The ultrastructure and in vitro growth properties of 5 tumorigenic mouse submandibular-gland epithelial cell lines were studied. In all lines, in vitro acinus formation occurred, and well differentiated epithelial cells showing epithelial microvilli and desmosomes and cytoplasmic tonofilaments were present. None of the cells showed specific ultratructural features of the normal differentiated submandibular-gland ducts. All the lines formed colonies in semi-solid agar and on confluent monolayers of BALB/c 3T3 cells, and all lacked density-dependent inhibition of growth, as demonstrated by a random distribution of [3H]TdR labelling throughout growing colonies. These 3 growth properties appear to be reliable in vitro markers for epithelial neoplastic transformation in this system, but colony-forming efficiency in agar is lower than that reported for many transformed mesenchymal cells
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