: 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,
• germ cells: reproductive cells, spermatozoain men, oocytesin women,
• 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 animal tissue.

Related sciences

• Cytology, the study of loose cells, for example cells taken from the cervix during a cervicovaginal smear (pap smear). The cells are directly spread on a glass slide and stained.
• Cell biologythe study of structures smaller than cells, within the cell itself, called organelles. It usually requires an electron microscopeand biochemistrytechniques.
• Anatomy, is the study of organsvisible by the naked eye; and
• Morphology, which studies entire organisms.

Histological artifacts

A histological artifact is a structure or feature that is absent in living tissues, but introduced during preparation or staining. Troubleshooting and minimizing artifacts is a major part of the discipline of histochemistry.

References

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.

See also

• Pathology
• Anatomical pathology
• Histopathology
• Biological staining
• Important publications in histology

External links

• The Histotechnician, from the ASCP web site
• Histology Protocols
• Immunohistochemistry - In Situ Hybridization
• Histowebaf:Histologie

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It uses material from the http://en.wikipedia.org/wiki/Histology Wikipedia article Histology.