Two-dimensional cell model.

a) An isolated cell is modeled as a disk. b) A cell is modeled as a disk segment when contacting other cell(s). An outer edge e_i is an arc or a circle, representing the boundary between cell c_i and the outside medium (denoted as c₀). An inner edge e_i,j occurs when a cell c_i is in contact with another cell c_j. Their shared boundary is modeled as a straight line segment. When two cells c_i and c_j make contact, their outer edges (arcs) e_i and e_j intersect at two vertices v_i,0,j and v_j,0,i, which are also the two end-points of the inner edge e_i,j. c) When three cells c_i,c_j and c_k intersect, they form a vertex v_i,j,k. d) A cell completely surrounded by other cells is represented as a polygon.