In vitro neoplastic transformation of mouse skin cells: morphology and ultrastructure of cells and tumors.

Mixed cultures of epidermal and dermal cells from term fetuses of Balb/cAn mice were exposed to high concentrations (50 μg/ml) of 7. 12-dimethylbenz(a)anthracene (DMBA) in medium containing Tween-80 or to medium with Tween-80 alone for 45 min. Within 5 weeks the cultures exposed to DMBA began to exhibit accelerated growth in vitro and an epithelioid morphology. These same changes occurred in the Tween-80-treated group starting around 15 weeks in culture. Both sets of cultures remained epithelioid in appearance and rapidly growing for over 9 months. Injections of cells into syngeneic hosts, beginning approximately 21 weeks after treatment, gave rise to undifferentiated tumors. Animals receiving carcinogen-treated cells had more tumors than those receiving vehicle-treated cells. Ultrastructural studies of the transformed cells in vitro suggested that they were of keratinocyte origin, but DMBA + Tween-80-treated cells were more differentiated than cells treated with Tween-80 alone. 80-100 A cytofilaments, presumably identical to tonofilaments in vivo, were organized into bundles which traversed both the cellular endoplasm and ectoplasm, either terminating on adherens-like junctional structures, or looping back into the cytoplasm after attaching peripherally to such structures. After transplantation into syngeneic hosts, transformed cells from both groups produced tumors which ultrastructurally resembled anaplastic squamous-cell carcinomas. When highly undifferentiated tumors derived from DMBA + Tween-80-treated cells were recultured, more differentiated features reappeared. This observation indicates an important role for the cellular millieu in the determination of phenotypic expression.