We describe a direct way of measuring contact inhibition of locomotion by analysing the changes in motion of pairs of colliding cells. This allows values to be assigned to each type of cell in mixed collisions and will enable certain hypotheses about the relationship between contact inhibition and invasion in culture to be tested critically. We find that fibrosarcoma (FS9) cells, on colliding with chick heart fibroblasts, show a reversed contact-inhibition response that we call contact promotion of locomotion. We also describe a measure of the lateral changes in motion that result from collisions between cells and show that this is dependent on the type of colliding cell but, unlike contact inhibition, it does not appear to be dependent on the type of cell with which it collides for the types studied here. Finally, we analyse how the total response is dependent on the dispositions and motions of the cells before collision and we find that FS9 cells, on colliding with fibroblasts, tend to turn towards the point of initial marginal contact. We conclude that the FS9 cells show a pronounced response on colliding with the fibroblasts, which is in contrast to the subjective impression that the FS9 cells do not respond much. These findings support the thesis of Abercrombie and colleagues, that the infiltration of a population of normal cells by a population of invasive cells in culture is dependent on the nature of the response of each cell type to collision with the other and that the invasive cells fail to show contact inhibition in these heterotypic collisions; but the findings further suggest that these invasive cells show an active invasionary response as opposed to merely failing to show contact inhibition.

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