8/ We also developed a second model of CIL where instead of crawling away from neighbors, cells reach out and grab new neighbors away from their current neighbors. This pulling model also results in a fluid under tension.

7/ Next, we figured the simplest way to fight the clumping instability is to direct that motion not randomly, but away from a cell’s neighbors, like contact inhibition of locomotion. This generates a tensioned fluid network of cells that, while directed, still flows diffusively.

6/ We first tried maintaining the cell network by letting the cells move with a random self-propelled walk commonly used to model cell motion. However, no set of parameters could generate the kind of material we see in experiments. It always clumps!

5/ But how can a network be under tension and flowing at the same time? Breaking one bond would cause a cell to be pulled towards its remaining neighbors. To model this, we use a simple but effective model of hysteretic sticking between cells.

4/ However, those stellate appendages made us wonder, is there tension across those arms? By ablating the PSM with a laser, we measure a significant retraction velocity, suggesting yes, the cellular network is under tension!

3/ We confirm that, just like in many other systems of body-axis elongation, when we track the relative motion of the cells in the avian PSM, they move diffusively like a fluid.

2/ First, notice how different the cell architecture in the avian presomitic mesoderm (PSM) is from confluent and bubble-like cells seen in other tissues. It looks more like a network of cells attached by stellate arms.