pleomorphism

Pleomorphism refers to the ability of an organism or cell to take on more than one form or shape. In biology, it denotes variation in size, shape, or internal structure within a single species, rather than the existence of separate species with distinct morphologies. The concept contrasts with monomorphism, where members of a lineage maintain a relatively uniform morphology.

In microbiology, pleomorphism is observed in certain bacteria, algae, and other microorganisms that can alter their appearance in response to environmental conditions such as nutrient availability, temperature, or growth phase. Some bacteria lack rigid cell walls, which can contribute to irregular shapes; others undergo developmental cycles that produce multiple forms. Classic examples include bacteria that vary between coccoid, rod-like, or filamentous forms, and the life-cycle forms of certain pathogens that alternate between different appearances.

In fungi and protozoa, pleomorphism manifests as dimorphism or switching between morphological states. Dimorphic pathogenic fungi may grow as mold at one temperature and as yeast at another, while Candida species can transition between yeast and hyphal forms. Such plasticity is often linked to pathogenicity and tissue invasion.

In pathology, pleomorphism is used to describe variation in size and shape among cells and their nuclei within a tissue sample. Pronounced cellular and nuclear pleomorphism is a hallmark examined by pathologists when evaluating malignant tumors, helping to assess tumor grade and behavior.

Overall, pleomorphism highlights the diversity of cellular form within a lineage and its relevance to biology, diagnosis, and disease.