Fig. 1.

An epithelium and its corresponding models. a An image of amnioserosa cells in a Drosophila embryo during early dorsal closure (courtesy of M. Shane Hutson). Primary functional and force-generating structures are shown in (b). c Illustrates the net interfacial tensions $\mathrm{\gamma }$ acting along the cell boundaries and the effective viscosity $\mathrm{\mu }$ of their cytoplasm. For explanatory purposes only, the cells are considered to be of two types and the tensions associated with different kinds of boundaries are labelled with subscripts. d Monoline model of the system in (c) and straight rod elements (shown in yellow) are used to represent each cell edge and to carry its interfacial tension $\mathrm{\gamma }$. Select dashpots representing the effective cytoplasm viscosity $\mathrm{\mu }$ are shown. Notice that nodes (red dots) exist only at the triple (or higher-order) junctions. e The associated polyline model and its segmented edges have multiple rod elements connected by intermediate nodes (shown in blue). Note how the segmented edges much more closely approximate the true cell shapes