micropapillary

Micropapillary is a histopathologic growth pattern observed in several carcinomas, defined by small, tight clusters of tumor cells that form micropapillae lacking a true fibrovascular core. These cell clusters often reside within clear spaces that resemble lymphovascular channels. A characteristic feature is reversed cell polarity, in which the apical surface of tumor cells faces outward. Immunohistochemical staining for polarity markers such as MUC1 or EMA can highlight this reversed orientation.

The pattern is most prominently described in invasive micropapillary carcinoma of the breast, but has also been reported in other sites, including lung adenocarcinoma, urothelial carcinoma, and ovarian serous carcinomas, among others. In many of these tumors, micropapillary features are associated with a higher likelihood of lymphovascular invasion and nodal metastasis, contributing to an overall impression of more aggressive disease in comparison with conventional histologic patterns. However, prognosis is influenced by the broader context of tumor type, stage, and treatment.

Diagnosis relies on light microscopy of biopsies or surgical specimens. Distinguishing true micropapillary architecture from retraction artifacts or other patterns can be challenging, and immunohistochemistry can aid characterization. In breast IMPC, markers that delineate cell polarity help confirm the distinctive inside-out arrangement. Recognition of micropapillary patterns informs pathologists about potential elevations in regional metastatic risk, guiding careful staging and multidisciplinary management.