Myoepithelial differentiation and basal lamina deposition in fibroadenoma and adenosis of the breast.

Sixteen cases of breast fibroadenomas and 11 of adenosis were studied ultrastructurally; emphasis was placed on the patterns of myoepithelial differentiation and the number and appearance of the basal laminae. Ducts of both fibroadenomas and adenosis showed well differentiated, peripherally arranged myoepithelial elements with conspicuous cytoplasmic filaments and numerous hemidesmosomes; focal myoepithelial multilayering occurred. Myoepithelial cells exhibited complex, convoluted cytoplasmic processes extending into the stroma and resulting in the formation of pseudocysts containing stromal material. Basal lamina deposition was invariably found; basal lamina reduplication was extremely frequent. Basal lamina discontinuities with cytoplasmic processes extending directly into the stroma were seen in adenosis. Myoepithelial features and conspicuous basal lamina deposition indicate advanced differentiation and correlate well with the benign prognosis of these lesions. The focal basal lamina gaps in adenosis may be significant in the long-term evolution of this and similar dysplastic processes.