Sarcomatoid carcinoma: an ultrastructural study with light microscopic and immunohistochemical correlation of 10 cases from various anatomic sites.

The histogenesis of sarcomatoid carcinoma has been an intriguing topic for pathologists for many years, and considerable evidence has accumulated in the fields of tissue culture, electron microscopy, and immunohistochemistry to support the concept that the sarcomatous cells derive by way of "divergent differentiation" (metaplasia) from the carcinomatous elements. We have studied a group of 10 cases of these tumors from various organs, using detailed ultrastructural analysis as well as light microscopic and immunohistochemical correlation. We found that there is an ultrastructural spectrum of differentiation from epithelial to mesenchymal type features and that the malignant spindle cells may be purely epithelial (3 cases), purely mesenchymal (3 cases), or a mixture of both (4 cases). Furthermore, individual cells may show biphasia, having desmosomes and tonofibrils as well as well developed rough endoplasmic reticulum and filaments with dense bodies. Electron microscopic and immunohistochemical results do not always correlate, illustrating the prudence of using several keratin antibodies, including wide-spectrum ones, and of performing electron microscopic examination on these tumors.