Solid papillary carcinoma of breast: an ultrastructural study.

Solid papillary carcinoma of the breast is a subset of papillary carcinoma, which occurs in older women and usually has a favorable prognosis. It is primarily intraductal but also is often associated with invasive carcinoma, especially mucinous carcinoma. Intracellular and extracellular mucin is also found in the in situ stage, in most tumors. In addition to forming solid papillary masses, the cells palisade around vessels in pseudorosettes and show minimal nuclear atypia. Some cells show neuroendocrine differentiation, based on argyophilia with Grimelius staining. Four examples of this neoplasm were studied electron microscopically. Myoepithelial cells were not found. Neoplastic cells had an ultrastructure that was generally similar to that of other types of mammary carcinoma. There were extracellular microlumens, but intracellular lumens and pseudolumens were few or absent. Secretory activity varied among cells, and those cells appearing active had a variety of granule types, including typical flocculent and "bull's-eye" mucinous granules, small dense-core granules, and large serous-like granules. Some of the dense-core granules were interpreted as neuroendocrine in nature, based on their abluminal or juxtavascular location, whereas others that were apical and subluminal were probably mucinous in type. The large serous-appearing granules were subluminal in some cells and diffuse in others and may also have represented a variant of mucinous granules. The results support earlier opinions that accurate interpretation of specific granular function at the electron microscopic level depends on cytochemical studies using uranaffin as a marker of neuroendocrine activity. Although mucinous granules are identified by their lack of staining with uranaffin, the nature of the serous-appearing granules would still not be answered by this method; that is, a negative reaction would not define whether the granules are truly serous, or simply another form of mucin. Regardless of limitations of this type, correlation and extrapolation of histochemical (Grimelius and Alcian blue) and immunohistochemical (chromogranin and synaptophysin) results with subcellular structure are still very useful in establishing cell type.