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Cellular adaptation
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<h2 id="atrophy">Atrophy</h2> <p class="note">Main article: <a href="/facts/Atrophy/QNxSoxDV">Atrophy</a></p> <p>Cellular atrophy is a decrease in cell size. If enough cells in an organ undergo atrophy the entire organ will decrease in size. <a href="/facts/Thymus/YYEJu7KA">Thymus</a> atrophy during early <a href="/facts/Human_development_(biology)/S97k10C2">human development</a> (childhood) is an example of physiologic atrophy. <a href="/facts/Skeletal_muscle/en1tVYFN">Skeletal muscle</a> atrophy is a common pathologic adaptation to skeletal muscle disuse (commonly called "disuse atrophy"). Tissue and organs especially susceptible to atrophy include skeletal muscle, cardiac muscle, <a href="/facts/Secondary_sex_characteristic/lV2Fl3uA">secondary</a> <a href="/facts/Sex_organ/c2ypY14E">sex organs</a>, and the <a href="/facts/Brain/EKla9NkJ">brain</a>.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p> <h2 id="hypertrophy">Hypertrophy</h2> <p class="note">Main article: <a href="/facts/Hypertrophy/4JolPfnS">Hypertrophy</a></p> <p>Cellular hypertrophy is an increase in cell size and volume. If enough cells of an organ hypertrophy the whole organ will increase in size. Hypertrophy may involve an increase in intracellular <a href="/facts/Protein/Jxmagu3E">protein</a> as well as <a href="/facts/Cytosol/VyXgygWm">cytosol</a> (intracellular fluid) and other cytoplasmic components. For example, <a href="/facts/Adipocyte/FuM7LlaK">adipocytes</a> (fat cells) may expand in size by depositing more <a href="/facts/Lipid/TdQR87HX">lipid</a> within cytoplasmic <a href="/facts/Vesicle_(biology_and_chemistry)/1KjHpjMM">vesicles</a>. Thus in human adults, increases in body fat tissue occurs mostly by increases in the size of adipocytes, not by increases in the number of adipocytes.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Hypertrophy may be caused by <a href="/facts/Biomechanics/H4qnnmu5">mechanical</a> signals (e.g., stretch) or <a href="/facts/Tropic_hormone/KY47Fnkc">trophic</a> signals (e.g., growth factors). An example of physiologic hypertrophy is in skeletal muscle with sustained <a href="/facts/Weight_bearing/lRHt7tWt">weight bearing</a> exercise. An example of pathologic hypertrophy is in cardiac muscle as a result of <a href="/facts/Hypertension/wlAvBfcf">hypertension</a>.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <h2 id="hyperplasia">Hyperplasia</h2> <p class="note">Main article: <a href="/facts/Hyperplasia/gDv6kB4j">Hyperplasia</a></p> <p>Hyperplasia is an increase in the number of cells.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> It is the result of increased cell <a href="/facts/Mitosis/ExxrSUs8">mitosis</a> or division (also referred to as <a href="/facts/Cell_proliferation/Jp3hWo2t">cell proliferation</a>). The two types of physiologic hyperplasia are <a href="/facts/Compensatory_growth_(organ)/nsb6yTDy">compensatory</a> and <a href="/facts/Hormonal/WFnBnINJ">hormonal</a>. Compensatory hyperplasia permits tissue and organ regeneration. It is common in epithelial cells of the <a href="/facts/Epidermis_(skin)/JvRABubP">epidermis</a> and <a href="/facts/Intestine/nK2zwqBS">intestine</a>, <a href="/facts/Liver/Rtp4Fk2b">liver</a> <a href="/facts/Hepatocytes/USpvDz1n">hepatocytes</a>, <a href="/facts/Bone_marrow/4dfudk0H">bone marrow</a> cells, and <a href="/facts/Fibroblasts/n08KsFll">fibroblasts</a>. It occurs to a lesser extent in <a href="/facts/Bone/kwJaR6IQ">bone</a>, <a href="/facts/Cartilage/vxy2qzKN">cartilage</a>, and <a href="/facts/Smooth_muscle/mpcpTsHp">smooth muscle</a> cells. Hormonal hyperplasia occurs mainly in organs that depend on <a href="/facts/Estrogen/dPEewnDV">estrogen</a>. For example, the estrogen-dependent uterine cells undergo hyperplasia and hypertrophy following pregnancy. Pathologic hyperplasia is an abnormal increase in cell division. A common pathologic hyperplasia in women occurs in the <a href="/facts/Endometrium/LcesXIXm">endometrium</a> and is called <a href="/facts/Endometriosis/wKS0I8mc">endometriosis</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h2 id="metaplasia">Metaplasia</h2> <p class="note">Main article: <a href="/facts/Metaplasia/k3S1XCrA">Metaplasia</a></p> <p>Metaplasia occurs when a cell of a certain type is replaced by another cell type, which may be less differentiated. It is a reversible process thought to be caused by stem cell reprogramming. <a href="/facts/Stem_cells/qgB9dFrI">Stem cells</a> are found in epithelia and <a href="/facts/Embryo/9QHnLVLN">embryonic</a> <a href="/facts/Mesenchyme/3QDGRD1C">mesenchyme</a> of <a href="/facts/Connective_tissue/mal3fc7k">connective tissue</a>. A prominent example of metaplasia involves the changes associated with the respiratory tract in response to inhalation of irritants, such as smog or smoke. The <a href="/facts/Bronchial/t83cUXou">bronchial</a> cells convert from <a href="/facts/Mucus/hnlBuHAy">mucus</a>-secreting, <a href="/facts/Ciliated/C9l436HM">ciliated</a>, <a href="/facts/Columnar/ecKHcHC3">columnar</a> epithelium to non-ciliated, <a href="/facts/Squamous/ecKHcHC3">squamous</a> epithelium incapable of secreting mucus. These transformed cells may become dysplastic or cancerous if the stimulus (e.g., cigarette smoking) is not removed. The most common example of metaplasia is <a href="/facts/Barrett%2527s_esophagus/NUKY2U7p">Barrett's esophagus</a>, when the non-keratinizing squamous epithelium of the esophagus undergoes metaplasia to become mucinous columnar cells, ultimately protecting the esophagus from acid reflux originating in the stomach. If stress persists, metaplasia can progress to dysplasia and eventually carcinoma; Barrett's esophagus, for example, can eventually progress to adenocarcinoma.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p> <h2 id="dysplasia">Dysplasia</h2> <p class="note">Main article: <a href="/facts/Dysplasia/sR1UKrbB">Dysplasia</a></p> <p>Dysplasia refers to abnormal changes in cellular shape, size, and/or organization. Dysplasia is not considered a true adaptation; rather, it is thought to be related to hyperplasia and is sometimes called "atypical hyperplasia". Tissues prone to dysplasia include <a href="/facts/Cervix/qw3gl8Pq">cervical</a> and <a href="/facts/Respiratory/IG8v5JEr">respiratory</a> epithelium, where it is strongly associated with the development of cancer; it may also be involved in the development of <a href="/facts/Breast_cancer/6hME580v">breast cancer</a>. Although dysplasia is reversible, if stress persists, then dysplasia progresses to irreversible carcinoma.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Cell_damage/I2vy7A9u">Cell damage</a></li></ul> <h2 id="notes-and-references">Notes and references</h2> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Cellular Adaptation". The Lecturio Medical Concept Library. 26 October 2020. Retrieved 7 July 2021. <a href="https://www.lecturio.com/concepts/cellular-adaptation/" target="_blank">https://www.lecturio.com/concepts/cellular-adaptation/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Cellular Adaptation". The Lecturio Medical Concept Library. 26 October 2020. Retrieved 7 July 2021. <a href="https://www.lecturio.com/concepts/cellular-adaptation/" target="_blank">https://www.lecturio.com/concepts/cellular-adaptation/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Miller M, Zachary J (17 February 2017). "Mechanisms and Morphology of Cellular Injury, Adaptation, and Death". Pathologic Basis of Veterinary Disease: 2–43.e19. doi:10.1016/B978-0-323-35775-3.00001-1. ISBN 9780323357753. PMC 7171462. <a href="9780323357753" target="_blank">9780323357753</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Hopkin, Michael (5 May 2008). "Fat cell numbers stay constant through adult life". Nature. Springer Nature: news.2008.800. doi:10.1038/news.2008.800. Archived from the original on 16 October 2019. <a href="https://web.archive.org/web/20191016014247/https://www.nature.com/news/2008/080505/full/news.2008.800.html" target="_blank">https://web.archive.org/web/20191016014247/https://www.nature.com/news/2008/080505/full/news.2008.800.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Miller M, Zachary J (17 February 2017). "Mechanisms and Morphology of Cellular Injury, Adaptation, and Death". Pathologic Basis of Veterinary Disease: 2–43.e19. doi:10.1016/B978-0-323-35775-3.00001-1. ISBN 9780323357753. PMC 7171462. <a href="9780323357753" target="_blank">9780323357753</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Hyperplasia: MedlinePlus Medical Encyclopedia". medlineplus.gov. <a href="https://medlineplus.gov/ency/article/003441.htm" target="_blank">https://medlineplus.gov/ency/article/003441.htm</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Miller M, Zachary J (17 February 2017). "Mechanisms and Morphology of Cellular Injury, Adaptation, and Death". Pathologic Basis of Veterinary Disease: 2–43.e19. doi:10.1016/B978-0-323-35775-3.00001-1. ISBN 9780323357753. PMC 7171462. <a href="9780323357753" target="_blank">9780323357753</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Miller M, Zachary J (17 February 2017). "Mechanisms and Morphology of Cellular Injury, Adaptation, and Death". Pathologic Basis of Veterinary Disease: 2–43.e19. doi:10.1016/B978-0-323-35775-3.00001-1. ISBN 9780323357753. PMC 7171462. <a href="9780323357753" target="_blank">9780323357753</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Porth, Carol Mattson (2005). Pathophysiology : Concepts of Altered Health States (PDF) (7th ed.). Philadelphia, Pa: Lippencott, Williams & Wilkins. p. 105. ISBN 0-7817-4988-3. Archived from the original (PDF) on 29 December 2016. Retrieved 28 December 2016. <a href="0-7817-4988-3" target="_blank">0-7817-4988-3</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> </ol>