  Dorsal closure is a process that occurs during embryogenesis of Drosophila
melanogaster. During dorsal closure, the amnioserosa (AS), a one-cell thick
epithelial tissue that fills the dorsal opening, shrinks as the lateral
epidermis sheets converge and eventually merge. During this process, the aspect
ratio of amnioserosa cells increases markedly. The standard 2-dimensional
vertex model, which successfully describes tissue sheet mechanics in multiple
contexts, would in this case predict that the tissue should fluidize via cell
neighbor changes. Surprisingly, however, the amnioserosa remains an elastic
solid with no such events. We here present a minimal extension to the vertex
model that explains how the amnioserosa can achieve this unexpected behavior.
We show that continuous shrinkage of the preferred cell perimeter and cell
perimeter polydispersity lead to the retention of the solid state of the
amnioserosa. Our model accurately captures measured cell shape and orientation
changes and predicts non-monotonic junction tension that we confirm with laser
ablation experiments.
