epithelioid

Epithelioid describes a cell morphology in which cells are polygonal with a relatively large, round to oval nucleus and abundant cytoplasm, giving an epithelial-like appearance. The term is descriptive and does not indicate true epithelial origin; epithelioid cells can arise from various lineages that adopt epithelial-like features.

In histology, epithelioid cells include epithelioid histiocytes, which are activated macrophages that assume an epithelial-like shape. They commonly form granulomas, aggregates of macrophages that may develop into multinucleated giant cells (such as Langhans-type). This arrangement is characteristic of granulomatous inflammation seen in infections (for example, tuberculosis) and certain noninfectious diseases (such as sarcoidosis), though it is not exclusive to any single condition.

The term is also used in tumor descriptions. Epithelioid morphology appears in neoplasms in which tumor cells resemble epithelial cells despite not necessarily arising from epithelial tissue. Examples include epithelioid sarcoma, epithelioid hemangioendothelioma, and epithelioid variants of melanoma or mesothelioma. These tumors typically show cohesive nests or sheets of rounded cells with ample cytoplasm and can mimic carcinomas, requiring immunohistochemical and molecular studies for proper classification.

Diagnostic considerations often rely on immunoprofiles and genetic features. Epithelioid sarcoma characteristically shows cytokeratin and EMA positivity with INI1 (SMARCB1) loss; epithelioid hemangioendothelioma expresses endothelial markers such as CD31 and ERG; epithelioid melanoma exhibits melanocytic markers like S-100, HMB-45, and Melan-A.