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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 . Histopathology , the microscopic study of diseased tissue, is an important tool of Anatomical Pathology since accurate diagnosis of Cancer and 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 , Biopsy or 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% Formaldehyde in water). Embedding The most common technique is wax embedding. The samples are 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. Sectioning The tissue is then sectioned into very thin (3 - 5 micrometer) sections using a ''' Microtome '''. These slices, thinner than the average Cell , are then placed on a glass slide for Staining . Staining A 3 to 5 micrometer slice of most tissues is almost completely transparent with very little or no visible detail. To see the tissue under a Microscope , the sections are Stained with one or more pigments. Hematoxylin and ''' Eosin ''' (abbreviated H&E) 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 Safranine , fast green FCF, silver salts and numerous natural and artificial Dyes originally developed to stain cloth fibers. The science of tissue staining is called histochemistry. Recently, ''' 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 Epoxy resin. 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 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. HISTOLOGICAL CLASSIFICATION OF ANIMAL TISSUES
Note that tissues from plant, fungus and microorganisms can also be examined histologically. Their structure is very different from animal tissue. RELATED SCIENCES
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
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