  Cell neighbor exchanges play a critical role in regulating tissue fluidity
during epithelial morphogenesis and repair. In vivo, these neighbor exchanges
are often hindered by the formation of transiently stable four-fold vertices,
which can develop into complex multicellular rosettes where five or more cell
junctions meet. Despite their importance, the mechanical origins of
multicellular rosettes have remained elusive, and current cellular models lack
the ability to explain their formation and maintenance. Here we present a
dynamic vertex model of epithelial tissues with strain-dependent tension
remodeling and mechanical memory dissipation. We show that an increase in cell
junction tension upon contraction and reduction in tension upon extension can
stabilize higher-order vertices, temporarily stalling cell rearrangements. On
the other hand, inducing mechanical memory dissipation via relaxation of
junction strain and stress promotes the resolution of higher-order vertices,
facilitating cell neighbor exchanges. We demonstrate that by tuning the rates
of tension remodeling and mechanical memory dissipation, we can control
topological transitions and tissue material properties, recapitulating complex
cellular topologies seen in developing organisms.
