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Pleomorphism in The American Heritage Dictionary is defined as the occurrence of two or more structural forms during a life cycle, especially of certain plants.
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 as Antoine Béchamp). Monomorphic theory, supported by Rudolf Virchow, Ferdinand Cohn, and Robert Koch, emerged to become the dominant paradigm 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.
One of the many areas in which Pasteur and Bechamp argued concerned Pleomorphism. Bechamp contended that bacteria could change forms. A rod-shaped bacteria could become spheroid, etc. Pasteur disagreed. In 1914, Madame Victor Henry of Pasteur Institute confirmed that Bechamp was correct and Pasteur wrong.
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.
The virions 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 the hepatitis B virus.