Topological primitives.

Possible topological changes when cells grow or when their boundaries move. (a) Edge insertion. When two isolated cells come into contact, we add an edge to represent the intersection plane of the boundary between these two cells. (b) Void removal. When three cells come into contact, the curve triangular empty space is replaced by a vertex where the inner edges meet. (c) Edge flip. When two previously disconnected cells expand to meet while pushing away two previously connected cells, we replace one inner edge with a new inner edge.