Selection of metastatic variants on the basis of clonal morphology in vitro

The highly metastatic mouse melanoma cell line F87 C16 T2 is heterogeneous for clonal morphology in vitro. Tight, intermediate and loose morphologies can be distinguished. The tight and loose morphologies are selectable on repeated subcloning and stable for at least 15 passages in vitro. Both types of clones and the cell lines derived from them produce tumours in mice but the loose cells produce faster growing primaries and more secondary deposits than tight cells. The loose cells grow only marginally faster than the tight cells in vitro and the saturation densities of both are similar. Direct cloning of cells dissociated from tumours shows that the tight and loose morphologies are stable in vivo and that there is no obvious selection for either cell type in primary or secondary tumours. Nevertheless injections of progressively fewer cells suggests that loose cells are up to ten times more efficient at producing tumours. Our findings are discussed in relation to current ideas on the importance of heterogeneity and selection in the production of metastases.