- Pleomorphism
Pleomorphism is the occurrence of two or more structural forms during a life cycle, especially of certain plants.
It can also apply at the species level.cite web |url=http://www.mercksource.com/pp/us/cns/cns_hl_dorlands_split.jsp?pg=/ppdocs/us/common/dorlands/dorland/six/000083390.htm |title=Dorlands Medical Dictionary:pleomorphism |format= |work= |accessdate=]
Bacteria
In the first decades of the 20th century, the term was used to refer to the supposed ability of
bacteria to change shape dramatically or to exist in a number of extreme morphological (changing) forms. This claim sparked a controversy among the microbiologists and split them into two schools: the monomorphists, who opposed the claim, and the pleomorphists (such asAntoine Béchamp ).Monomorphic theory, supported by
Louis Pasteur ,Rudolf Virchow ,Ferdinand Cohn , andRobert Koch , emerged to become the dominantparadigm in modern medical science: it is now almost universally accepted that each bacterial cell is derived from a previously existing cell of practically the same size and shape.The modern-day definition of pleomorphism in the context of bacteria is now a "variation" of size or shape of the cell or cell nuclei, rather than a "change" of shape as previously.
Neoplasms
The term is also used in cytology to describe variability in the size and shape of cells and/or their nuclei. It is a feature characteristic of
malignant neoplasms .Furthermore, the tumors themselves can express variable appearance, and can then be noted pleomorphic, e.g.
Pleomorphic adenoma .Virus
The
virion s of certain viruses are sometimes seen to express pleomorphism, in the sense that they can show variable appearances. However, this characteristic is in fact not a true pleomorphic characteristic, since one and the same virion doesn't change shape, although its successors might take another shape. One example is thehepatitis B virus.References
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