  The spontaneous formation of vortices is a hallmark of collective cellular
activity. Here, we study the onset and persistence of coherent angular motion
(CAMo) as a function of the number of cells $N$ confined in circular
micropatterns. We find that the persistence of CAMo increases with $N$ but
exhibits a pronounced discontinuity accompanied by a geometric rearrangement of
cells to a configuration containing a central cell. Computer simulations based
on a generalized Potts model reproduce the emergence of vortex states and show
in agreement with experiment that their stability depends on the interplay of
spatial arrangement and internal polarization of neighboring cells. Hence, the
distinct migrational states in finite size ensembles reveal significant insight
into the local interaction rules guiding collective migration.
