Editing Histology (section)

===Fixation ===
{{main|Fixation (histology)}}
[[File:Stigmatella personata thin section.jpg|thumb|300px|right|Histologic section of a fossilized invertebrate. [[Ordovician]] [[bryozoan]].]]

Chemical [[Fixation (histology)|fixatives]] are used to preserve and maintain the structure of tissues and cells; fixation also hardens tissues which aids in cutting the thin sections of tissue needed for observation under the microscope.<ref name="Leeson and Leeson, 1981" /><ref name="Bancroft and Stevens, 1982" /> Fixatives generally preserve tissues (and cells) by irreversibly cross-linking proteins.<ref name="Bancroft and Stevens, 1982" /> The most widely used fixative for light microscopy is 10% neutral buffered [[formalin]], or NBF (4% [[formaldehyde]] in [[phosphate buffered saline]]).<ref name="Wick, 2019" /><ref name="Bancroft and Stevens, 1982" /><ref name="Ross and Pawlina, 2016" />

For electron microscopy, the most commonly used fixative is [[glutaraldehyde]], usually as a 2.5% solution in [[phosphate buffered saline]].<ref name="Ross and Pawlina, 2016" /> Other fixatives used for electron microscopy are [[osmium tetroxide]] or [[uranyl acetate]].<ref name="Ross and Pawlina, 2016" />

The main action of these [[aldehyde]] fixatives is to cross-link amino groups in proteins through the formation of [[methylene bridge]]s ({{chem2|\sCH2\s}}), in the case of formaldehyde, or by C<sub>5</sub>H<sub>10</sub> cross-links in the case of glutaraldehyde. This process, while preserving the structural integrity of the cells and tissue can damage the biological functionality of proteins, particularly [[enzymes]].

Formalin fixation leads to degradation of mRNA, miRNA, and DNA as well as denaturation and modification of proteins in tissues. However, extraction and analysis of nucleic acids and proteins from formalin-fixed, paraffin-embedded tissues is possible using appropriate protocols.<ref name="Weiss et al., 2011" /><ref name="Bennike et al., 2016" />