Rigidity and glass transitions in biological tissues

In multicellular organisms, properly programmed collective motion is required to form tissues and organs, and this programming breaks down in diseases like cancer. Recent experimental work highlights that some organisms tune the global mechanical properties of a tissue across a fluid-solid transition to allow or prohibit cell motion and control processes such as body axis elongation. What is the physical origin of such rigidity transitions? Is it similar to zero-temperature jamming transitions in particulate matter, or glass transitions in molecular or colloidal materials? Over the past decade, our group and others have shown that models for confluent tissues, where there are no gaps or overlaps between cells, exhibit a rigidity transition due to geometric incompatibility. A similar transition is also seen in models for biopolymer networks. I will use a newly developed framework for “higher-order rigidity” to discuss similarities and differences between rigidity in particulate matter and rigidity in confluent tissues and fiber networks. I will also discuss recent work to test which mechanisms are operating in real biological systems.