anaplastic

Anaplastic, in pathology, describes cells or tissues that have lost the differentiated features of their cell type, appearing primitive or undifferentiated. The term is most often applied to neoplasms, where anaplasia indicates a high level of cellular dedifferentiation and is commonly associated with malignancy.

Morphologic features include marked cellular pleomorphism, enlarged and hyperchromatic nuclei with increased mitotic activity, abnormal mitotic figures, high nuclear-to-cytoplasmic ratio, and loss of normal tissue architecture and polarity.

Clinical significance: Anaplasia is a hallmark of high-grade malignant tumors. Its presence suggests aggressive behavior and a poorer prognosis compared with well-differentiated cancers. Some tumors are described as "anaplastic" or "undifferentiated" when they lack recognizable tissue-specific differentiation.

Examples: Anaplastic thyroid carcinoma is a rare but extremely aggressive thyroid cancer characterized by pronounced anaplasia. Anaplasia can occur in a variety of cancers, including sarcomas and carcinomas, and may reflect dedifferentiation of a previously differentiated tumor. In some cases, tumors may have focal areas of anaplasia within a previously differentiated lesion.

Diagnosis and terminology: Pathologists diagnose anaplasia on histopathologic examination, often supported by immunohistochemistry and molecular studies to define lineage and to differentiate from truly poorly differentiated or sarcomatoid patterns. The term "anaplastic" is descriptive and does not correspond to a single, uniform histologic entity.