**2.4 Cell–cell adhesion dynamics dictate the structure of multicellular assemblies**

The assembly of multicellular structures does not just depend on the strength of the underlying cell–cell adhesions but also their dynamics. If cell–cell adhesions are dynamic, meaning that formed protein–protein interactions constantly form and disassemble within the chemical equilibrium, cells can move with respect to each other and maximize the number of adhesive contacts they form. This scenario is observed in mixtures of iLID and Nano expressing cells, which assembled into spherical and compact clusters. If cell–cell adhesions are not dynamic, meaning that once protein– protein interactions form that they do not reverse, cells stick to the first cell they meet and cannot move to areas with potential higher numbers of adhesions. For example, mixtures of nMagHigh and pMagHigh or nMag and pMag expressing cells assemble into ramified branched structures, which are kinetically trapped. Optogenetics allows for the altering of the dynamics of the cell–cell adhesion by turning light on and off. The ramified structures formed with nMag and pMag cells could then be converted into compact spheres under pulsed illumination (5 min on, 5 min off), allowing the adhesions to dissipate and the cells to move.
