Unstable Cellular Differentiation in Adenosquamous Cell Carcinoma23

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Abstract
For the determination of whether two or more stem lines with fixed or stable differentiation are present in a growing mixed adenosquamous cell carcinoma, two different mixed tumors, derived from transformed F344 rat tracheal epithelium, were dissociated and ten single cells were isolated from each tumor. Each of these single cells was allowed to grow 27 population doublings to form a clonal population. All clones were then inoculated into animals for the assessment of in vivo differentiation. All cell inocula produced carcinomas within 7–70 days. Of the ten single cell clones isolated from the first tumor, seven clones produced mixed adenosquamous carcinomas and three produced adenocarcinomas in which no squamous cell component was found. From the second tumor, eight clones produced mixed tumors and two produced squamous cell carcinomas in which no adenocarcinoma component was found. The mixed tumors derived from clones contained widely varying proportions of adeno and squamous components. Recloning of the cloned lines yielded populations that produced mixed adenosquamous cell carcinomas upon inoculation in vivo. Comparison of tumor phenotypes from clones of early and late passages further confirmed the instability of differentiation in these neoplastic epithelial cells. These studies strongly suggest that the mixture of differentiated cell types in adenosquamous cell carcinomas is not the result of a mixture of stem cells with one fixed phenotype within the tumor, but rather is the result of an instability of differentiation; the neoplastic cells can express both types of differentiation.