Squamous metaplasia of the breast. An ultrastructural and immunologic evaluation.

Squamous metaplasia in the breast is rare. This case of an intracystic papilloma of the breast with prominent squamous change was found by electron microscopy and immunohistochemistry to contain evidence of squamous differentiation within myoepithelial cells. The findings suggest that squamous metaplasia of the breast may result from myoepithelial cell differentiation in a manner analogous to the development of squamous lesions in the cervix and salivary gland. The possible relationship of this finding to the development of squamous lesions of the breast is discussed.