The development of aneuploid clones from diploid progenitor cells is a regular characteristic of head and neck squamous cell carcinoma progression. While the significance of aneuploidy formation for the acquisition of invasive and metastatic behavior is well documented, little is known about the contribution of diploid tumor cells after aneuploid clones have emerged. To distinguish diploid cells of epithelial origin from benign cellular components, we applied multiparameter flow cytometry of DNA content and cytokeratin (CK) expression to 36 primary tumors. Twenty-seven carcinomas accommodated aneuploid cell lines that stained positive for CK. All diploid cell populations obtained from aneuploid carcinomas contained CK-positive subpopulations as did all of nine tumors that consisted exclusively of diploid cells. The proportions of CK-positive diploid cells ranged between 6% and 80%, independent of whether they were achieved from entirely diploid or from aneuploid carcinomas. CK-gated diploid and aneuploid cell populations showed largely identical S-phase fractions. These results emphasize that diploid tumor cells regularly persist after the development of aneuploid clones and significantly contribute to local tumor progression. Despite the presence of diploid epithelial cells in aneuploid primary tumors, exclusively the aneuploid clones of eight corresponding lymph node metastases were CK-positive. This provides further evidence of a largely reduced metastatic potential of diploid tumor cells.

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