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Histology (from the Greek '''') is the study of Tissue sectioned as a thin slice, using a Microtome . It can be described as microscopic Anatomy . The photographing of stained cells is called '''histography'''. 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 scientists who perform the preparation of histological sections are Histotechnicians, Histology Technicians (HT), Histology Technologists (HTL), Medical Scientists, Medical Laboratory Technicians or Biomedical Scientist s. Their field of study is called '''histotechnology'''. SOURCE OF TISSUE Histological examination of tissues starts with Surgery , Biopsy or Autopsy (or Necropsy , in the case of animal tissues). Engineered Tissue is also used for histological examination, some labs are able to use cells and engineer a tissue that would act as an equivalent to real tissue. The equivalent would not resemble real tissue, but would be a useful model for studying the interactions of cells with infection and many other conditions. TECHNICAL PROCEDURE Fixation See Also: Fixation (histology) The tissues are mechanically and biochemically stabilized in a fixative. The most common fixative is neutral buffered formalin (10% Formaldehyde in Phosphate Buffered Saline (PBS)). A good fixative is when it is less toxic to the person handling it, and when it does not damage and harden the tissue being preserved. Processing The most common technique is wax processing. The samples are immersed in multiple baths of progressively more concentrated Ethanol to dehydrate the tissue, followed by a clearing agent such as, Xylene or Histoclear, and finally hot molten Paraffin Wax (impregnation). During this 12 to 16 hour process, paraffin wax will replace the xylene: Embedding Soft, moist tissues are turned into a hard paraffin block, which is then placed in a mold containing more molten wax (embedded) and allowed to cool and harden. Embedding can also be accomplished using frozen, non-fixed tissue in a freezing medium. This freezing medium is liquid at room temperature but when cooled will solidify. Non-fixed tissue allows for procedures such as in-situ hybridizations for specific MRNA that would have been destroyed during the fixing process. It also allows for very short turnaround where that is needed, as with a patient currently undergoing Surgery . Sectioning The tissue is then sectioned into very thin (2 - 8 micrometer) sections using a Microtome . These slices, usually thinner than the average Cell , are then placed on a glass Slide for Staining . Frozen tissue embedded in a freezing medium is cut on a microtome in a cooled machine called a Cryostat . Staining Routine staining:This is done to give contrast to the tissue being examined, as without staining it is very difficult to see differences in Cell Morphology . Hematoxylin and Eosin (abbreviated H&E) are the most commonly used stains in histology and histopathology. Hematoxylin colors Nuclei blue, eosin colors the Cytoplasm pink. To see the tissue under a , oil red o, congo red, fast green FCF, silver salts and numerous natural and artificial Dye s, that were usually originated from the development dyes for the textile industry. Histochemistry refers to the science of using chemical reactions between laboratory chemicals and components within tissue. A commonly performed histochemical technique is the Perls Prussian Blue reaction, used to demonstrate iron deposits in diseases like Hemochromatosis . Histology samples have often been examined by radioactive techniques. In Historadiography a slide (sometimes stained histochemically) is X-rayed. More commonly, Autoradiography is used to visualize the locations to which a radioactive substance has been transported within the body, such as cells in S Phase (undergoing DNA Replication ) which incorporate tritiated Thymidine , or sites to which radiolabeled Nucleic Acid probes bind in In Situ Hybridization . For autoradiography on a microscopic level the slide is typically dipped into liquid nuclear tract emulsion, which dries to form the exposure film. Individual silver grains in the film are visualized with Dark Field Microscopy . Recently, , or when the stain is a Fluorescent molecule, Immunofluorescence . This technique has greatly increased the ability to identify categories of cells under a microscope. Other advanced techniques can be combined with this, such as nonradioactive In Situ Hybridization to identify specific DNA or RNA molecules with fluorescent probes or tags that can be used for immunofluorescence and Enzyme-linked Fluorescence amplification (especially Alkaline Phosphatase and Tyramide Signal Amplification ). Fluorescence Microscopy and Confocal Microscopy are used to detect fluorescent signals with good intracellular detail. Digital Camera s are increasingly used to capture histological and histopathological images. COMMON LABORATORY STAINS Table sourced from 1 The Nissl Method and Golgi's Method are useful in identifying Neuron s. Alternative techniques Alternative techniques include Cryosection . The tissue is frozen and cut using a Cryostat . Tissue staining methods are similar to those of wax sections. Plastic embedding is commonly used in the preparation of material for electron microscopy. Tissues are embedded 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 the histologists, Camillo Golgi and Santiago Ramon 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 There are four basic types of tissues: muscle tissue, nervous tissue, connective tissue, and epithelial tissue. All tissue types are subtypes of these four basic tissue types (for example blood cells are classified as connective tissue since they generally originate inside bone marrow).
Note that tissues from plant, fungus and microorganisms can also be examined histologically. Their structure is very different from animal tissue. RELATED SCIENCES
ARTIFACTS See Artifacts (Histology) for the main article and list of known histological artifacts Artifacts are structures or features in tissue that interfere with normal histological examination. These are not always present in normal tissue and can come from outside sources. Artifacts interfere with histology by changing the tissues appearance and hiding structures. These can be divided into two categories: Pre-histology These are features and structures that have being introduced prior to the collection of the tissues. A common example of these include: ink from tattoos and freckles (melanin) in skin samples. Post-histology Artifacts can result from tissue processing. Processing commonly lead to changes like shrinkage, color changes in different tissues types and alterations of the structures in the tissue. Because these are caused in a laboratory the majority of post histology artifacts can be avoided or removed after being discovered. A common example is mercury pigment left behind after using Bouin's Fixative to fix a section. 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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