Printed in Great Britain OBSERVATIONS ON THE SORTING-OUT OF EMBRYONIC CELLS IN MONOLAYER CULTURE

Two problems are raised concerning the movement of cells during tissue-specific sorting-out of chick embryo cells in mixed aggregates, (i) A possible expectation from the hypothesis of 'contact inhibition ' is that cells which are entirely surrounded by other cells in monolayer should be held stationary. Cells within solid aggregates, being totally surrounded by others, might also not be expected to move. How is it then that cell movement takes place within solid aggregates during sorting-out? (ii) Are the movements of cells within sorting aggregates 'passive', being driven by adhesive differentials, or 'active', being merely guided by such differentials? In order to study these questions, sorting-out experiments with chick embryonic limb bud mesenchyme and liver cells were carried out in monolayer culture, permitting direct observation of cell movements. Cell behaviour was observed by time-lapse cinematography. Sorting-out of these cells in monolayer began before and continued after the cells had spread to confluency. During sorting, liver cells showed ruffling activity even when they appeared to be totally surrounded by other cells. Both cell types showed contact inhibition as judged by the criterion of monolayering, fo