Signet ring cells in breast carcinoma. An immunohistochemical and ultrastructural study.

Two cases of breast carcinoma composed predominantly of neoplastic cells with a signet ring appearance, one a case of invasive lobular carcinoma (ILC) and the other a case of invasive ductal carcinoma (IDC), were examined electron microscopically and immunohistochemically. In signet ring cells in the ILC, mucin was demonstrated ultrastructurally in the intracytoplasmic lumina and also to a small degree in the cytoplasmic mucous granules, whereas in signet ring cells in the IDC, mucin was found only in the cytoplasmic mucous granules. Immunohistochemically, signet ring cells in the ILC were intensely positive for gross cystic disease fluid protein (GCDFP-15), but those in the IDC showed no immunoreaction for GCDFP-15. Thus ultrastructural features and GCDFP-15 immunoreactivity appear to be useful for distinguishing between the two different types of signet ring cells.