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.^[1]

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 as Antoine Béchamp).

Monomorphic theory, supported by Louis Pasteur, 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.

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 histology and 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 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.

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